>>>>> "Alex" == S Alexander Jacobson <[EMAIL PROTECTED]> writes:
Alex> I wrote the following function that attempts to generalize show by
Alex> allowing the user to choose the function to stringify constructor
Alex> arguments.
>> stringArgs' sep stringer1 (MyFoo x i s) =
>> x' ++sep++i' ++ sep ++ s'
>> where
>> x'=stringer' x
>> i'=stringer'' i
>> s'=stringer''' s
>> stringer' o=stringer o
>> stringer'' o=stringer o
>> stringer''' o =stringer o
>> stringer o=stringer1 o --!! replacing stringer1 w/ show works!
Alex> This function generates a type error because stringer is being used as
Int-> Char and as Char->Char. But, if I replace stringer1 in the last
Alex> line, with the function `show`, hugs allows this function.
OK, show :: forall a . Show a => a -> String [if my memory serves correctly]
so it is polymorphic in the type a.
However, the parameters of a function cannot (currently) have a
polymorphic type. So when you want to pass show to stringArgs you will
have to use one particular instance of this type. In the present case,
you get Show a => a -> String [note that the forall is gone], but you
cannot resolve the overloading because you do not know what a is. Hence
Alex> However, if I use show and attempt to call this function using foo:
>> foo x = stringArgs "\n" show (MyFoo "asd" 12 "asd")
Alex> I get an ambiguous type signature error.
the ambiguity. If Haskell included first-class polymorphism (which is
on the list for Haskell 2 and which is present in some form in Hugs
1.3c) and you provided an explicit signature, then your function and
your example would work out. Modulo syntax, this is what is would look
like:
stringArgs' :: String -> (forall a . Show a => a -> String) -> MyFooType -> String
stringArgs' sep stringer (MyFoo x i s) =
x' ++sep++i' ++ sep ++ s'
where
x'=stringer x
i'=stringer i
s'=stringer s
And your function call would look just like the one above.
-Peter
