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jean-christophe mincke

26 May 2011 26 May '11

6:01 a.m.

Hello, Could anyone help me understand what is wrong with the definition of f2 in the code below? class C a b where convert :: a -> b convertToInt :: (C a Int) => a -> Int convertToInt x = convert x f1 x = convertToInt x f2 = \x -> convertToInt x f3 :: (C a Int) => a -> Int f3 = \x -> convertToInt x Thank you Regards J-C

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Christopher Done

26 May 26 May

6:10 a.m.

The problem is the monomorphism restriction: http://www.haskell.org/haskellwiki/Monomorphism_restriction http://www.haskell.org/ghc/docs/7.0.2/html/users_guide/monomorphism.html http://stackoverflow.com/questions/tagged/monomorphism-restriction http://www.cs.auckland.ac.nz/references/haskell/haskell-intro-html/pitfalls.... This kicks everyone in the butt at least once. It would be good if GHC could point it out, as mine (6.12.3) just complains about no instance. Maybe GHC7 does point it out. It's a waste of people's time otherwise.

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Brandon Allbery

6:14 a.m.

On Thu, May 26, 2011 at 06:10, Christopher Done wrote:

This kicks everyone in the butt at least once. It would be good if GHC could point it out, as mine (6.12.3) just complains about no instance. Maybe GHC7 does point it out. It's a waste of people's time otherwise.

I think the Haskell standards committee is moving in the direction of eliminating it as being the wrong solution to the problem.

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jean-christophe mincke

27 May 27 May

5:20 a.m.

OK thanks everybody ! On Thu, May 26, 2011 at 12:14 PM, Brandon Allbery wrote:

On Thu, May 26, 2011 at 06:10, Christopher Done wrote:

...
This kicks everyone in the butt at least once. It would be good if GHC could point it out, as mine (6.12.3) just complains about no instance. Maybe GHC7 does point it out. It's a waste of people's time otherwise.

I think the Haskell standards committee is moving in the direction of eliminating it as being the wrong solution to the problem.

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Brandon Allbery

26 May 26 May

6:12 a.m.

On Thu, May 26, 2011 at 06:01, jean-christophe mincke wrote:

f1 x = convertToInt x f2 = \x -> convertToInt x

Absent useful things like error messages, I'll assume that you tripped over the monomorphism restriction. That is, when you have a name that doesn't take a direct parameter, there are additional constraints on that name's type. (The inner lambda is not considered for this purpose, so while in theory f1 and f2 should be identical, in practice they are not unless you use {-# LANGUAGE NoMonomorphismRestriction #-}.

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Brandon Allbery
Christopher Done
jean-christophe mincke