Source

semantic / doc / wisent.texi

   1
   2
   3
   4
   5
   6
   7
   8
   9
  10
  11
  12
  13
  14
  15
  16
  17
  18
  19
  20
  21
  22
  23
  24
  25
  26
  27
  28
  29
  30
  31
  32
  33
  34
  35
  36
  37
  38
  39
  40
  41
  42
  43
  44
  45
  46
  47
  48
  49
  50
  51
  52
  53
  54
  55
  56
  57
  58
  59
  60
  61
  62
  63
  64
  65
  66
  67
  68
  69
  70
  71
  72
  73
  74
  75
  76
  77
  78
  79
  80
  81
  82
  83
  84
  85
  86
  87
  88
  89
  90
  91
  92
  93
  94
  95
  96
  97
  98
  99
 100
 101
 102
 103
 104
 105
 106
 107
 108
 109
 110
 111
 112
 113
 114
 115
 116
 117
 118
 119
 120
 121
 122
 123
 124
 125
 126
 127
 128
 129
 130
 131
 132
 133
 134
 135
 136
 137
 138
 139
 140
 141
 142
 143
 144
 145
 146
 147
 148
 149
 150
 151
 152
 153
 154
 155
 156
 157
 158
 159
 160
 161
 162
 163
 164
 165
 166
 167
 168
 169
 170
 171
 172
 173
 174
 175
 176
 177
 178
 179
 180
 181
 182
 183
 184
 185
 186
 187
 188
 189
 190
 191
 192
 193
 194
 195
 196
 197
 198
 199
 200
 201
 202
 203
 204
 205
 206
 207
 208
 209
 210
 211
 212
 213
 214
 215
 216
 217
 218
 219
 220
 221
 222
 223
 224
 225
 226
 227
 228
 229
 230
 231
 232
 233
 234
 235
 236
 237
 238
 239
 240
 241
 242
 243
 244
 245
 246
 247
 248
 249
 250
 251
 252
 253
 254
 255
 256
 257
 258
 259
 260
 261
 262
 263
 264
 265
 266
 267
 268
 269
 270
 271
 272
 273
 274
 275
 276
 277
 278
 279
 280
 281
 282
 283
 284
 285
 286
 287
 288
 289
 290
 291
 292
 293
 294
 295
 296
 297
 298
 299
 300
 301
 302
 303
 304
 305
 306
 307
 308
 309
 310
 311
 312
 313
 314
 315
 316
 317
 318
 319
 320
 321
 322
 323
 324
 325
 326
 327
 328
 329
 330
 331
 332
 333
 334
 335
 336
 337
 338
 339
 340
 341
 342
 343
 344
 345
 346
 347
 348
 349
 350
 351
 352
 353
 354
 355
 356
 357
 358
 359
 360
 361
 362
 363
 364
 365
 366
 367
 368
 369
 370
 371
 372
 373
 374
 375
 376
 377
 378
 379
 380
 381
 382
 383
 384
 385
 386
 387
 388
 389
 390
 391
 392
 393
 394
 395
 396
 397
 398
 399
 400
 401
 402
 403
 404
 405
 406
 407
 408
 409
 410
 411
 412
 413
 414
 415
 416
 417
 418
 419
 420
 421
 422
 423
 424
 425
 426
 427
 428
 429
 430
 431
 432
 433
 434
 435
 436
 437
 438
 439
 440
 441
 442
 443
 444
 445
 446
 447
 448
 449
 450
 451
 452
 453
 454
 455
 456
 457
 458
 459
 460
 461
 462
 463
 464
 465
 466
 467
 468
 469
 470
 471
 472
 473
 474
 475
 476
 477
 478
 479
 480
 481
 482
 483
 484
 485
 486
 487
 488
 489
 490
 491
 492
 493
 494
 495
 496
 497
 498
 499
 500
 501
 502
 503
 504
 505
 506
 507
 508
 509
 510
 511
 512
 513
 514
 515
 516
 517
 518
 519
 520
 521
 522
 523
 524
 525
 526
 527
 528
 529
 530
 531
 532
 533
 534
 535
 536
 537
 538
 539
 540
 541
 542
 543
 544
 545
 546
 547
 548
 549
 550
 551
 552
 553
 554
 555
 556
 557
 558
 559
 560
 561
 562
 563
 564
 565
 566
 567
 568
 569
 570
 571
 572
 573
 574
 575
 576
 577
 578
 579
 580
 581
 582
 583
 584
 585
 586
 587
 588
 589
 590
 591
 592
 593
 594
 595
 596
 597
 598
 599
 600
 601
 602
 603
 604
 605
 606
 607
 608
 609
 610
 611
 612
 613
 614
 615
 616
 617
 618
 619
 620
 621
 622
 623
 624
 625
 626
 627
 628
 629
 630
 631
 632
 633
 634
 635
 636
 637
 638
 639
 640
 641
 642
 643
 644
 645
 646
 647
 648
 649
 650
 651
 652
 653
 654
 655
 656
 657
 658
 659
 660
 661
 662
 663
 664
 665
 666
 667
 668
 669
 670
 671
 672
 673
 674
 675
 676
 677
 678
 679
 680
 681
 682
 683
 684
 685
 686
 687
 688
 689
 690
 691
 692
 693
 694
 695
 696
 697
 698
 699
 700
 701
 702
 703
 704
 705
 706
 707
 708
 709
 710
 711
 712
 713
 714
 715
 716
 717
 718
 719
 720
 721
 722
 723
 724
 725
 726
 727
 728
 729
 730
 731
 732
 733
 734
 735
 736
 737
 738
 739
 740
 741
 742
 743
 744
 745
 746
 747
 748
 749
 750
 751
 752
 753
 754
 755
 756
 757
 758
 759
 760
 761
 762
 763
 764
 765
 766
 767
 768
 769
 770
 771
 772
 773
 774
 775
 776
 777
 778
 779
 780
 781
 782
 783
 784
 785
 786
 787
 788
 789
 790
 791
 792
 793
 794
 795
 796
 797
 798
 799
 800
 801
 802
 803
 804
 805
 806
 807
 808
 809
 810
 811
 812
 813
 814
 815
 816
 817
 818
 819
 820
 821
 822
 823
 824
 825
 826
 827
 828
 829
 830
 831
 832
 833
 834
 835
 836
 837
 838
 839
 840
 841
 842
 843
 844
 845
 846
 847
 848
 849
 850
 851
 852
 853
 854
 855
 856
 857
 858
 859
 860
 861
 862
 863
 864
 865
 866
 867
 868
 869
 870
 871
 872
 873
 874
 875
 876
 877
 878
 879
 880
 881
 882
 883
 884
 885
 886
 887
 888
 889
 890
 891
 892
 893
 894
 895
 896
 897
 898
 899
 900
 901
 902
 903
 904
 905
 906
 907
 908
 909
 910
 911
 912
 913
 914
 915
 916
 917
 918
 919
 920
 921
 922
 923
 924
 925
 926
 927
 928
 929
 930
 931
 932
 933
 934
 935
 936
 937
 938
 939
 940
 941
 942
 943
 944
 945
 946
 947
 948
 949
 950
 951
 952
 953
 954
 955
 956
 957
 958
 959
 960
 961
 962
 963
 964
 965
 966
 967
 968
 969
 970
 971
 972
 973
 974
 975
 976
 977
 978
 979
 980
 981
 982
 983
 984
 985
 986
 987
 988
 989
 990
 991
 992
 993
 994
 995
 996
 997
 998
 999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
2023
2024
2025
2026
2027
2028
2029
2030
2031
2032
2033
2034
2035
2036
2037
2038
2039
2040
2041
2042
2043
2044
2045
2046
2047
2048
2049
2050
2051
2052
2053
2054
\input texinfo  @c -*-texinfo-*-
@c %**start of header
@setfilename wisent.info
@set TITLE  Wisent Parser Development
@set AUTHOR Eric M. Ludlam, David Ponce, and Richard Y. Kim
@settitle @value{TITLE}

@c *************************************************************************
@c @ Header
@c *************************************************************************

@c Merge all indexes into a single index for now.
@c We can always separate them later into two or more as needed.
@syncodeindex vr cp
@syncodeindex fn cp
@syncodeindex ky cp
@syncodeindex pg cp
@syncodeindex tp cp

@c @footnotestyle separate
@c @paragraphindent 2
@c @@smallbook
@c %**end of header

@copying
This manual documents the Wisent parser generator.

