Re: [Haskell-beginners] Further constraining types

7 Aug 2011

      On Sun, 07 Aug 2011 08:08:31 +0200, Antoine Latter   
wrote:
...
You can use non-standard GHC extensions:
Given a function:
someView :: Natural a -> a
you can pattern match like so:
case myNat of
  (someView -> 5) -> "It's a five!!!"
This requires {-# LANGUAGE ViewPatterns #-} at the top of your source
file to enable.
Not using any extension:
   case fromNatural myNat of
     5 -> "It's a five!!!"

The function fromNatural is, of course, imported from the module defining  
Natural.

Regards,
Henk-Jan van Tuyl

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