A Hindley-Milner polymorphic typing system. Implements type inference via unification.
[dependencies]
polytype = "7.0"polytype provides the
TypeScheme and
Type enums, the
Context
struct, and the
tp! and
ptp! macros which
help to concisely create types and type schemes.
Unification:
let mut ctx = Context::default();
// t1: list(int → α) ; t2: list(β → bool)
let t1 = tp!(list(tp!(@arrow[tp!(int), tp!(0)])));
let t2 = tp!(list(tp!(@arrow[tp!(1), tp!(bool)])));
ctx.unify(&t1, &t2).expect("unifies");
let t1 = t1.apply(&ctx);
let t2 = t2.apply(&ctx);
assert_eq!(t1, t2); // list(int → bool)Apply a type context:
let mut ctx = Context::default();
// assign t0 to int
ctx.extend(0, tp!(int));
let t = tp!(list(tp!(0)));
assert_eq!(t.to_string(), "list(t0)");
let t = t.apply(&ctx);
assert_eq!(t.to_string(), "list(int)");Instantiate a TypeScheme:
let mut ctx = Context::default();
// ∀α. list(α)
let scheme = ptp!(3; tp!(list(tp!(3))));
// They instantiate to new fresh type variables
let t1 = scheme.instantiate(&mut ctx);
let t2 = scheme.instantiate(&mut ctx);
assert_eq!(t1.to_string(), "list(t0)");
assert_eq!(t2.to_string(), "list(t1)");See the documentation for more details.
By default polytype includes a type parser that can be invoked with
str::parse.
This can be disabled with default-features = false.