Copyright @copyright{} 2001, 2002, 2003, 2004, 2007 David Ponce

Some texts are borrowed or adapted from the manual of Bison version
1.35.  The text in section entitled ``Understanding the automaton'' is
adapted from the section ``Understanding Your Parser'' in the manual
of Bison version 1.49.

Copyright @copyright{} 1988, 1989, 1990, 1991, 1992, 1993, 1995, 1998,
1999, 2000, 2001, 2002, 2003, 2004 Free Software Foundation, Inc.

@quotation
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1 or
any later version published by the Free Software Foundation; with the
Invariant Sections being list their titles, with the Front-Cover Texts
being list, and with the Back-Cover Texts being list.  A copy of the
license is included in the section entitled ``GNU Free Documentation
License''.
@end quotation
@end copying

@ifinfo
@dircategory Emacs
@direntry
* Semantic Wisent parser development: (wisent).
@end direntry
@end ifinfo

@iftex
@finalout
@end iftex

@c @setchapternewpage odd
@c @setchapternewpage off

@ifinfo
This file documents Application Development with Semantic.
@emph{Infrastructure for parser based text analysis in Emacs}

Copyright @copyright{} 2001, 2002, 2003, 2004 @value{AUTHOR}
@end ifinfo

@titlepage
@sp 10
@title @value{TITLE}
@author by @value{AUTHOR}
@vskip 0pt plus 1 fill
Copyright @copyright{} 2001, 2002, 2003, 2004 @value{AUTHOR}
@page
@vskip 0pt plus 1 fill
@insertcopying
@end titlepage
@page

@c MACRO inclusion
@include semanticheader.texi
@paragraphindent none


@c *************************************************************************
@c @ Document
@c *************************************************************************
@contents

@node top
@top @value{TITLE}

Wisent (the European Bison ;-) is an Emacs Lisp implementation of the
GNU Compiler Compiler Bison.

This manual describes how to use Wisent to develop grammars for
programming languages, and how to use grammars to parse language
source in Emacs buffers.

It also describes how Wisent is used with the @semantic{} tool set
described in the @ref{Top, Semantic Manual, Semantic Manual, semantic}.

@menu
* Wisent Overview::             
* Wisent Grammar::              
* Wisent Parsing::              
* Wisent Semantic::             
* GNU Free Documentation License::  
* Index::                       
@end menu

@node Wisent Overview
@chapter Wisent Overview

@dfn{Wisent} (the European Bison) is an implementation in Emacs Lisp
of the GNU Compiler Compiler Bison. Its code is a port of the C code
of GNU Bison 1.28 & 1.31.

For more details on the basic concepts for understanding Wisent, it is
worthwhile to read the @ref{Top, Bison Manual, bison}.
@ifhtml
@uref{http://www.gnu.org/manual/bison/html_node/index.html}.
@end ifhtml

Wisent can generate compilers compatible with the @semantic{} tool set.
See the @ref{Top, Semantic Manual, , semantic}.

It benefits from these Bison features:

@itemize @bullet
@item 
It uses a fast but not so space-efficient encoding for the parse
tables, described in Corbett's PhD thesis from Berkeley:
@quotation
@cite{Static Semantics in Compiler Error Recovery}@*
June 1985, Report No. UCB/CSD 85/251.
@end quotation

@item 
For generating the lookahead sets, Wisent uses the well-known
technique of F. DeRemer and A. Pennello they described in:
@quotation
@cite{Efficient Construction of LALR(1) Lookahead Sets}@*
October 1982, ACM TOPLS Vol 4 No 4.
@end quotation

@item 
Wisent resolves shift/reduce conflicts using operator precedence and
associativity.

@item 
Parser error recovery is accomplished using rules which match the
special token @code{error}.
@end itemize

Nevertheless there are some fundamental differences between Bison and
Wisent.

@itemize
@item
Wisent is intended to be used in Emacs.  It reads and produces Emacs
Lisp data structures.  All the additional code used in grammars is
Emacs Lisp code.

@item
Contrary to Bison, Wisent does not generate a parser which combines
Emacs Lisp code and grammar constructs.  They exist separately.
Wisent reads the grammar from a Lisp data structure and then generates
grammar constructs as tables.  Afterward, the derived tables can be
included and byte-compiled in separate Emacs Lisp files, and be used
at a later time by the Wisent's parser engine.

@item
Wisent allows multiple start nonterminals and allows a call to the
parsing function to be made for a particular start nonterminal.  For
example, this is particularly useful to parse a region of an Emacs
buffer.  @semantic{} heavily depends on the availability of this feature.
@end itemize

@node Wisent Grammar
@chapter Wisent Grammar

@cindex context-free grammar
@cindex rule
In order for Wisent to parse a language, it must be described by a
@dfn{context-free grammar}.  That is a grammar specified as rules that
can be applied regardless of context.  For more information, see
@ref{Language and Grammar, , , bison}, in the Bison manual.

@cindex terminal
@cindex nonterminal
The formal grammar is formulated using @dfn{terminal} and
@dfn{nonterminal} items.  Terminals can be Emacs Lisp symbols or
characters, and nonterminals are symbols only.

@cindex token
Terminals (also known as @dfn{tokens}) represent the lexical
elements of the language like numbers, strings, etc..

For example @samp{PLUS} can represent the operator @samp{+}.

Nonterminal symbols are described by rules:

@example
@group
RESULT @equiv{} COMPONENTS@dots{}
@end group
@end example

@samp{RESULT} is a nonterminal that this rule describes and
@samp{COMPONENTS} are various terminals and nonterminals that are put
together by this rule.

For example, this rule:

@example
@group
exp @equiv{} exp PLUS exp
@end group
@end example

Says that two groupings of type @samp{exp}, with a @samp{PLUS} token
in between, can be combined into a larger grouping of type @samp{exp}.
 
@menu
* Grammar format::              
* Example::                     
* Compiling a grammar::         
* Conflicts::                   
@end menu

@node Grammar format, Example, Wisent Grammar, Wisent Grammar
@comment  node-name,  next,  previous,  up
@section Grammar format

@cindex grammar format
To be acceptable by Wisent a context-free grammar must respect a
particular format.  That is, must be represented as an Emacs Lisp list
of the form:

@code{(@var{terminals} @var{assocs} . @var{non-terminals})}

@table @var
@item terminals
Is the list of terminal symbols used in the grammar.

@cindex associativity
@item assocs
Specify the associativity of @var{terminals}.  It is @code{nil} when
there is no associativity defined, or an alist of
@w{@code{(@var{assoc-type} . @var{assoc-value})}} elements.

@var{assoc-type} must be one of the @code{default-prec},
@code{nonassoc}, @code{left} or @code{right} symbols.  When
@var{assoc-type} is @code{default-prec}, @var{assoc-value} must be
@code{nil} or @code{t} (the default).  Otherwise it is a list of
tokens which must have been previously declared in @var{terminals}.

For details, see @ref{Contextual Precedence, , , bison}, in the
Bison manual.

@item non-terminals
Is the list of nonterminal definitions.  Each definition has the form:

@code{(@var{nonterm} . @var{rules})}

Where @var{nonterm} is the nonterminal symbol defined and
@var{rules} the list of rules that describe this nonterminal.  Each
rule is a list:

@code{(@var{components} [@var{precedence}] [@var{action}])}

Where:

@table @var
@item components
Is a list of various terminals and nonterminals that are put together
by this rule.

For example,

@example
@group
(exp ((exp ?+ exp))          ;; exp: exp '+' exp
     )                       ;;    ;
@end group
@end example

Says that two groupings of type @samp{exp}, with a @samp{+} token in
between, can be combined into a larger grouping of type @samp{exp}.
 
@cindex grammar coding conventions
By convention, a nonterminal symbol should be in lower case, such as
@samp{exp}, @samp{stmt} or @samp{declaration}.  Terminal symbols
should be upper case to distinguish them from nonterminals: for
example, @samp{INTEGER}, @samp{IDENTIFIER}, @samp{IF} or
@samp{RETURN}.  A terminal symbol that represents a particular keyword
in the language is conventionally the same as that keyword converted
to upper case.  The terminal symbol @code{error} is reserved for error
recovery.

