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

Hi, oh yes, I was going to add the instance and then I realized it doesn't work. Please feel free to fix. Thanks! -Iavor On Fri, Jan 3, 2014 at 3:25 PM, Gabor Greif wrote:

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@git.haskell.org wrote:

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Repository : ssh://git@git.haskell.org/base

On branch : master Link :

http://ghc.haskell.org/trac/ghc/changeset/c5c8c4dfbdc8493bcfaa804751eff2a9a4...

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commit c5c8c4dfbdc8493bcfaa804751eff2a9a41cc07a Author: Iavor S. Diatchki Date: Fri Jan 3 15:11:34 2014 -0800

Add functions to compare Nat and Symbol types for equality.

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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

+--------------------------------------------------------------------------------

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+ +-- | 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

--------------------------------------------------------------------------------

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-- 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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