ghc-typelits-knownnat-0.1: README.md

# ghc-typelits-knownnat

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A type checker plugin for GHC that can derive "complex" `KnownNat`
constraints from other simple/variable `KnownNat` constraints. i.e. without this
plugin, you must have both a `KnownNat n` and a `KnownNat (n+2)` constraint in
the type signature of the following function:

```
f :: forall n . (KnownNat n, KnownNat (n+2)) => Proxy n -> Integer
f _ = natVal (Proxy :: Proxy n) + natVal (Proxy :: Proxy (n+2))
```

Using the plugin you can omit the `KnownNat (n+2)` constraint:

```
f :: forall n . KnownNat n => Proxy n -> Integer
f _ = natVal (Proxy :: Proxy n) + natVal (Proxy :: Proxy (n+2))
```

The plugin can only derive `KnownNat` constraints consisting of:

* Type-level naturals
* Type variables
* Applications of the arithmetic expression: `{+,*,^}`.

i.e. it _cannot_ derive a `KnownNat (n-1)` constraint from a `KnownNat n`
constraint

To use the plugin, add the

```
OPTIONS_GHC -fplugin GHC.TypeLits.KnownNat.Solver
```

Pragma to the header of your file.