@cindex middle-rule actions
Scattered among the components can be @dfn{middle-rule} actions.
Usually only @var{action} is provided (@pxref{action}).

If @var{components} in a rule is @code{nil}, it means that the rule
can match the empty string.  For example, here is how to define a
comma-separated sequence of zero or more @samp{exp} groupings:

@example
@group
(expseq  (nil)               ;; expseq: ;; empty
         ((expseq1))         ;;       | expseq1
         )                   ;;       ;

(expseq1 ((exp))             ;; expseq1: exp
         ((expseq1 ?, exp))  ;;        | expseq1 ',' exp
         )                   ;;        ;
@end group
@end example

@cindex precedence level
@item precedence
Assign the rule the precedence of the given terminal item, overriding
the precedence that would be deduced for it, that is the one of the
last terminal in it.  Notice that only terminals declared in
@var{assocs} have a precedence level.  The altered rule precedence
then affects how conflicts involving that rule are resolved.

@var{precedence} is an optional vector of one terminal item.

Here is how @var{precedence} solves the problem of unary minus.
First, declare a precedence for a fictitious terminal symbol named
@code{UMINUS}.  There are no tokens of this type, but the symbol
serves to stand for its precedence:

@example
@dots{}
((default-prec t) ;; This is the default
 (left '+' '-')
 (left '*')
 (left UMINUS))
@end example

Now the precedence of @code{UMINUS} can be used in specific rules:

@example
@group
(exp    @dots{}                  ;; exp:    @dots{}
         ((exp ?- exp))      ;;         | exp '-' exp
        @dots{}                  ;;         @dots{}
         ((?- exp) [UMINUS]) ;;         | '-' exp %prec UMINUS
        @dots{}                  ;;         @dots{}
        )                    ;;         ;
@end group
@end example

If you forget to append @code{[UMINUS]} to the rule for unary minus,
Wisent silently assumes that minus has its usual precedence.  This
kind of problem can be tricky to debug, since one typically discovers
the mistake only by testing the code.

Using @code{(default-prec nil)} declaration makes it easier to
discover this kind of problem systematically.  It causes rules that
lack a @var{precedence} modifier to have no precedence, even if the
last terminal symbol mentioned in their components has a declared
precedence.

If @code{(default-prec nil)} is in effect, you must specify
@var{precedence} for all rules that participate in precedence conflict
resolution.  Then you will see any shift/reduce conflict until you
tell Wisent how to resolve it, either by changing your grammar or by
adding an explicit precedence.  This will probably add declarations to
the grammar, but it helps to protect against incorrect rule
precedences.

The effect of @code{(default-prec nil)} can be reversed by giving
@code{(default-prec t)}, which is the default.

For more details, see @ref{Contextual Precedence, , , bison}, in the
Bison manual.

It is important to understand that @var{assocs} declarations defines
associativity but also assign a precedence level to terminals.  All
terminals declared in the same @code{left}, @code{right} or
@code{nonassoc} association get the same precedence level.  The
precedence level is increased at each new association.

On the other hand, @var{precedence} explicitly assign the precedence
level of the given terminal to a rule.

@cindex semantic actions
@item @anchor{action}action
An action is an optional Emacs Lisp function call, like this:

@code{(identity $1)}

The result of an action determines the semantic value of a rule.

From an implementation standpoint, the function call will be embedded
in a lambda expression, and several useful local variables will be
defined:

@table @code
@vindex $N
@item $@var{n}
Where @var{n} is a positive integer.  Like in Bison, the value of
@code{$@var{n}} is the semantic value of the @var{n}th element of
@var{components}, starting from 1.  It can be of any Lisp data
type.

@vindex $region@var{n}
@item $regionN
Where @var{n} is a positive integer.  For each @code{$@var{n}}
variable defined there is a corresponding @code{$region@var{n}}
variable.  Its value is a pair @code{(@var{start-pos} .
@var{end-pos})} that represent the start and end positions (in the
lexical input stream) of the @code{$@var{n}} value.  It can be
@code{nil} when the component positions are not available, like for an
empty string component for example.

@vindex $region
@item $region
Its value is the leftmost and rightmost positions of input data
matched by all @var{components} in the rule.  This is a pair
@code{(@var{leftmost-pos} .  @var{rightmost-pos})}.  It can be
@code{nil} when components positions are not available.

@vindex $nterm
@item $nterm
This variable is initialized with the nonterminal symbol
(@var{nonterm}) the rule belongs to.  It could be useful to improve
error reporting or debugging.  It is also used to automatically
provide incremental re-parse entry points for @semantic{} tags
(@pxref{Wisent Semantic}).

@vindex $action
@item $action
The value of @code{$action} is the symbolic name of the current
semantic action (@pxref{Debugging actions}).
@end table

When an action is not specified a default value is supplied, it is
@code{(identity $1)}.  This means that the default semantic value of a
rule is the value of its first component.  Excepted for a rule
matching the empty string, for which the default action is to return
@code{nil}.
@end table
@end table

@node Example, Compiling a grammar, Grammar format, Wisent Grammar
@comment  node-name,  next,  previous,  up
@section Example

@cindex grammar example
Here is an example to parse simple infix arithmetic expressions.  See
@ref{Infix Calc, , , bison}, in the Bison manual for details.

@lisp
@group
'(
  ;; Terminals
  (NUM)
  
  ;; Terminal associativity & precedence
  ((nonassoc ?=)
   (left ?- ?+)
   (left ?* ?/)
   (left NEG)
   (right ?^))
  
  ;; Rules
  (input
   ((line))
   ((input line)
    (format "%s %s" $1 $2))
   )

  (line
   ((?;)
    (progn ";"))
   ((exp ?;)
    (format "%s;" $1))
   ((error ?;)
    (progn "Error;")))
   )

  (exp
   ((NUM)
    (string-to-number $1))
   ((exp ?= exp)
    (= $1 $3))
   ((exp ?+ exp)
    (+ $1 $3))
   ((exp ?- exp)
    (- $1 $3))
   ((exp ?* exp)
    (* $1 $3))
   ((exp ?/ exp)
    (/ $1 $3))
   ((?- exp) [NEG]
    (- $2))
   ((exp ?^ exp)
    (expt $1 $3))
   ((?\( exp ?\))
    (progn $2))
   )
  )
@end group
@end lisp

In the bison-like @dfn{WY} format (@pxref{Wisent Semantic}) the
grammar looks like this:

@example
@group
%token <number> NUM

%nonassoc '=' ;; comparison
%left '-' '+'
%left '*' '/'
%left NEG     ;; negation--unary minus
%right '^'    ;; exponentiation

%%

input:
    line
  | input line
    (format "%s %s" $1 $2)
  ;

line:
    ';'
    @{";"@}
  | exp ';'
    (format "%s;" $1)
  | error ';'
    @{"Error;"@}
  ;

exp:
    NUM
    (string-to-number $1)
  | exp '=' exp
    (= $1 $3)
  | exp '+' exp
    (+ $1 $3)
  | exp '-' exp
    (- $1 $3)
  | exp '*' exp
    (* $1 $3)
  | exp '/' exp
    (/ $1 $3)
  | '-' exp %prec NEG
    (- $2)
  | exp '^' exp
    (expt $1 $3)
  | '(' exp ')'
    @{$2@}
  ;

%%
@end group
@end example

@node Compiling a grammar, Conflicts, Example, Wisent Grammar
@comment  node-name,  next,  previous,  up
@section Compiling a grammar

@cindex automaton
After providing a context-free grammar in a suitable format, it must
be translated into a set of tables (an @dfn{automaton}) that will be
used to derive the parser.  Like Bison, Wisent translates grammars that
must be @dfn{LALR(1)}.

@cindex LALR(1) grammar
@cindex look-ahead token
A grammar is @acronym{LALR(1)} if it is possible to tell how to parse
any portion of an input string with just a single token of look-ahead:
the @dfn{look-ahead token}.  See @ref{Language and Grammar, , ,
bison}, in the Bison manual for more information.

