polytype

A Hindley-Milner polymorphic typing system. Implements type inference via unification.

Usage

[dependencies]
polytype = "1.1"

Provided by polytype are the Type enum and the Context struct.

Unification:

let mut ctx = Context::default();

let tbool = Type::Constructed("bool", vec![]);
let tint = Type::Constructed("int", vec![]);
fn tlist(tp: Type) -> Type {
    Type::Constructed("list", vec![Box::new(tp)])
}

// t1: list(int → α) ; t2: list(β → bool)
let t1 = tlist(Type::Arrow(Arrow::new(tint, Type::Variable(0))));
let t2 = tlist(Type::Arrow(Arrow::new(Type::Variable(1), tbool)));
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.unify(&Type::Variable(0), &Type::Constructed("int", vec![])).expect("unifies");

let t = Type::Constructed("list", vec![Box::new(Type::Variable(0))]);
assert_eq!(format!("{}", &t), "list(t0)");
let t = t.apply(&ctx);
assert_eq!(format!("{}", &t), "list(int)");

Independent instantiation:

let mut ctx = Context::default();

let t1 = Type::Constructed("list", vec![Box::new(Type::Variable(3))]);
let t2 = Type::Constructed("list", vec![Box::new(Type::Variable(3))]);

let t1 = t1.instantiate_indep(&mut ctx);
let t2 = t2.instantiate_indep(&mut ctx);
assert_eq!(format!("{}", &t1), "list(t0)");
assert_eq!(format!("{}", &t2), "list(t1)");

Dependent instantiation:

use std::collections::HashMap;

let mut ctx = Context::default();

let t1 = Type::Constructed("list", vec![Box::new(Type::Variable(3))]);
let t2 = Type::Constructed("list", vec![Box::new(Type::Variable(3))]);

let mut bindings = HashMap::new();
let t1 = t1.instantiate(&mut ctx, &mut bindings);
let t2 = t2.instantiate(&mut ctx, &mut bindings);
assert_eq!(format!("{}", &t1), "list(t0)");
assert_eq!(format!("{}", &t2), "list(t0)");

See the documentation for more details.