Can anyone help with the following problem. I want to define a class
called for example 'AType' with a member function called 'typename'
which will return a string indicating the type of the value applied to
the function. A default definition of this member function returns the
string "Default" for types declared as members of the class but which do
not define a string to return. For example, this class listed below
allows me to do the following (as expected).
> typename 'a'
"Char"
> typename "Hello"
"[Char]"
> typename ('a',"Hello")
"(Char,[Char])"
> typename True
"Default"
My problem though is with members of the 'Num' class, as an overloading
ambiguity problem arises with the following expression.
> typename 1
"Error ...... etc."
I can circumvent the problem by supplying an explicit type signature
> typename (1::Int)
"Int"
but this is not a satisfactory solution for the class I require. I thought
that the module 'default declaration' keyword might help but I am still
getting errors. I would (1) preferably like to be able to distinguish
between all members of the Num class, but (2) would be happy enough if
all members of the Num class were lumped together and a default type
string could be declared eg "Number".
Can anyone suggest how I might solve this problem. (Note there is no
such problem if I run my script through Gofer!).
many thanks,
Aiden McCaughey
School of Computing & Mathematics
University of Ulster
EMAIL: [EMAIL PROTECTED]
TEL: (44) 504 265621 ext 5334
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module AType where
class AType a where
typename :: a -> String
typename _ = "Default"
instance AType Int where
typename _ = "Int"
instance AType Char where
typename _ = "Char"
instance AType [a] where
typename (x:xs) = "[" ++ (typename x) ++ "]"
instance (AType a, AType b) => AType (a,b) where
typename (a,b) = "("++ typename a ++ "," ++ typename b ++ ")"
instance AType Bool
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