@cindex grammar compilation
Grammar translation (compilation) is achieved by the function:

@cindex compiling a grammar
@vindex wisent-single-start-flag
@findex wisent-compile-grammar
@defun wisent-compile-grammar grammar &optional start-list
Compile @var{grammar} and return an @acronym{LALR(1)} automaton.

Optional argument @var{start-list} is a list of start symbols
(nonterminals).  If @code{nil} the first nonterminal defined in the
grammar is the default start symbol.  If @var{start-list} contains
only one element, it defines the start symbol.  If @var{start-list}
contains more than one element, all are defined as potential start
symbols, unless @code{wisent-single-start-flag} is non-@code{nil}.  In
that case the first element of @var{start-list} defines the start
symbol and others are ignored.

The @acronym{LALR(1)} automaton is a vector of the form:

@code{[@var{actions gotos starts functions}]}

@table @var
@item actions
A state/token matrix telling the parser what to do at every state
based on the current look-ahead token.  That is shift, reduce, accept
or error.  See also @ref{Wisent Parsing}.

@item gotos
A state/nonterminal matrix telling the parser the next state to go to
after reducing with each rule.

@item starts
An alist which maps the allowed start symbols (nonterminals) to
lexical tokens that will be first shifted into the parser stack.

@item functions
An obarray of semantic action symbols.  A semantic action is actually
an Emacs Lisp function (lambda expression).
@end table
@end defun

@node Conflicts, , Compiling a grammar, Wisent Grammar
@comment  node-name,  next,  previous,  up
@section Conflicts

Normally, a grammar should produce an automaton where at each state
the parser has only one action to do (@pxref{Wisent Parsing}).

@cindex ambiguous grammar
In certain cases, a grammar can produce an automaton where, at some
states, there are more than one action possible.  Such a grammar is
@dfn{ambiguous}, and generates @dfn{conflicts}.

@cindex deterministic automaton
The parser can't be driven by an automaton which isn't completely
@dfn{deterministic}, that is which contains conflicts.  It is
necessary to resolve the conflicts to eliminate them.  Wisent resolves
conflicts like Bison does.

@cindex grammar conflicts
@cindex conflicts resolution
There are two sorts of conflicts:

@table @dfn
@cindex shift/reduce conflicts
@item shift/reduce conflicts
When either a shift or a reduction would be valid at the same state.

Such conflicts are resolved by choosing to shift, unless otherwise
directed by operator precedence declarations.
See @ref{Shift/Reduce , , , bison}, in the Bison manual for more
information.

@cindex reduce/reduce conflicts
@item reduce/reduce conflicts
That occurs if there are two or more rules that apply to the same
sequence of input.  This usually indicates a serious error in the
grammar.

Such conflicts are resolved by choosing to use the rule that appears
first in the grammar, but it is very risky to rely on this.  Every
reduce/reduce conflict must be studied and usually eliminated.  See
@ref{Reduce/Reduce , , , bison}, in the Bison manual for more
information.
@end table

@menu
* Grammar Debugging::           
* Understanding the automaton::  
@end menu

@node Grammar Debugging
@subsection Grammar debugging

@cindex grammar debugging
@cindex grammar verbose description
To help writing a new grammar, @code{wisent-compile-grammar} can
produce a verbose report containing a detailed description of the
grammar and parser (equivalent to what Bison reports with the
@option{--verbose} option).

To enable the verbose report you can set to non-@code{nil} the
variable:

@vindex wisent-verbose-flag
@deffn Option wisent-verbose-flag
non-@code{nil} means to report verbose information on generated parser.
@end deffn

Or interactively use the command:

@findex wisent-toggle-verbose-flag
@deffn Command wisent-toggle-verbose-flag
Toggle whether to report verbose information on generated parser.
@end deffn

The verbose report is printed in the temporary buffer
@code{*wisent-log*} when running interactively, or in file
@file{wisent.output} when running in batch mode.  Different
reports are separated from each other by a line like this:

@example
@group
*** Wisent @var{source-file} - 2002-06-27 17:33
@end group
@end example

where @var{source-file} is the name of the Emacs Lisp file from which
the grammar was read.  See @ref{Understanding the automaton}, for
details on the verbose report.

@table @strong
@item Please Note
To help debugging the grammar compiler itself, you can set this
variable to print the content of some internal data structures:

@vindex wisent-debug-flag
@defvar wisent-debug-flag
non-@code{nil} means enable some debug stuff.
@end defvar
@end table

@node Understanding the automaton
@subsection Understanding the automaton

@cindex understanding the automaton
This section (took from the manual of Bison 1.49) describes how to use
the verbose report printed by @code{wisent-compile-grammar} to
understand the generated automaton, to tune or fix a grammar.

We will use the following example:

@example
@group
(let ((wisent-verbose-flag t)) ;; Print a verbose report!
  (wisent-compile-grammar
   '((NUM STR)                          ; %token NUM STR

     ((left ?+ ?-)                      ; %left '+' '-'; 
      (left ?*))                        ; %left '*'

     (exp                               ; exp:
      ((exp ?+ exp))                    ;    exp '+' exp
      ((exp ?- exp))                    ;  | exp '-' exp
      ((exp ?* exp))                    ;  | exp '*' exp
      ((exp ?/ exp))                    ;  | exp '/' exp
      ((NUM))                           ;  | NUM
      )                                 ;  ;

     (useless                           ; useless:
      ((STR))                           ;    STR
      )                                 ;  ;
     )
   'nil)                                ; no %start declarations
  )
@end group
@end example

When evaluating the above expression, grammar compilation first issues
the following two clear messages:

@example
@group
Grammar contains 1 useless nonterminals and 1 useless rules
Grammar contains 7 shift/reduce conflicts
@end group
@end example

The @samp{*wisent-log*} buffer details things!

The first section reports conflicts that were solved using precedence
and/or associativity:

@example
@group
Conflict in state 7 between rule 1 and token '+' resolved as reduce.
Conflict in state 7 between rule 1 and token '-' resolved as reduce.
Conflict in state 7 between rule 1 and token '*' resolved as shift.
Conflict in state 8 between rule 2 and token '+' resolved as reduce.
Conflict in state 8 between rule 2 and token '-' resolved as reduce.
Conflict in state 8 between rule 2 and token '*' resolved as shift.
Conflict in state 9 between rule 3 and token '+' resolved as reduce.
Conflict in state 9 between rule 3 and token '-' resolved as reduce.
Conflict in state 9 between rule 3 and token '*' resolved as reduce.
@end group
@end example

The next section reports useless tokens, nonterminal and rules (note
that useless tokens might be used by the scanner):

@example
@group
Useless nonterminals:

   useless


Terminals which are not used:

   STR


Useless rules:

#6     useless: STR;
@end group
@end example

The next section lists states that still have conflicts:

@example
@group
State 7 contains 1 shift/reduce conflict.
State 8 contains 1 shift/reduce conflict.
State 9 contains 1 shift/reduce conflict.
State 10 contains 4 shift/reduce conflicts.
@end group
@end example

The next section reproduces the grammar used:

@example
@group
Grammar

  Number, Rule
  1       exp -> exp '+' exp
  2       exp -> exp '-' exp
  3       exp -> exp '*' exp
  4       exp -> exp '/' exp
  5       exp -> NUM
@end group
@end example

And reports the uses of the symbols:

@example
@group
Terminals, with rules where they appear

$EOI (-1)
error (1)
NUM (2) 5
STR (3) 6
'+' (4) 1
'-' (5) 2
'*' (6) 3
'/' (7) 4


Nonterminals, with rules where they appear

exp (8)
    on left: 1 2 3 4 5, on right: 1 2 3 4
@end group
@end example

The report then details the automaton itself, describing each state
with it set of @dfn{items}, also known as @dfn{pointed rules}.  Each
item is a production rule together with a point (marked by @samp{.})
that the input cursor.

@example
@group
state 0

    NUM	shift, and go to state 1

    exp	go to state 2
@end group
@end example

State 0 corresponds to being at the very beginning of the parsing, in
the initial rule, right before the start symbol (@samp{exp}).  When
the parser returns to this state right after having reduced a rule
that produced an @samp{exp}, it jumps to state 2.  If there is no such
transition on a nonterminal symbol, and the lookahead is a @samp{NUM},
then this token is shifted on the parse stack, and the control flow
jumps to state 1.  Any other lookahead triggers a parse error.

