polytype 1.2.3

A Hindley-Milner polymorphic typing system.
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# polytype

[![Build Status](https://travis-ci.org/lucasem/polytype-rs.svg?branch=master)](https://travis-ci.org/lucasem/polytype-rs)
[![crates.io](https://img.shields.io/crates/v/polytype.svg)](https://crates.io/crates/polytype)
[![docs.rs](https://docs.rs/polytype/badge.svg)](https://docs.rs/polytype)

A [Hindley-Milner](https://wikipedia.org/wiki/Hindley–Milner_type_system) polymorphic typing system.
Implements type inference via unification.

## Usage

```toml
[dependencies]
polytype = "1.2"
```

Provided by **`polytype`** are the
[`Type`](https://docs.rs/polytype/1.2.3/polytype/enum.Type.html) enum and
[`Context`](https://docs.rs/polytype/1.2.3/polytype/struct.Context.html)
struct, and the macros
[`tp!`](https://docs.rs/polytype/1.2.3/polytype/macro.tp.html) and
[`arrow!`](https://docs.rs/polytype/1.2.3/polytype/macro.arrow.html) which
help to concisely create
types.

Unification:

```rust
let mut ctx = Context::default();

// t1: list(int → α) ; t2: list(β → bool)
let t1 = tp!(list(arrow![tp!(int), tp!(0)]));
let t2 = tp!(list(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:

```rust
let mut ctx = Context::default();
// assign t0 to int
ctx.unify(&tp!(0), &tp!(int)).expect("unifies");

let t = tp!(list(tp!(0)));
assert_eq!(format!("{}", &t), "list(t0)");
let t = t.apply(&ctx);
assert_eq!(format!("{}", &t), "list(int)");
```

Independent instantiation:

```rust
let mut ctx = Context::default();

let t1 = tp!(list(tp!(3)));
let t2 = tp!(list(tp!(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:

```rust
use std::collections::HashMap;

let mut ctx = Context::default();

let t1 = tp!(list(tp!(3)));
let t2 = tp!(list(tp!(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](https://docs.rs/polytype) for more details.