Hello,
I am writing a function which returns an exception/error string should an
unexpected parameter is passed in. Borrowing from the Maybe module I have the
following:
------------------------------------------------------------
data Result a = Ex String | Value a deriving Show
-- Testing the Result a type
first_h :: [a] -> Result a
first_h [] = Ex "No list"
first_h (x:xs) = Value x
-- Trying out the first_h
main :: IO()
main = print (first_h [])
------------------------------------------------------------
Which the compiler complains:
Ambiguous type variable(s) `a' in the constraint `Show a'
arising from use of `print' at atype.hs:8
In the definition of `main': print (first_h [])
This is understandable since it is unable to unify the empty list with a concrete list
of type 'a' i.e. there are an infinite types which would match. My question is how can
I indicate to the compiler that it does not matter which type the empty list is since
the return result is of a concreate type.
Thanks
Tee
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