In the state 1...

@example
@group
state 1

    exp  ->  NUM .   (rule 5)

    $default	reduce using rule 5 (exp)
@end group
@end example

the rule 5, @samp{exp: NUM;}, is completed.  Whatever the lookahead
(@samp{$default}), the parser will reduce it.  If it was coming from
state 0, then, after this reduction it will return to state 0, and
will jump to state 2 (@samp{exp: go to state 2}).

@example
@group
state 2

    exp  ->  exp . '+' exp   (rule 1)
    exp  ->  exp . '-' exp   (rule 2)
    exp  ->  exp . '*' exp   (rule 3)
    exp  ->  exp . '/' exp   (rule 4)

    $EOI	shift, and go to state 11
    '+'	shift, and go to state 3
    '-'	shift, and go to state 4
    '*'	shift, and go to state 5
    '/'	shift, and go to state 6
@end group
@end example

In state 2, the automaton can only shift a symbol.  For instance,
because of the item @samp{exp -> exp . '+' exp}, if the lookahead if
@samp{+}, it will be shifted on the parse stack, and the automaton
control will jump to state 3, corresponding to the item
@samp{exp -> exp . '+' exp}:

@example
@group
state 3

    exp  ->  exp '+' . exp   (rule 1)

    NUM	shift, and go to state 1

    exp	go to state 7
@end group
@end example

Since there is no default action, any other token than those listed
above will trigger a parse error.

The interpretation of states 4 to 6 is straightforward:

@example
@group
state 4

    exp  ->  exp '-' . exp   (rule 2)

    NUM	shift, and go to state 1

    exp	go to state 8



state 5

    exp  ->  exp '*' . exp   (rule 3)

    NUM	shift, and go to state 1

    exp	go to state 9



state 6

    exp  ->  exp '/' . exp   (rule 4)

    NUM	shift, and go to state 1

    exp	go to state 10
@end group
@end example

As was announced in beginning of the report, @samp{State 7 contains 1
shift/reduce conflict.}:

@example
@group
state 7

    exp  ->  exp . '+' exp   (rule 1)
    exp  ->  exp '+' exp .   (rule 1)
    exp  ->  exp . '-' exp   (rule 2)
    exp  ->  exp . '*' exp   (rule 3)
    exp  ->  exp . '/' exp   (rule 4)

    '*'	shift, and go to state 5
    '/'	shift, and go to state 6

    '/'	[reduce using rule 1 (exp)]
    $default	reduce using rule 1 (exp)
@end group
@end example

Indeed, there are two actions associated to the lookahead @samp{/}:
either shifting (and going to state 6), or reducing rule 1.  The
conflict means that either the grammar is ambiguous, or the parser
lacks information to make the right decision.  Indeed the grammar is
ambiguous, as, since we did not specify the precedence of @samp{/},
the sentence @samp{NUM + NUM / NUM} can be parsed as @samp{NUM + (NUM
/ NUM)}, which corresponds to shifting @samp{/}, or as @samp{(NUM +
NUM) / NUM}, which corresponds to reducing rule 1.

Because in @acronym{LALR(1)} parsing a single decision can be made,
Wisent arbitrarily chose to disable the reduction, see
@ref{Conflicts}.  Discarded actions are reported in between square
brackets.

Note that all the previous states had a single possible action: either
shifting the next token and going to the corresponding state, or
reducing a single rule.  In the other cases, i.e., when shifting
@emph{and} reducing is possible or when @emph{several} reductions are
possible, the lookahead is required to select the action.  State 7 is
one such state: if the lookahead is @samp{*} or @samp{/} then the
action is shifting, otherwise the action is reducing rule 1.  In other
words, the first two items, corresponding to rule 1, are not eligible
when the lookahead is @samp{*}, since we specified that @samp{*} has
higher precedence that @samp{+}.  More generally, some items are
eligible only with some set of possible lookaheads.

States 8 to 10 are similar:

@example
@group
state 8

    exp  ->  exp . '+' exp   (rule 1)
    exp  ->  exp . '-' exp   (rule 2)
    exp  ->  exp '-' exp .   (rule 2)
    exp  ->  exp . '*' exp   (rule 3)
    exp  ->  exp . '/' exp   (rule 4)

    '*'	shift, and go to state 5
    '/'	shift, and go to state 6

    '/'	[reduce using rule 2 (exp)]
    $default	reduce using rule 2 (exp)



state 9

    exp  ->  exp . '+' exp   (rule 1)
    exp  ->  exp . '-' exp   (rule 2)
    exp  ->  exp . '*' exp   (rule 3)
    exp  ->  exp '*' exp .   (rule 3)
    exp  ->  exp . '/' exp   (rule 4)

    '/'	shift, and go to state 6

    '/'	[reduce using rule 3 (exp)]
    $default	reduce using rule 3 (exp)



state 10

    exp  ->  exp . '+' exp   (rule 1)
    exp  ->  exp . '-' exp   (rule 2)
    exp  ->  exp . '*' exp   (rule 3)
    exp  ->  exp . '/' exp   (rule 4)
    exp  ->  exp '/' exp .   (rule 4)

    '+'	shift, and go to state 3
    '-'	shift, and go to state 4
    '*'	shift, and go to state 5
    '/'	shift, and go to state 6

    '+'	[reduce using rule 4 (exp)]
    '-'	[reduce using rule 4 (exp)]
    '*'	[reduce using rule 4 (exp)]
    '/'	[reduce using rule 4 (exp)]
    $default	reduce using rule 4 (exp)
@end group
@end example

Observe that state 10 contains conflicts due to the lack of precedence
of @samp{/} wrt @samp{+}, @samp{-}, and @samp{*}, but also because the
associativity of @samp{/} is not specified.

Finally, the state 11 (plus 12) is named the @dfn{final state}, or the
@dfn{accepting state}:

@example
@group
state 11

    $EOI	shift, and go to state 12



state 12

    $default	accept
@end group
@end example

The end of input is shifted @samp{$EOI shift,} and the parser exits
successfully (@samp{go to state 12}, that terminates).

@node Wisent Parsing
@chapter Wisent Parsing

@cindex bottom-up parser
@cindex shift-reduce parser
The Wisent's parser is what is called a @dfn{bottom-up} or
@dfn{shift-reduce} parser which repeatedly:

@table @dfn
@cindex shift
@item shift
That is pushes the value of the last lexical token read (the
look-ahead token) into a value stack, and reads a new one.

@cindex reduce
@item reduce
That is replaces a nonterminal by its semantic value.  The values of
the components which form the right hand side of a rule are popped
from the value stack and reduced by the semantic action of this rule.
The result is pushed back on top of value stack.
@end table

The parser will stop on:

@table @dfn
@cindex accept
@item accept
When all input has been successfully parsed.  The semantic value of
the start nonterminal is on top of the value stack.

@cindex syntax error
@item error
When a syntax error (an unexpected token in input) has been detected.
At this point the parser issues an error message and either stops or
calls a recovery routine to try to resume parsing.
@end table

@cindex table-driven parser
The above elementary actions are driven by the @acronym{LALR(1)}
automaton built by @code{wisent-compile-grammar} from a context-free
grammar.

The Wisent's parser is entered by calling the function:

@findex wisent-parse
@defun wisent-parse automaton lexer &optional error start
Parse input using the automaton specified in @var{automaton}.

@table @var
@item automaton
Is an @acronym{LALR(1)} automaton generated by
@code{wisent-compile-grammar} (@pxref{Wisent Grammar}).

@item lexer
Is a function with no argument called by the parser to obtain the next
terminal (token) in input (@pxref{Writing a lexer}).

@item error
Is an optional reporting function called when a parse error occurs.
It receives a message string to report.  It defaults to the function
@code{wisent-message} (@pxref{Report errors}).

@item start
Specify the start symbol (nonterminal) used by the parser as its goal.
It defaults to the start symbol defined in the grammar
(@pxref{Wisent Grammar}).
@end table
@end defun

The following two normal hooks permit to do some useful processing
respectively before to start parsing, and after the parser terminated.

