# polytype
[](https://travis-ci.org/lucasem/polytype-rs)
[](https://crates.io/crates/polytype)
[](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 = "6.2"
```
**`polytype`** provides the
[`TypeSchema`](https://docs.rs/polytype/~6/polytype/enum.TypeSchema.html) and
[`Type`](https://docs.rs/polytype/~6/polytype/enum.Type.html) enums, the
[`Context`](https://docs.rs/polytype/~6/polytype/struct.Context.html)
struct, and the
[`tp!`](https://docs.rs/polytype/~6/polytype/macro.tp.html) and
[`ptp!`](https://docs.rs/polytype/~6/polytype/macro.ptp.html) macros which
help to concisely create types and type schemas.
Unification:
```rust
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:
```rust
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 `TypeSchema`:
```rust
let mut ctx = Context::default();
// ∀α. list(α)
let schema = ptp!(3; tp!(list(tp!(3))));
// They instantiate to new fresh type variables
let t1 = schema.instantiate(&mut ctx);
let t2 = schema.instantiate(&mut ctx);
assert_eq!(t1.to_string(), "list(t0)");
assert_eq!(t2.to_string(), "list(t1)");
```
See the [documentation](https://docs.rs/polytype) for more details.
## Features
By default `polytype` includes a type parser that can be invoked with `Type::parse`.
This can be disabled with `default-features = false`.