# ghc-typelits-knownnat

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 derive `KnownNat`

constraints for types consisting of:

- Type variables, when there is a corresponding
`KnownNat`

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

- Type functions, when there is either:
- a matching given
`KnownNat`

constraint; or
- a corresponding
`KnownNat<N>`

instance for the type function

To elaborate the latter points, given the type family `Min`

:

```
type family Min (a :: Nat) (b :: Nat) :: Nat where
Min 0 b = 0
Min a b = If (a <=? b) a b
```

the plugin can derive a `KnownNat (Min x y + 1)`

constraint given only a
`KnownNat (Min x y)`

constraint:

```
g :: forall x y . (KnownNat (Min x y)) => Proxy x -> Proxy y -> Integer
g _ _ = natVal (Proxy :: Proxy (Min x y + 1))
```

And, given the type family `Max`

:

```
type family Max (a :: Nat) (b :: Nat) :: Nat where
Max 0 b = b
Max a b = If (a <=? b) b a
```

and corresponding `KnownNat2`

instance:

```
instance (KnownNat a, KnownNat b) => KnownNat2 "TestFunctions.Max" a b where
natSing2 = let x = natVal (Proxy @a)
y = natVal (Proxy @b)
z = max x y
in SNatKn z
{-# INLINE natSing2 #-}
```

the plugin can derive a `KnownNat (Max x y + 1)`

constraint given only a
`KnownNat x`

and `KnownNat y`

constraint:

```
h :: forall x y . (KnownNat x, KnownNat y) => Proxy x -> Proxy y -> Integer
h _ _ = natVal (Proxy :: Proxy (Max x y + 1))
```

To use the plugin, add the

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

Pragma to the header of your file.