Q: Resolving ambiguous type variable

[Hal Daume III]

In short, you cannot.

What if your main were:

main = getArgs >>= print . first_h

The compiler doesn't know the difference and so it needs a type.

Simple fix:

> main = print (first_h ([] :: [Char]))


--
Hal Daume III

 "Computer science is no more about computers    | hdaume@isi.edu
  than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume

On Fri, 17 May 2002 tty@SAFe-mail.net wrote:

> 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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