haskellpattern-matchingpattern-synonyms

Why can't I pattern match against a ratio in Haskell?

I'm trying to pattern match against a ratio:

isValid :: Ratio Int -> Bool
isValid (num % den) = ...

However, this yields:

Parse error in pattern: num % den

Interestingly, the Data.Ratio package defines the numerator and denominator functions in this way, but with the :% operator:

numerator   (x :% _)    =  x
denominator (_ :% y)    =  y

However, I don't have access to this latter operator.

Could anyone explain why my pattern match doesn't work and how I could fix it?

Solution

  • While in this case you can just import the operator from GHC.Ratio, there is a general solution starting from GHC 7.8: pattern synonyms.

    pattern num :% denom <- ((\x -> (numerator x, denominator x)) -> (num, denom))