On 2010 Oct 16, at 00:51, Ivan Lazar Miljenovic wrote:
On 16 October 2010 09:47, Jacek Generowicz
<jacek.generow...@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).
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.
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.
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