On 2010 Oct 16, at 01:39, Daniel Fischer wrote:
On Saturday 16 October 2010 01:18:55, Jacek Generowicz wrote:
On 2010 Oct 16, at 01:14, Jacek Generowicz wrote:
On 2010 Oct 16, at 00:51, Ivan Lazar Miljenovic wrote:
"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).
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?
An in both of the explanations above, it should then complain about
the lack of an instance of View, rather than about ambiguous type
variables, n'est-ce pas?
Non.
If you write
x = view 2
the type checker looks at the expression 2 and it sees it has the type
Num a => a
(because integer literals are actually shorthand for (fromInteger
literal)
and fromInteger :: Num a => Integer -> a).
Then (or before that) it looks at the context in which the 2 appears.
That context is that the function view is applied to it.
view :: View v => v -> String
So, in order for the expression 2 to be well typed, the type checker
finds
the two constraints
2 :: Num a => a
2 :: View v => v
These constraints have to be unified (which is very easy here),
resulting
in
2 :: (Num a, View a) => a
But there's no way to find out what type a is/has to be.
Hence a is an ambiguous type variable.
Exactly. Which is why I made the point that the two explanations
offered by Christopher and Ivan (both of which suggested that the
problem was related to a missing View instance) imply that the error
report should mention missing View instances: As the error message
does not mention them, I took this as further evidence that those
suggestions were not correct.
Now, under certain circumstances such ambiguous type variables are
resolved
by defaulting. If you replace view with show, you get a Show
constraint
instead of the View constraint and then defaulting may (and must)
happen.
But since View is defined outside the standard libraries, the language
report says defaulting mustn't take place, so it doesn't
Yup. That much was clear from you last message.
(it may work if
you specify
{-# LANGUAGE ExtendedDefaultRules #-}
at the top of your module).
Interesting. Thanks.
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