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Castile / README.markdown

Castile

This is the reference distribution for Castile, an unremarkable programming language.

The current version of Castile is 0.3-PRE. It is not only subject to change, it is pretty much guaranteed to change.

Unlike most of my programming languages, there is nothing that could really be described as innovative or experimental or even particularly unusual about Castile. It is not a particularly comfortable programming experience, often forcing the programmer to be explicit and verbose.

The reference implementation is slightly less unremarkable than the language itself, if only for the fact that it compiles to four different target languages: Javascript, Ruby, a hypothetical stack machine called "stackmac" (a stackmac emulator ships with this distribution,) and (coming soon) C.

Castile's influences might include:

  • C: Most of Castile's syntax follows C, but it is generally more permissive (semicolons are optional, types of local variables and return types for functions do not have to be declared, etc.) It has a type system (where structs are the only types with name equivalence) which can be applied statically. It has function values, but not closures.

  • Rust: There is a union type, to which values must be explicitly promoted (with as) and extracted (with typecase ... is.) This is like Rust's Enum, which is (to quote its tutorial) "much like the 'tagged union' pattern in C, but with better static guarantees." Along with structs, this provides something similar to algebraic data typing, as seen in languages such as Haskell, Scala, etc.

  • Eightebed: A few years back I realized that pointers that can assume a null value are really a variant type, like Haskell's Maybe. Of course, in most languages with pointers, the property of being null isn't captured by the type; you can go ahead and dereference a pointer in C or Java, whether it's valid or not. In Castile, this is captured with a union type which includes void, and typecase generalizes Eightebed's ifvalid.

  • Python: The first time a local variable is assigned counts as its declaration as a local.

  • Ruby: The last expression in a function body is the return value of that function; no explicit return is needed there. (But unlike Ruby, and more like Pascal or linted C, all other expressions in statement position within a block must have void type.)

  • Erlang (or any other purely functional language): There are no language-level pointers; sharing, if it happens at all, must be orchestrated by the implementation. Global variables and function arguments are not mutable, and neither are the fields of structs. (But unlike Erlang, local variables are mutable.)

Some lines of research underneath all this are, if all we have is a relatively crude language, but we make it typesafe and give it a slightly nicer type system, does it suffice to make programming tolerable? Do tolerable ways of managing memory without a full garbage collector present themselves? Does having a simple compiler which can be target many backends provide any advantages?

Also unlike most of my programming languages (with the exceptions of ILLGOL and Bhuna), Castile was largely "designed by building" -- I wrote an interpreter, and the language it happens to accept, I called Castile. I wrote the interpreter in a very short span of time; most of it was done within 24 hours of starting (but consider that I ripped off some of the scanning/parsing code from ALPACA.) A few days later, I extended the implementation to also allow compiling to Javascript, and a few days after that, I added a Ruby backend (why not, eh?), and over the next few days, the stackmac backend and emulator.

This document contains what is as close as there is to a specification of the language, in the form of a Falderal test suite. The interpreter and all compilers pass all the tests, but there are known shortcomings in at least the compilers (no name mangling, etc.)

The eg directory contains a few example Castile programs, including a string tokenizer.

One area where the Castile implementation is not entirely unremarkable is that the typechecker is not required to be run. Unchecked Castile is technically a different language from Castile; there are Castile programs which would result in an error, where the Unchecked Castile program would not (because it never executes the part of the program with a bad type.) However, Unchecked Castile programs should be otherwise well-behaved; any attempt to execute code which would have resulted in a type failure, will result in a crash. Note, however, that this only applies to the evaluator, not any of the compiler backends. Compiling Unchecked Castile will simply not work (the backend will crash when it can't see any types.)

