[commit: packages/base] master: Add functions to compare Nat and Symbol types for equality. (c5c8c4d)

[Gabor Greif]

Iavor,

this is great! Just out of curiosity, you import TestEquality but
never reference it. Is this an oversight, should I nuke it?

Cheers,

    Gabor

On 1/4/14, git at git.haskell.org <git at git.haskell.org> wrote:
> Repository : ssh://git@git.haskell.org/base
>
> On branch  : master
> Link       :
> http://ghc.haskell.org/trac/ghc/changeset/c5c8c4dfbdc8493bcfaa804751eff2a9a41cc07a/base
>
>>---------------------------------------------------------------
>
> commit c5c8c4dfbdc8493bcfaa804751eff2a9a41cc07a
> Author: Iavor S. Diatchki <diatchki at galois.com>
> Date:   Fri Jan 3 15:11:34 2014 -0800
>
>     Add functions to compare Nat and Symbol types for equality.
>
>
>>---------------------------------------------------------------
>
> c5c8c4dfbdc8493bcfaa804751eff2a9a41cc07a
>  GHC/TypeLits.hs |   23 ++++++++++++++++++++++-
>  1 file changed, 22 insertions(+), 1 deletion(-)
>
> diff --git a/GHC/TypeLits.hs b/GHC/TypeLits.hs
> index f3ba70e..129beb3 100644
> --- a/GHC/TypeLits.hs
> +++ b/GHC/TypeLits.hs
> @@ -26,6 +26,8 @@ module GHC.TypeLits
>    , KnownSymbol, symbolVal
>    , SomeNat(..), SomeSymbol(..)
>    , someNatVal, someSymbolVal
> +  , sameNat, sameSymbol
> +
>
>      -- * Functions on type nats
>    , type (<=), type (<=?), type (+), type (*), type (^), type (-)
> @@ -40,7 +42,8 @@ import GHC.Read(Read(..))
>  import GHC.Prim(magicDict)
>  import Data.Maybe(Maybe(..))
>  import Data.Proxy(Proxy(..))
> -import Data.Type.Equality(type (==))
> +import Data.Type.Equality(type (==), TestEquality(..), (:~:)(Refl))
> +import Unsafe.Coerce(unsafeCoerce)
>
>  -- | (Kind) This is the kind of type-level natural numbers.
>  data Nat
> @@ -167,6 +170,23 @@ type family (m :: Nat) ^ (n :: Nat) :: Nat
>  type family (m :: Nat) - (n :: Nat) :: Nat
>
>
> +--------------------------------------------------------------------------------
> +
> +-- | We either get evidence that this function was instantiated with the
> +-- same type-level numbers, or 'Nothing'.
> +sameNat :: (KnownNat a, KnownNat b) =>
> +           Proxy a -> Proxy b -> Maybe (a :~: b)
> +sameNat x y
> +  | natVal x == natVal y = Just (unsafeCoerce Refl)
> +  | otherwise            = Nothing
> +
> +-- | We either get evidence that this function was instantiated with the
> +-- same type-level symbols, or 'Nothing'.
> +sameSymbol :: (KnownSymbol a, KnownSymbol b) =>
> +              Proxy a -> Proxy b -> Maybe (a :~: b)
> +sameSymbol x y
> +  | symbolVal x == symbolVal y  = Just (unsafeCoerce Refl)
> +  | otherwise                   = Nothing
>
> --------------------------------------------------------------------------------
>  -- PRIVATE:
> @@ -187,3 +207,4 @@ withSSymbol :: (KnownSymbol a => Proxy a -> b)
>              -> SSymbol a      -> Proxy a -> b
>  withSSymbol f x y = magicDict (WrapS f) x y
>
> +
>
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