CODEWITHSUNDEEP
haskellpattern-matching
Evg
Evg

Reputation: 3080

haskell pattern match numbers (deconstruction)

Can pattern matching in Haskell be used for deconstructing numbers in this way:

f (n + 1) = n

I expect predicessor for ex: f 6 = 5, f 5 = 4 etc.

I found such kind of pattern matching usage here: https://wiki.haskell.org/Continuation

 facCPS :: a -> (a -> r) -> r
 facCPS 0 k = k 1
 facCPS n'@(n + 1) k = facCPS n $ \ret -> k (n' * ret)
 facCPS 4 (+ 2) :: Integral a => a

but in my ghci it does not work:

Prelude> f (n + 1) = n :4:12: error: Parse error in pattern: n + 1

Maybe some options need to be added? can I use somehow use pattern matching in this way?

Upvotes: 5

Views: 169

Answers (1)

Marc Talbot
Marc Talbot

Reputation: 2059

Using the example from the wiki that you quoted:

{-# LANGUAGE NPlusKPatterns #-}
fac :: Integral a => a -> a
fac 0 = 1
fac n'@(n + 1) = n' * fac n

This was removed from the language in 2010, but Haskell remains a very configurable language. :)

Upvotes: 4

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