Grammar

Program ::= {Defn [";"]}.
Defn    ::= "fun" ident "(" [Arg {"," Arg}] ")" Body
          | "struct" ident "{" {ident ":" TExpr [";"]} "}"
          | ident (":" TExpr0 | "=" Literal).
Arg     ::= ident [":" TExpr1].
Body    ::= "{" {Stmt [";"]} "}".
Stmt    ::= "while" Expr0 Block
          | "typecase" ident "is" TExpr0 Block
          | "do" Expr0
          | "return" Expr0
          | If
          | Expr0.
Block   ::= "{" {Stmt [";"]} "}".
If      ::= "if" Expr0 Block ["else" (Block | If)].
Expr0   ::= Expr1 {("and" | "or") Expr1} ["as" TExpr0].
Expr1   ::= Expr2 {(">" | ">=" | "<" | "<=" | "==" | "!=") Expr2}.
Expr2   ::= Expr3 {("+" | "-") Expr3}.
Expr3   ::= Expr4 {("*" | "/") Expr4}.
Expr4   ::= Expr5 {"(" [Expr0 {"," Expr0}] ")" | "." ident}.
Expr5   ::= "make" ident "(" [ident ":" Expr0 {"," ident ":" Expr0}] ")"
          | "(" Expr0 ")"
          | "not" Expr1
          | Literal
          | ident ["=" Expr0].
Literal ::= strlit
          | ["-"] intlit
          | "true" | "false" | "null"
          | "fun" "(" [Arg {"," Arg}] ")" Body.
TExpr0  ::= TExpr1 [{"," TExpr1} "->" TExpr1].
TExpr1  ::= TExpr2 {"|" TExpr2}.
TExpr2  ::= "integer"
          | "boolean"
          | "void"
          | "(" TExpr0 ")"
          | ident.

Examples

-> Tests for functionality "Run Castile Program"

Rudiments

Minimal correct program.

| fun main() {}
=

A program may evaluate to a value.

| fun main() { 160 }
= 160

The function named main is the one that is evaluated when the program is run.

| fun foobar(a, b, c) { 100 }
| fun main() { 120 }
| fun f() { 140 }
= 120

main should have no formal arguments.

| fun main(a, b, c) {
|   120
| }
? type mismatch

But other functions may.

| fun foobar(a, b) { b }
| fun main() { foobar(100, 200) }
= 200

Defined function names must be unique.

| fun dup() { 1 }
| fun dup() { 2 }
? duplicate

Formal argument names must be unique.

| fun f(g, g) {}
| fun main() { 1 }
? defined

Functions must be defined before they are referenced.

| fun main() { f(7) }
| fun f(g) { g }
? undefined

Either that, or forward-declared.

| f : integer -> integer
| fun main() { f(7) }
| fun f(g) { g * 2 }
= 14

If forward-declared, types must match.

| f : integer -> string
| fun main() { f(7) }
| fun f(g) { g * 2 }
? type mismatch

Arguments must match...

| fun f(g, h) { g * 2 + h * 2 }
| fun main() { f(7) }
? argument mismatch

| fun f(g, h) { g * 2 + h * 2 }
| fun main() { f(7,8,9) }
? argument mismatch

Statements

Statements are commands that have the type void and are executed for their side-effects. So, in general, statements may not be expressions. The exception is that the last statement in a block may be an expression; the result of that expression is the value of the block.

| fun main() {
|   20 * 8
| }
= 160

| fun main() {
|   20 + 3 * 8;
|   20 * 8
| }
? type mismatch

An if/else lets you make decisions.

| fun main() {
|   a = 0;
|   if 3 > 2 {
|     a = 70
|   } else {
|     a = 80
|   }
|   a
| }
= 70

An if need not have an else.

| fun main() {
|   a = 60
|   if 3 > 2 {
|     a = 70
|   }
|   a
| }
= 70

if always typechecks to void, one branch or two.

| fun main() {
|   a = 60
|   if 3 > 2 {
|     a = 70
|   }
| }
=

| fun main() {
|   a = 60
|   if 3 > 2 {
|     a = 70
|   } else {
|     a = 90
|   }
| }
=

If an if does have an else, the part after else must be either a block (already shown) or another if.

| fun main() {
|   if 2 > 3 {
|     return 60
|   } else if 4 > 5 {
|     return 0
|   } else {
|     return 1
|   }
| }
= 1