@vindex wisent-pre-parse-hook
@defvar wisent-pre-parse-hook
Normal hook run just before entering the @var{LR} parser engine.
@end defvar

@vindex wisent-post-parse-hook
@defvar wisent-post-parse-hook
Normal hook run just after the @var{LR} parser engine terminated.
@end defvar

@menu
* Writing a lexer::             
* Actions goodies::             
* Report errors::               
* Error recovery::              
* Debugging actions::           
@end menu

@node Writing a lexer
@section What the parser must receive

It is important to understand that the parser does not parse
characters, but lexical tokens, and does not know anything about
characters in text streams!

@cindex lexical analysis
@cindex lexer
@cindex scanner
Reading input data to produce lexical tokens is performed by a lexer
(also called a scanner) in a lexical analysis step, before the syntax
analysis step performed by the parser.  The parser automatically calls
the lexer when it needs the next token to parse.

@cindex lexical tokens
A Wisent's lexer is an Emacs Lisp function with no argument.  It must
return a valid lexical token of the form:

@code{(@var{token-class value} [@var{start} . @var{end}])}

@table @var
@item token-class
Is a category of lexical token identifying a terminal as specified in
the grammar (@pxref{Wisent Grammar}).  It can be a symbol or a character
literal.

@item value
Is the value of the lexical token.  It can be of any valid Emacs Lisp
data type.

@item start
@itemx end
Are the optionals beginning and end positions of @var{value} in the
input stream.
@end table

When there are no more tokens to read the lexer must return the token
@code{(list wisent-eoi-term)} to each request.

@vindex wisent-eoi-term
@defvar wisent-eoi-term
Predefined constant, End-Of-Input terminal symbol.
@end defvar

@code{wisent-lex} is an example of a lexer that reads lexical tokens
produced by a @semantic{} lexer, and translates them into lexical tokens
suitable to the Wisent parser.  See also @ref{Wisent Lex}.

To call the lexer in a semantic action use the function
@code{wisent-lexer}.  See also @ref{Actions goodies}.

@node Actions goodies
@section Variables and macros useful in grammar actions.

@vindex wisent-input
@defvar wisent-input
The last token read.
This variable only has meaning in the scope of @code{wisent-parse}.
@end defvar

@findex wisent-lexer
@defun wisent-lexer
Obtain the next terminal in input.
@end defun

@findex wisent-region
@defun wisent-region &rest positions
Return the start/end positions of the region including
@var{positions}.  Each element of @var{positions} is a pair
@w{@code{(@var{start-pos} .  @var{end-pos})}} or @code{nil}.  The
returned value is the pair @w{@code{(@var{min-start-pos} .
@var{max-end-pos})}} or @code{nil} if no @var{positions} are
available.
@end defun

@node Report errors
@section The error reporting function

@cindex error reporting
When the parser encounters a syntax error it calls a user-defined
function.  It must be an Emacs Lisp function with one argument: a
string containing the message to report.

By default the parser uses this function to report error messages:

@findex wisent-message
@defun wisent-message string &rest args
Print a one-line message if @code{wisent-parse-verbose-flag} is set.
Pass @var{string} and @var{args} arguments to @dfn{message}.
@end defun

@table @strong
@item Please Note:
@code{wisent-message} uses the following function to print lexical
tokens:

@defun wisent-token-to-string token
Return a printed representation of lexical token @var{token}.
@end defun

The general printed form of a lexical token is:

@w{@code{@var{token}(@var{value})@@@var{location}}}
@end table

To control the verbosity of the parser you can set to non-@code{nil}
this variable:

@vindex wisent-parse-verbose-flag
@deffn Option wisent-parse-verbose-flag
non-@code{nil} means to issue more messages while parsing.
@end deffn

Or interactively use the command:

@findex wisent-parse-toggle-verbose-flag
@deffn Command wisent-parse-toggle-verbose-flag
Toggle whether to issue more messages while parsing.
@end deffn

When the error reporting function is entered the variable
@code{wisent-input} contains the unexpected token as returned by the
lexer.

The error reporting function can be called from a semantic action too
using the special macro @code{wisent-error}.  When called from a
semantic action entered by error recovery (@pxref{Error recovery}) the
value of the variable @code{wisent-recovering} is non-@code{nil}.

@node Error recovery
@section Error recovery

@cindex error recovery
The error recovery mechanism of the Wisent's parser conforms to the
one Bison uses.  See @ref{Error Recovery, , , bison}, in the Bison
manual for details.

@cindex error token
To recover from a syntax error you must write rules to recognize the
special token @code{error}.  This is a terminal symbol that is
automatically defined and reserved for error handling.

When the parser encounters a syntax error, it pops the state stack
until it finds a state that allows shifting the @code{error} token.
After it has been shifted, if the old look-ahead token is not
acceptable to be shifted next, the parser reads tokens and discards
them until it finds a token which is acceptable.

@cindex error recovery strategy
Strategies for error recovery depend on the choice of error rules in
the grammar.  A simple and useful strategy is simply to skip the rest
of the current statement if an error is detected:

@example
@group
(stmnt (( error ?; )) ;; on error, skip until ';' is read
       )
@end group
@end example

It is also useful to recover to the matching close-delimiter of an
opening-delimiter that has already been parsed:

@example
@group
(primary (( ?@{ expr  ?@} ))
         (( ?@{ error ?@} ))
         @dots{}
         )
@end group
@end example

@cindex error recovery actions
Note that error recovery rules may have actions, just as any other
rules can.  Here are some predefined hooks, variables, functions or
macros, useful in such actions:

@vindex wisent-nerrs
@defvar wisent-nerrs
The number of parse errors encountered so far.
@end defvar

@vindex wisent-recovering
@defvar wisent-recovering
non-@code{nil} means that the parser is recovering.
This variable only has meaning in the scope of @code{wisent-parse}.
@end defvar

@findex wisent-error
@defun wisent-error msg
Call the user supplied error reporting function with message
@var{msg} (@pxref{Report errors}).

For an example of use, @xref{wisent-skip-token}.
@end defun

@findex wisent-errok
@defun wisent-errok
Resume generating error messages immediately for subsequent syntax
errors.

The parser suppress error message for syntax errors that happens
shortly after the first, until three consecutive input tokens have
been successfully shifted.

Calling @code{wisent-errok} in an action, make error messages resume
immediately.  No error messages will be suppressed if you call it in
an error rule's action.

For an example of use, @xref{wisent-skip-token}.
@end defun

@findex wisent-clearin
@defun wisent-clearin
Discard the current lookahead token.
This will cause a new lexical token to be read.

In an error rule's action the previous lookahead token is reanalyzed
immediately.  @code{wisent-clearin} may be called to clear this token.

For example, suppose that on a parse error, an error handling routine
is called that advances the input stream to some point where parsing
should once again commence.  The next symbol returned by the lexical
scanner is probably correct.  The previous lookahead token ought to
be discarded with @code{wisent-clearin}.

For an example of use, @xref{wisent-skip-token}.
@end defun

@findex wisent-abort
@defun wisent-abort
Abort parsing and save the lookahead token.
@end defun

@findex wisent-set-region
@defun wisent-set-region start end
Change the region of text matched by the current nonterminal.
@var{start} and @var{end} are respectively the beginning and end
positions of the region occupied by the group of components associated
to this nonterminal.  If @var{start} or @var{end} values are not a
valid positions the region is set to @code{nil}.

For an example of use, @xref{wisent-skip-token}.
@end defun

@vindex wisent-discarding-token-functions
@defvar wisent-discarding-token-functions
List of functions to be called when discarding a lexical token.
These functions receive the lexical token discarded.
When the parser encounters unexpected tokens, it can discards them,
based on what directed by error recovery rules.  Either when the
parser reads tokens until one is found that can be shifted, or when an
semantic action calls the function @code{wisent-skip-token} or
@code{wisent-skip-block}.
For language specific hooks, make sure you define this as a local
hook.

For example, in @semantic{}, this hook is set to the function
@code{wisent-collect-unmatched-syntax} to collect unmatched lexical
tokens (@pxref{Useful functions}).
@end defvar

@findex wisent-skip-token
@defun wisent-skip-token
@anchor{wisent-skip-token}
Skip the lookahead token in order to resume parsing.
Return nil.
Must be used in error recovery semantic actions.

