[Haskell-cafe] Is Show special? Of course not but...

[Cristiano Paris]

Hi cafè,

given the following toy code:

---------------
module Main where

class Dumb p where
	dumb :: p -> String

newtype Branded a b = Branded b

unbrand :: Branded a b -> b
unbrand (Branded x) = x

wrong :: Dumb a => b -> Branded a b
wrong = Branded

right :: Show a => b -> Branded a b
right = Branded
---------------

Why:

---------------
quarry:Haskell paris$ ghci -O1
GHCi, version 6.12.3: http://www.haskell.org/ghc/  :? for help
*Main> unbrand $ right True
True
*Main> unbrand $ right "Foo"
"Foo"
---------------

but:

---------------
*Main> unbrand $ wrong True

<interactive>:1:10:
    Ambiguous type variable `a' in the constraint:
      `Dumb a' arising from a use of `wrong' at <interactive>:1:10-19
    Probable fix: add a type signature that fixes these type variable(s)
---------------

?

Maybe it's a dumb question but... thank you for any explanation...

-- 
Cristiano

GPG Key: 4096R/C17E53C6 2010-02-22
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