No dangling else problem.

| fun main() {
|   if 2 > 3 {
|     return 60
|   } else if 4 < 5 {
|     return 99
|   } else {
|     return 1
|   }
| }
= 99

while loops.

| fun main() {
|   a = 0 b = 4
|   while b > 0 {
|     a = a + b
|     b = b - 1
|   }
|   a
| }
= 10

A while itself has void type.

| fun main() {
|   a = 0; b = 4;
|   while b > 0 {
|     a = a + b;
|     b = b - 1;
|   }
| }
=

break may be used to prematurely exit a while.

| fun main() {
|   a = 0; b = 0;
|   while true {
|     a = a + b;
|     b = b + 1;
|     if (b > 4) { break; }
|   }
|   a
| }
= 10

Expressions

Precedence.

| fun main() {
|   2 + 3 * 4  /* not 20 */
| }
= 14

Unary negation.

| fun main() {
|   -3
| }
= -3

| fun main() {
|   2+-5
| }
= -3

Minus sign must be right in front of a number.

| fun main() {
|   -(4)
| }
? Expected

Unary not.

| fun main() {
|   not (4 > 3)
| }
= False

Precedence of unary not.

| fun main() {
|   not true or true
| }
= True

| fun main() {
|   not 3 > 4
| }
= True

Local Variables

Local variables.

| fun main() {
|   a = 6;
|   b = 7;
|   a + b
| }
= 13

Local variables can be assigned functions.

| fun ancillary(x) { x * 2 }
| fun main() {
|   a = ancillary;
|   a(7)
| }
= 14

Local variables can be assigned.

| fun main() {
|   a = 6;
|   a = a + 12;
|   a
| }
= 18

| fun main() {
|   a = 6;
|   z = 99;
|   a
| }
= 6

| fun main() {
|   z = 6;
|   a
| }
? undefined

Local variables cannot occur in expressions until they are defined by an initial assignment.

| fun main() {
|   z = a * 10;
|   a = 10;
|   z
| }
? undefined

A local variables may not be defined inside an if or while or typecase block, as it might not be executed.

| fun main() {
|   if (4 > 5) {
|     a = 10;
|   } else {
|     b = 11;
|   }
|   b
| }
? within control

| fun main() {
|   b = false;
|   while b {
|     a = 10;
|   }
|   a
| }
? within control

| fun main() {
|   a = 55 as integer|string;
|   typecase a is string {
|     b = 7
|   }
|   a
| }
? within control

Assignment, though it syntactically may occur in expressions, has a type of void, so it can only really happen at the statement level.

| fun main() {
|   a = 0; b = 0;
|   a = b = 9;
| }
? type mismatch

Variables in upper scopes may be modified.

| fun main() {
|   a = 0
|   if 3 > 2 {
|     a = 4;
|   }
|   a
| }
= 4

Non-local Values

Literals may appear at the toplevel. Semicolons are optional at toplevel.

| factor = 5;
| fun main() {
|   6 * factor
| }
= 30

Toplevel literals may not be updated. (And thus

| factor = 5
| fun main() {
|   factor = 7
| }
? shadows

Toplevel literals may be function literals (the syntax we've been using is just sugar.)

| main = fun() {
|   7
| }
= 7

Truth and falsehood are builtin toplevels.

| fun main() {
|   true or false
| }
= True

| fun main() {
|   false and true
| }
= False

So is null, which is the single value of void type.

| fun wat(x: void) { 3 }
| fun main() {
|   wat(null)
| }
= 3

More on Functions

Function arguments may not be updated.

| fun foo(x) {
|   x = x + 14;
|   x
| }
| fun main() {
|   foo(7)
| }
? shadows

Factorial can be computed.

| factorial : integer -> integer
| fun factorial(a) {
|   if a == 0 {
|     return 1
|   } else {
|     return a * factorial(a - 1)
|   }
| }
| fun main() {
|   factorial(6)
| }
= 720

Literal functions.

