[Haskell-cafe] Ambiguous type variable

[Ivan Lazar Miljenovic]

On 16 October 2010 09:47, Jacek Generowicz <jacek.generowicz at cern.ch> wrote:
> -- Given a definition of view which is essentially a synonym for show:
>
> class View a where
>    view :: a -> String
>
> instance View Int where
>    view = show
>
> -- why does "show 2" compile, while "view 2" gives an
> -- 'Ambiguous type variable' error
>
> fine                  = view (2::Int)
> noProblem             = show 2
> ambiguousTypeVariable = view 2

"2" is a generic number.  If you don't specify a type, it usually
defaults to Integer.  All Num instances that come in the Prelude have
Show instances, so no matter which gets picked "show 2" works.
However, when you say "view 2" ghc/ghci doesn't know that you want 2
to be an Int (as that's the only type you have an instance for View
for).

-- 
Ivan Lazar Miljenovic
Ivan.Miljenovic at gmail.com
IvanMiljenovic.wordpress.com