Re: [Haskell-cafe] TypeLits & Typeable

Hi guys, Yep, we know about this and, I believe, the plan is to add custom rules to the constraint solver to solve `Typable n` constraints (where n is a number or symbol). Just for the record, the other design choice was to add instance `Typeable (n :: Symbol)`, but that conflicted with some of the polymorphic instances already present in the library, so we decided to go for the custom constraint solver rules. This should not be hard to do, I just need to sit down and do it---my current priority has been to catch up the type-nats solver with HEAD and clean up things for merging. -Iavor On Mon, Aug 26, 2013 at 1:19 AM, José Pedro Magalhães wrote:

Hi Nicolas,

It's not intentional, but Iavor is aware of this, and we want to change it. I'm CC-ing him as he might know more about what the current plan is.

Cheers, Pedro

On Sat, Aug 24, 2013 at 3:20 PM, Nicolas Trangez wrote:

...

Hello Cafe,

I was playing around with TypeLits in combination with Typeable (using GHC 7.7.7.20130812 FWIW), but was surprised to find Symbols aren't Typeable, and as such the following doesn't work. Is this intentional, or am I missing something?

Thanks,

Nicolas

{-# LANGUAGE DataKinds, KindSignatures, DeriveFunctor, DeriveDataTypeable #-} module Main where

import Data.Typeable import GHC.TypeLits

data NoSymbol n a b = NoSymbol a b deriving (Typeable)

data WithSymbol (n :: Symbol) a b = WithSymbol a b deriving (Typeable)

data Sym deriving (Typeable)

main :: IO () main = do print $ typeOf (undefined :: NoSymbol Sym Int Int)

let d = undefined :: WithSymbol "sym" Int Int {- print $ typeOf d

No instance for (Typeable Symbol "sym") arising from a use of ‛typeOf’ -}

return ()

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