| fun main() {
|   inc = fun(x) { x + 1 };
|   inc(7)
| }
= 8

| fun main() {
|   fun(x){ x + 1 }(9)
| }
= 10

| fun main() {
|   a = 99;
|   a = fun(x){ x + 1 }(9);
|   a
| }
= 10

Literal functions can have local variables, loops, etc.

| fun main() {
|   z = 99;
|   z = fun(x) {
|     a = x;  b = x;
|     while a > 0 {
|       b = b + a; a = a - 1;
|     }
|     return b
|   }(9);
|   z
| }
= 54

Literal functions can define other literal functions...

| fun main() {
|   fun(x){ fun(y){ fun(z){ z + 1 } } }(4)(4)(10)
| }
= 11

Literal functions can access globals.

| oid = 19
| fun main() {
|   fun(x){ x + oid }(11);
| }
= 30

Literal functions cannot access variables declared in enclosing scopes.

| fun main() {
|   oid = 19;
|   fun(x){ x + oid }(11);
| }
? undefined

Literal functions cannot access arguments declared in enclosing scopes.

| fun main() {
|   fun(x){ fun(y){ fun(z){ y + 1 } } }(4)(4)(10)
| }
? undefined

Functions can be passed to functions and returned from functions.

| fun doubble(x) { x * 2 }
| fun triple(x) { x * 3 }
| fun apply_and_add_one(f: (integer -> integer), x) { f(x) + 1 }
| fun sellect(a) { if a > 10 { return doubble } else { return triple } }
| fun main() {
|   t = sellect(5);
|   d = sellect(15);
|   p = t(10);
|   apply_and_add_one(d, p)
| }
= 61

To overcome the syntactic ambiguity with commas, function types in function definitions must be in parens.

| fun add(x, y) { x + y }
| fun mul(x, y) { x * y }
| fun do_it(f: (integer, integer -> integer), g) {
|   f(3, g)
| }
| fun main() {
|   do_it(mul, 4) - do_it(add, 4)
| }
= 5

return may be used to prematurely return a value from a function.

| fun foo(y) {
|   x = y
|   while x > 0 {
|     if x < 5 {
|       return x;
|     }
|     x = x - 1;
|   }
|   17
| }
| fun main() {
|   foo(10) + foo(0)
| }
= 21

Type of value returned must jibe with value of function's block.

| fun foo(x) {
|   return "string";
|   17
| }
| fun main() {
|   foo(10) + foo(0)
| }
? type mismatch

Type of value returned must jibe with other return statements.

| fun foo(x) {
|   if x > 0 {
|     return "string";
|   } else {
|     return 17
|   }
| }
| fun main() {
|   foo(10) + foo(0)
| }
? type mismatch

Builtins

The usual.

| fun main() {
|   print("Hello, world!")
| }
= Hello, world!

Some standard functions are builtin and available as toplevels.

| fun main() {
|   a = "hello";
|   b = len(a);
|   while b > 0 {
|     print(a);
|     b = b - 1;
|     a = substr(a, 1, b)
|   }
| }
= hello
= ello
= llo
= lo
= o

The + operator is not string concatenation. concat is.

| fun main() {
|   print("hello " + "world")
| }
? type mismatch

| fun main() {
|   print(concat("hello ", "world"))
| }
= hello world

The builtin toplevels are functions and functions need parens.

| fun main() {
|   print "hi"
| }
? type mismatch

Note that the above was the motivation for requiring statements to have void type; if non-void exprs could be used anywhere, that would just throw away the function value print (b/c semicolons are optional) and return 'hi'.