It typically looks like this:

@lisp
@group
(wisent-message "%s: skip %s" $action
                (wisent-token-to-string wisent-input))
(run-hook-with-args
 'wisent-discarding-token-functions wisent-input)
(wisent-clearin)
(wisent-errok)))
@end group
@end lisp
@end defun

@findex wisent-skip-block
@defun wisent-skip-block
Safely skip a block in order to resume parsing.
Return nil.
Must be used in error recovery semantic actions.

A block is data between an open-delimiter (syntax class @code{(}) and
a matching close-delimiter (syntax class @code{)}):

@example
@group
(a parenthesized block)
[a block between brackets]
@{a block between braces@}
@end group
@end example

The following example uses @code{wisent-skip-block} to safely skip a
block delimited by @samp{LBRACE} (@code{@{}) and @samp{RBRACE}
(@code{@}}) tokens, when a syntax error occurs in
@samp{other-components}:

@example
@group
(block ((LBRACE other-components RBRACE))
       ((LBRACE RBRACE))
       ((LBRACE error)
        (wisent-skip-block))
       )
@end group
@end example
@end defun

@node Debugging actions
@section Debugging semantic actions

@cindex semantic action symbols
Each semantic action is represented by a symbol interned in an
@dfn{obarray} that is part of the @acronym{LALR(1)} automaton
(@pxref{Compiling a grammar}).  @code{symbol-function} on a semantic
action symbol return the semantic action lambda expression.

A semantic action symbol name has the form
@code{@var{nonterminal}:@var{index}}, where @var{nonterminal} is the
name of the nonterminal symbol the action belongs to, and @var{index}
is an action sequence number within the scope of @var{nonterminal}.
For example, this nonterminal definition:

@example
@group
input:
   line                     [@code{input:0}]
 | input line
   (format "%s %s" $1 $2)   [@code{input:1}]
 ;
@end group
@end example

Will produce two semantic actions, and associated symbols:

@table @code
@item input:0
A default action that returns @code{$1}.

@item input:1
That returns @code{(format "%s %s" $1 $2)}.
@end table

@cindex debugging semantic actions
Debugging uses the Lisp debugger to investigate what is happening
during execution of semantic actions.
Three commands are available to debug semantic actions.  They receive
two arguments:

@itemize @bullet
@item The automaton that contains the semantic action.

@item The semantic action symbol.
@end itemize

@findex wisent-debug-on-entry
@deffn Command wisent-debug-on-entry automaton function
Request @var{automaton}'s @var{function} to invoke debugger each time it is called.
@var{function} must be a semantic action symbol that exists in @var{automaton}.
@end deffn

@findex wisent-cancel-debug-on-entry
@deffn Command wisent-cancel-debug-on-entry automaton function
Undo effect of @code{wisent-debug-on-entry} on @var{automaton}'s @var{function}.
@var{function} must be a semantic action symbol that exists in @var{automaton}.
@end deffn

@findex wisent-debug-show-entry
@deffn Command wisent-debug-show-entry automaton function
Show the source of @var{automaton}'s semantic action @var{function}.
@var{function} must be a semantic action symbol that exists in @var{automaton}.
@end deffn

@node Wisent Semantic
@chapter How to use Wisent with Semantic

@cindex tags
This section presents how the Wisent's parser can be used to produce
@dfn{tags} for the @semantic{} tool set.

@semantic{} tags form a hierarchy of Emacs Lisp data structures that
describes a program in a way independent of programming languages.
Tags map program declarations, like functions, methods, variables,
data types, classes, includes, grammar rules, etc..

@cindex WY grammar format
To use the Wisent parser with @semantic{} you have to define
your grammar in @dfn{WY} form, a grammar format very close
to the one used by Bison.

Please @inforef{top, Semantic Grammar Framework Manual, grammar-fw}
for more information on @semantic{} grammars.

@menu
* Grammar styles::              
* Wisent Lex::                  
@end menu

@node Grammar styles
@section Grammar styles

@cindex grammar styles
@semantic{} parsing heavily depends on how you wrote the grammar.
There are mainly two styles to write a Wisent's grammar intended to be
used with the @semantic{} tool set: the @dfn{Iterative style} and the
@dfn{Bison style}.  Each one has pros and cons, and in certain cases
it can be worth a mix of the two styles!

@menu
* Iterative style::             
* Bison style::                 
* Mixed style::                 
* Start nonterminals::          
* Useful functions::            
@end menu

@node Iterative style, Bison style, Grammar styles, Grammar styles
@subsection Iterative style

@cindex grammar iterative style
The @dfn{iterative style} is the preferred style to use with @semantic{}.
It relies on an iterative parser back-end mechanism which parses start
nonterminals one at a time and automagically skips unexpected lexical
tokens in input.

Compared to rule-based iterative functions (@pxref{Bison style}),
iterative parsers are better in that they can handle obscure errors
more cleanly.

@cindex raw tag
Each start nonterminal must produces a @dfn{raw tag} by calling a
@code{TAG}-like grammar macro with appropriate parameters.  See also
@ref{Start nonterminals}.

@cindex expanded tag
Then, each parsing iteration automatically translates a raw tag into
@dfn{expanded tags}, updating the raw tag structure with internal
properties and buffer related data.

After parsing completes, it results in a tree of expanded tags.

The following example is a snippet of the iterative style Java grammar
provided in the @semantic{} distribution in the file
@file{wisent-java-tags.wy}.

@example
@group
@dots{}
;; Alternate entry points
;;    - Needed by partial re-parse
%start formal_parameter
@dots{}
;;    - Needed by EXPANDFULL clauses
%start formal_parameters
@dots{}

formal_parameter_list
  : PAREN_BLOCK
    (EXPANDFULL $1 formal_parameters)
  ;

formal_parameters
  : LPAREN
    ()
  | RPAREN
    ()
  | formal_parameter COMMA
  | formal_parameter RPAREN
  ;

formal_parameter
  : formal_parameter_modifier_opt type variable_declarator_id
    (VARIABLE-TAG $3 $2 nil :typemodifiers $1)
  ;
@end group
@end example

@findex EXPANDFULL
It shows the use of the @code{EXPANDFULL} grammar macro to parse a
@samp{PAREN_BLOCK} which contains a @samp{formal_parameter_list}.
@code{EXPANDFULL} tells to recursively parse @samp{formal_parameters}
inside @samp{PAREN_BLOCK}.  The parser iterates until it digested all
available input data inside the @samp{PAREN_BLOCK}, trying to match
any of the @samp{formal_parameters} rules:

@itemize
@item @samp{LPAREN}

@item @samp{RPAREN}

@item @samp{formal_parameter COMMA}

@item @samp{formal_parameter RPAREN}
@end itemize

At each iteration it will return a @samp{formal_parameter} raw tag,
or @code{nil} to skip unwanted (single @samp{LPAREN} or @samp{RPAREN}
for example) or unexpected input data.  Those raw tags will be
automatically expanded by the iterative back-end parser.

@node Bison style
@subsection Bison style

@cindex grammar bison style
What we call the @dfn{Bison style} is the traditional style of Bison's
grammars.  Compared to iterative style, it is not straightforward to
use grammars written in Bison style in @semantic{}.  Mainly because such
grammars are designed to parse the whole input data in one pass, and
don't use the iterative parser back-end mechanism (@pxref{Iterative
style}).  With Bison style the parser is called once to parse the
grammar start nonterminal.

The following example is a snippet of the Bison style Java grammar
provided in the @semantic{} distribution in the file
@file{wisent-java.wy}.

@example
@group
%start formal_parameter
@dots{}

formal_parameter_list
  : formal_parameter_list COMMA formal_parameter
    (cons $3 $1)
  | formal_parameter
    (list $1)
  ;

formal_parameter
  : formal_parameter_modifier_opt type variable_declarator_id
    (EXPANDTAG
     (VARIABLE-TAG $3 $2 :typemodifiers $1)
     )
  ;
@end group
@end example

The first consequence is that syntax errors are not automatically
handled by @semantic{}.  Thus, it is necessary to explicitly handle
them at the grammar level, providing error recovery rules to skip
unexpected input data.

