Re: [Haskell-cafe] Ambiguous type variable

On Saturday 16 October 2010 01:14:51, Jacek Generowicz wrote:

On 2010 Oct 16, at 00:51, Ivan Lazar Miljenovic wrote:

...

On 16 October 2010 09:47, Jacek Generowicz

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).

Which implies that defining all instances of Num to be instances of View should do the trick, and that doesn't seem to work. See below.

http://www.haskell.org/onlinereport/haskell2010/haskellch4.html#x10-790004.3... Defaulting only takes place when all involved classes are defined in the Prelude or the standard libraries. Your View class isn't, hence there's no defaulting. It works in ghci because ghci uses extended default rules (otherwise it would have to give too many `ambiguous type variable' messages).

On 2010 Oct 16, at 00:51, Christopher Done wrote:

...

Don't integral literals default to Integer, of which there is a Show instance but no View instance?

Hmm, it doesn't seem to be that simple.

The phenomenology seems to be:

As far as entering "view 2" into ghci is concerned, you need 'instance View Integer' or 'instance View Double'.

To get "x = view 2" to compile in ghc, having all of Int, Integer, Float and Double as instances of View is still not enough.

I did all this in an environment where I had not imported any other Num instances, and ":i Num" in ghci showed only the 4 aforementioned types as instances.

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