Struct Types

Record types. You can define them:

| struct person { name: string; age: integer }
| main = fun() {}
=

And make them.

| struct person { name: string; age: integer }
| main = fun() {
|   j = make person(name:"Jake", age:23);
|   print("ok")
| }
= ok

And extract the fields from them.

| struct person { name: string; age: integer }
| main = fun() {
|   j = make person(name:"Jake", age:23);
|   print(j.name)
|   if j.age > 20 {
|     print("Older than twenty")
|   } else {
|     print("Underage")
|   }
| }
= Jake
= Older than twenty

Structs must be defined somewhere.

| main = fun() {
|   j = make person(name:"Jake", age:23);
|   j
| }
? undefined

Structs need not be defined before use.

| main = fun() {
|   j = make person(name:"Jake", age:23);
|   j.age
| }
| struct person { name: string; age: integer }
= 23

Structs may not contain structs which don't exist.

| struct person { name: string; age: foobar }
| main = fun() { 333 }
? undefined

Types must match when making a struct.

| struct person { name: string; age: integer }
| main = fun() {
|   j = make person(name:"Jake", age:"Old enough to know better");
|   j.age
| }
? type mismatch

| struct person { name: string; age: integer }
| main = fun() {
|   j = make person(name:"Jake");
|   j.age
| }
? argument mismatch

| struct person { name: string }
| main = fun() {
|   j = make person(name:"Jake", age:23);
|   j.age
| }
? argument mismatch

Order of field initialization when making a struct doesn't matter.

| struct person { name: string; age: integer }
| main = fun() {
|   j = make person(age: 23, name:"Jake");
|   j.age
| }
= 23

Structs can be tested for equality. (Since structs are immutable, it doesn't matter if this is structural equality or identity.)

/| struct person { age: integer; name: string }
/| main = fun() {
/|   j = make person(age: 23, name:"Jake");
/|   k = make person(age: 23, name:"Jake");
/|   j == k
/| }
/= True

/| struct person { name: string; age: integer }
/| main = fun() {
/|   j = make person(age: 23, name:"Jake");
/|   k = make person(name:"Jake", age: 23);
/|   j == k
/| }
/= True

/| struct person { age: integer; name: string }
/| main = fun() {
/|   j = make person(age: 23, name:"Jake");
/|   k = make person(age: 23, name:"John");
/|   j == k
/| }
/= False

Structs can be passed to functions.

| struct person { name: string; age: integer }
| fun wat(bouncer: person) { bouncer.age }
| main = fun() {
|   j = make person(name:"Jake", age:23);
|   wat(j)
| }
= 23

Structs have name equivalence, not structural.

| struct person { name: string; age: integer }
| struct city { name: string; population: integer }
| fun wat(hometown: city) { hometown }
| main = fun() {
|   j = make person(name:"Jake", age:23);
|   wat(j)
| }
? type mismatch

Struct fields must all be unique.

| struct person { name: string; name: string }
| main = fun() {
|   j = make person(name:"Jake", name:"Smith");
| }
? defined

Values can be retrieved from structs.

| struct person { name: string; age: integer }
| fun age(bouncer: person) { bouncer.age }
| main = fun() {
|   j = make person(name:"Jake", age:23);
|   age(j)
| }
= 23

| struct person { name: string }
| fun age(bouncer: person) { bouncer.age }
| main = fun() {
|   j = make person(name:"Jake");
|   age(j)
| }
? undefined

Different structs may have the same field name in different positions.

| struct person { name: string; age: integer }
| struct city { population: integer; name: string }
| main = fun() {
|   j = make person(name:"Jake", age:23);
|   w = make city(population:600000, name:"Winnipeg");
|   print(j.name)
|   print(w.name)
| }
= Jake
= Winnipeg

Can't define the same struct multiple times.

| struct person { name: string; age: integer }
| struct person { name: string; age: string }
| fun main() { 333 }
? duplicate

Structs may refer to themselves.

| struct recursive {
|   next: recursive;
| }
| fun main() { 333 }
= 333

| struct odd {
|   next: even;
| }
| struct even {
|   next: odd;
| }
| fun main() { 333 }
= 333

But you can't actually make one of these infinite structs.

| struct recursive {
|   next: recursive;
| }
| fun main() { make recursive(next:make recursive(next:"nooo")) }
? type mismatch

Union Types

Values of union type are created with the type promotion operator, as .... Type promotion has a very low precedence, and can be applied to any expression.