The second consequence is that the iterative parser can't do automatic
tag expansion, except for the start nonterminal value.  It is
necessary to explicitly expand tags from concerned semantic actions by
calling the grammar macro @code{EXPANDTAG} with a raw tag as
parameter.  See also @ref{Start nonterminals}, for incremental
re-parse considerations.

@node Mixed style
@subsection Mixed style

@cindex grammar mixed style
@example
@group
%start grammar
;; Reparse
%start prologue epilogue declaration nonterminal rule
@dots{}

%%

grammar:
    prologue
  | epilogue
  | declaration
  | nonterminal
  | PERCENT_PERCENT
  ;
@dots{}

nonterminal:
    SYMBOL COLON rules SEMI
    (TAG $1 'nonterminal :children $3)
  ;

rules:
    lifo_rules
    (apply 'nconc (nreverse $1))
  ;

lifo_rules:
    lifo_rules OR rule
    (cons $3 $1)
  | rule
    (list $1)
  ;

rule:
    rhs
    (let* ((rhs $1)
           name type comps prec action elt)
      @dots{}
      (EXPANDTAG
       (TAG name 'rule :type type :value comps :prec prec :expr action)
       ))
  ;
@end group
@end example

This example shows how iterative and Bison styles can be combined in
the same grammar to obtain a good compromise between grammar
complexity and an efficient parsing strategy in an interactive
environment.

@samp{nonterminal} is parsed using iterative style via the main
@samp{grammar} rule.  The semantic action uses the @code{TAG} macro to
produce a raw tag, automagically expanded by @semantic{}.

But @samp{rules} part is parsed in Bison style! Why?

Rule delimiters are the colon (@code{:}), that follows the nonterminal
name, and a final semicolon (@code{;}).  Unfortunately these
delimiters are not @code{open-paren}/@code{close-paren} type, and the
Emacs' syntactic analyzer can't easily isolate data between them to
produce a @samp{RULES_PART} parenthesis-block-like lexical token.
Consequently it is not possible to use @code{EXPANDFULL} to iterate in
@samp{RULES_PART}, like this:

@example
@group
nonterminal:
    SYMBOL COLON rules SEMI
    (TAG $1 'nonterminal :children $3)
  ;

rules:
    RULES_PART  ;; @strong{Map a parenthesis-block-like lexical token}
    (EXPANDFULL $1 'rules)
  ;

rules:
    COLON
    ()
    OR
    ()
    SEMI
    ()
    rhs
    rhs
    (let* ((rhs $1)
           name type comps prec action elt)
      @dots{}
      (TAG name 'rule :type type :value comps :prec prec :expr action)
      )
  ;
@end group
@end example

In such cases, when it is difficult for Emacs to obtain
parenthesis-block-like lexical tokens, the best solution is to use the
traditional Bison style with error recovery!

In some extreme cases, it can also be convenient to extend the lexer,
to deliver new lexical tokens, to simplify the grammar.

@node Start nonterminals
@subsection Start nonterminals

@cindex start nonterminals
@cindex @code{reparse-symbol} property
When you write a grammar for @semantic{}, it is important to carefully
indicate the start nonterminals.  Each one defines an entry point in
the grammar, and after parsing its semantic value is returned to the
back-end iterative engine.  Consequently:

@strong{The semantic value of a start nonterminal must be a produced
by a TAG like grammar macro}.

Start nonterminals are declared by @code{%start} statements.  When
nothing is specified the first nonterminal that appears in the grammar
is the start nonterminal.

Generally, the following nonterminals must be declared as start
symbols:

@itemize @bullet
@item The main grammar entry point
@quotation
Of course!
@end quotation

@item nonterminals passed to @code{EXPAND}/@code{EXPANDFULL}
@quotation
These grammar macros recursively parse a part of input data, based on
rules of the given nonterminal.

For example, the following will parse @samp{PAREN_BLOCK} data using
the @samp{formal_parameters} rules:

@example
@group
formal_parameter_list
  : PAREN_BLOCK
    (EXPANDFULL $1 formal_parameters)
  ;
@end group
@end example

The semantic value of @samp{formal_parameters} becomes the value of
the @code{EXPANDFULL} expression.  It is a list of @semantic{} tags
spliced in the tags tree.

Because the automaton must know that @samp{formal_parameters} is a
start symbol, you must declare it like this:

@example
@group
%start formal_parameters
@end group
@end example
@end quotation
@end itemize

@cindex incremental re-parse
@cindex reparse-symbol
The @code{EXPANDFULL} macro has a side effect it is important to know,
related to the incremental re-parse mechanism of @semantic{}: the
nonterminal symbol parameter passed to @code{EXPANDFULL} also becomes
the @code{reparse-symbol} property of the tag returned by the
@code{EXPANDFULL} expression.

When buffer's data mapped by a tag is modified, @semantic{}
schedules an incremental re-parse of that data, using the tag's
@code{reparse-symbol} property as start nonterminal.

@strong{The rules associated to such start symbols must be carefully
reviewed to ensure that the incremental parser will work!}

Things are a little bit different when the grammar is written in Bison
style.  

@strong{The @code{reparse-symbol} property is set to the nonterminal
symbol the rule that explicitly uses @code{EXPANDTAG} belongs to.}

For example:

@example
@group
rule:
    rhs
    (let* ((rhs $1)
           name type comps prec action elt)
      @dots{}
      (EXPANDTAG
       (TAG name 'rule :type type :value comps :prec prec :expr action)
       ))
  ;
@end group
@end example

Set the @code{reparse-symbol} property of the expanded tag to
@samp{rule}.  A important consequence is that:

@strong{Every nonterminal having any rule that calls @code{EXPANDTAG}
in a semantic action, should be declared as a start symbol!}

@node Useful functions
@subsection Useful functions

Here is a description of some predefined functions it might be useful
to know when writing new code to use Wisent in @semantic{}:

@findex wisent-collect-unmatched-syntax
@defun wisent-collect-unmatched-syntax input
Add @var{input} lexical token to the cache of unmatched tokens, in
variable @code{semantic-unmatched-syntax-cache}.

See implementation of the function @code{wisent-skip-token} in
@ref{Error recovery}, for an example of use.
@end defun

@node Wisent Lex
@section The Wisent Lex lexer

@findex semantic-lex
The lexical analysis step of @semantic{} is performed by the general
function @code{semantic-lex}.  For more information, @inforef{Writing
Lexers, ,semantic-langdev}.

@code{semantic-lex} produces lexical tokens of the form:

@example
@group
@code{(@var{token-class start} . @var{end})}
@end group
@end example

@table @var
@item token-class
Is a symbol that identifies a lexical token class, like @code{symbol},
@code{string}, @code{number}, or @code{PAREN_BLOCK}.

@item start
@itemx end
Are the start and end positions of mapped data in the input buffer.
@end table
 
The Wisent's parser doesn't depend on the nature of analyzed input
stream (buffer, string, etc.), and requires that lexical tokens have a
different form (@pxref{Writing a lexer}):

@example
@group
@code{(@var{token-class value} [@var{start} . @var{end}])}
@end group
@end example

@cindex lexical token mapping
@code{wisent-lex} is the default Wisent's lexer used in @semantic{}.

@vindex wisent-lex-istream
@findex wisent-lex
@defun wisent-lex
Return the next available lexical token in Wisent's form.

The variable @code{wisent-lex-istream} contains the list of lexical
tokens produced by @code{semantic-lex}.  Pop the next token available
and convert it to a form suitable for the Wisent's parser.
@end defun

Mapping of lexical tokens as produced by @code{semantic-lex} into
equivalent Wisent lexical tokens is straightforward:

@example
@group
(@var{token-class start} . @var{end})
     @result{} (@var{token-class value start} . @var{end})
@end group
@end example

@var{value} is the input @code{buffer-substring} from @var{start} to
@var{end}.

@node GNU Free Documentation License
@appendix GNU Free Documentation License

@include fdl.texi

@node Index
@unnumbered Index
@printindex cp

@iftex
@contents
@summarycontents
@end iftex

@bye

@c Following comments are for the benefit of ispell.

@c  LocalWords:  Wisent automagically wisent Wisent's LALR obarray