[Haskell-cafe] Is Show special? Of course not but...
daniel.is.fischer at googlemail.com
Fri Feb 11 20:00:11 CET 2011
On Friday 11 February 2011 19:33:27, Cristiano Paris wrote:
> 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
> quarry:Haskell paris$ ghci -O1
> GHCi, version 6.12.3: http://www.haskell.org/ghc/ :? for help
> *Main> unbrand $ right True
> *Main> unbrand $ right "Foo"
> *Main> unbrand $ wrong True
> 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...
It's because there's no way to determine the type variable a (in either
wrong or right).
In such cases, under some circumstances, the type variable gets defaulted
(the ambiguous contexts all must have the form (C a), at least one numeric
class [Num, Integral, Fractional, ...] is involved, all involved classes
come from the Prelude or the standard libraries) as specified in the report
(section 4.3, iirc).
These conditions are not met by your 'right' function, but ghci uses
extended default rules, a type variable a with a Show constraint [and no
other] gets defaulted to (). But ghci has no rule how to default your Dumb
class, so it reports the ambiguous type variable there.
[Without extended defaulting rules, ambiguous type variable errors would be
too frequent at the ghci prompt.]
If you try to compile the module, both should raise an ambiguous type
variable error, since the compiler follows the standard defaulting rules
(unless you enable -XExtendedDefaultRules).
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