The type after the as must be a union.

| fun main() {
|   a = 20;
|   b = 30;
|   a + b as integer
| }
? bad cast

The type after the as must be one of the types in the union.

| fun main() {
|   a = 20;
|   b = 30;
|   a + b as string|void
| }
? bad cast

The type after the as must be the type of the expression.

| fun main() {
|   a = 20;
|   b = 30;
|   c = a + b as integer|string
|   print("ok")
| }
= ok

Values of union type can be passed to functions.

| fun foo(a, b: integer|string) {
|   a + 1
| }
| main = fun() {
|   a = 0;
|   a = foo(a, 333 as integer|string);
|   a = foo(a, "hiya" as integer|string);
|   a
| }
= 2

Order of types in a union doesn't matter.

| fun foo(a, b: integer|string) {
|   a + 1
| }
| main = fun() {
|   a = 0;
|   a = foo(a, 333 as integer|string);
|   a = foo(a, "hiya" as string|integer);
|   a
| }
= 2

The typecase construct can operate on the "right" type of a union.

| fun foo(a, b: integer|string) {
|   r = a;
|   typecase b is integer {
|     r = r + b;
|   };
|   typecase b is string {
|     r = r + len(b);
|   };
|   r
| }
| main = fun() {
|   a = 0;
|   a = foo(a, 333 as integer|string);
|   a = foo(a, "hiya" as integer|string);
|   a
| }
= 337

The expression in a typecase must be a variable.

| main = fun() {
|   a = 333 as integer|string;
|   typecase 333 is integer {
|     print("what?")
|   };
| }
? identifier

The expression in a typecase can be an argument.

| fun wat(j: integer|string) {
|   typecase j is integer {
|     print("integer")
|   };
| }
| main = fun() {
|   wat(444 as integer|string)
| }
= integer

The expression in a typecase cannot effectively be a global, as globals must be literals and there is no way (right now) to make a literal of union type.

Inside a typecase the variable cannot be updated.

| main = fun() {
|   a = 333 as integer|string;
|   typecase a is integer {
|     a = 700;
|   };
| }
? cannot assign

The union can include void.

| main = fun() {
|   j = null as void|integer;
|   typecase j is void {
|     print("nothing there")
|   };
| }
= nothing there

Struct Types + Union Types

Union types may be used to make fields of a struct "nullable", so that you can in actuality create recursive, but finite, data structures.

| struct list {
|   value: string;
|   next: list|integer;
| }
| main = fun() {
|   l = make list(
|     value: "first",
|     next: make list(
|       value: "second",
|       next:0 as list|integer
|     ) as list|integer)
|   s = l.next
|   typecase s is list {
|     print(s.value)
|   }
| }
= second

You may want to use helper functions to hide this ugliness.

| struct list {
|   value: string;
|   next: list|void;
| }
| fun singleton(v: string) {
|   make list(value:v, next:null as list|void)
| }
| fun cons(v: string, l: list) {
|   make list(value:v, next:l as list|void)
| }
| fun nth(n, l: list) {
|   u = l as list|void;
|   v = u;
|   k = n;
|   while k > 1 {
|     typecase u is void { break; }
|     typecase u is list { v = u.next; }
|     u = v;
|     k = k - 1;
|   }
|   return u
| }
| main = fun() {
|   l = cons("first", singleton("second"));
|   g = nth(2, l);
|   typecase g is list { print(g.value); }
| }
= second

Structs may be empty.

| struct red { }
| fun show(color: red) {
|   print("hi")
| }
| main = fun() {
|   show(make red());
| }
= hi

In combination with unions, this lets us create "typed enums".

| struct red { }
| struct green { }
| struct blue { }
| fun show(color: red|green|blue) {
|   typecase color is red { print("red"); }
|   typecase color is green { print("green"); }
|   typecase color is blue { print("blue"); }
| }
| main = fun() {
|   show(make red() as red|green|blue);
|   show(make blue() as red|green|blue);
| }
= red
= blue
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