(***********************************************************************)
(* *)
(* OCaml *)
(* *)
(* Xavier Leroy, projet Cristal, INRIA Rocquencourt *)
(* *)
(* Copyright 1996 Institut National de Recherche en Informatique et *)
(* en Automatique. All rights reserved. This file is distributed *)
(* under the terms of the Q Public License version 1.0. *)
(* *)
(***********************************************************************)
(* $Id: equations.ml 12800 2012-07-30 18:59:07Z doligez $ *)
(****************** Equation manipulations *************)
open Terms
type rule =
{ number: int;
numvars: int;
lhs: term;
rhs: term }
(* standardizes an equation so its variables are 1,2,... *)
let mk_rule num m n =
let all_vars = union (vars m) (vars n) in
let counter = ref 0 in
let subst =
List.map (fun v -> incr counter; (v, Var !counter)) (List.rev all_vars) in
{ number = num;
numvars = !counter;
lhs = substitute subst m;
rhs = substitute subst n }
(* checks that rules are numbered in sequence and returns their number *)
let check_rules rules =
let counter = ref 0 in
List.iter (fun r -> incr counter;
if r.number <> !counter
then failwith "Rule numbers not in sequence")
rules;
!counter
let pretty_rule rule =
print_int rule.number; print_string " : ";
pretty_term rule.lhs; print_string " = "; pretty_term rule.rhs;
print_newline()
let pretty_rules rules = List.iter pretty_rule rules
(****************** Rewriting **************************)
(* Top-level rewriting. Let eq:L=R be an equation, M be a term such that L<=M.
With sigma = matching L M, we define the image of M by eq as sigma(R) *)
let reduce l m r =
substitute (matching l m) r
(* Test whether m can be reduced by l, i.e. m contains an instance of l. *)
let can_match l m =
try let _ = matching l m in true
with Failure _ -> false
let rec reducible l m =
can_match l m ||
(match m with
| Term(_,sons) -> List.exists (reducible l) sons
| _ -> false)
(* Top-level rewriting with multiple rules. *)
let rec mreduce rules m =
match rules with
[] -> failwith "mreduce"
| rule::rest ->
try
reduce rule.lhs m rule.rhs
with Failure _ ->
mreduce rest m
(* One step of rewriting in leftmost-outermost strategy,
with multiple rules. Fails if no redex is found *)
let rec mrewrite1 rules m =
try
mreduce rules m
with Failure _ ->
match m with
Var n -> failwith "mrewrite1"
| Term(f, sons) -> Term(f, mrewrite1_sons rules sons)
and mrewrite1_sons rules = function
[] -> failwith "mrewrite1"
| son::rest ->
try
mrewrite1 rules son :: rest
with Failure _ ->
son :: mrewrite1_sons rules rest
(* Iterating rewrite1. Returns a normal form. May loop forever *)
let rec mrewrite_all rules m =
try
mrewrite_all rules (mrewrite1 rules m)
with Failure _ ->
m