Hi Zsolt, fs :: (((a, a) -> a) -> t) -> (t, t) fs g = (g fst, g snd) examples = (fs fmap, fs liftA, fs liftM, fs id, fs ($(1,"2")), fs ((,)id), fs (:[]), fs repeat) No instance for (Num [Char]) arising from the literal `1' at M.hs:6:54 Possible fix: add an instance declaration for (Num [Char]) In the expression: 1 In the second argument of `($)', namely `(1, "2")' In the first argument of `fs', namely `($ (1, "2"))' Anyways, this signature is not what intended. I want it to work for all tuples, regardless of their element types. Thanks Vladimir On 6/7/09, Zsolt Dollenstein wrote:

On Sun, Jun 7, 2009 at 9:17 AM, Vladimir Reshetnikov wrote:

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Hi Zsolt,

It does not compiles with GHC without type annotations.

It does with mine: The Glorious Glasgow Haskell Compilation System, version 6.10.2

Anyway, try this: fs :: (((a, a) -> a) -> t) -> (t, t)

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Thanks, Vladimir

On 6/7/09, Zsolt Dollenstein wrote:

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Hi Vladimir,

On Sun, Jun 7, 2009 at 12:06 AM, Vladimir Reshetnikov wrote:

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Hi,

I have the following code:

-------------------------------------------------------------------------------- fs g = (g fst, g snd) examples = (fs fmap, fs liftA, fs liftM, fs id, fs ($(1,"2")), fs ((,)id), fs (:[]), fs repeat) --------------------------------------------------------------------------------

The idea is that fs accepts a polymorphic function as its argument. What type signature can I specify for f in order to compile this code?

Have you tried putting the above into ghci for example, then asking for ":t fs"? Or am I misunderstanding your point?

Cheers, Zsolt

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If it is not possible in Haskell, is there another language with static typing which allows this?

Thanks, Vladimir _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe