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n+k patterns

posted Apr 1, 2009, 8:56 AM by Wei Hu
I never thought n+k patterns was any special. For example, aren't the two definitions equivalent?

fac1  0    =  1
fac1 (n+1) = (n+1) * fac1 n

fac2 0 = 1
fac2 n = n * fac2 (n-1)

In fact, they aren't. GHCi infers fac1's type as (Integral t) => t -> t, and fac2's as (Num t) => t -> t. Why's that? The answer is in the Haskell report.

Matching an n+k pattern (where n is a variable and k is a positive integer literal) against a value v succeeds if x >= k, resulting in the binding of n to x - k, and fails otherwise.
An n+k pattern can only be matched against a value in the class Integral.

And the formal semantics says
case v of { x+k -> e; _ -> e' }
= if v >= k then (\x -> e) (v-k) else e'
where k is a numeric literal

That's why fac3 terminates while fac4 diverges:

fac3 (n+1) = (n+1) * fac3 n      
fac3  0    =  1

fac4 n = n * fac4 (n-1)
fac4 0 = 1

And that's also why ghci warns about fac1's and fac3's pattern matches are non-exhaustive.

As the Haskell report points out, many people feel that n+k patterns should not be used. These patterns may be removed or changed in future versions of Haskell .
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