I'm working on a desugaring library for Haskell. Desugaring NPlusKPatterns currently takes up the most code, since a proper translation involves generating two guards. Meanwhile, most other pattern-related language extensions are defined in terms of ViewPatterns, which are trivial to desugar.

In my laziness to fix this otherwise, I propose that the following function is added to the Prelude:

> maySubtractIntegral :: Integral a => a -> a -> Maybe a

> maySubtractIntegral k nplusk = if n >= 0 then Just n else Nothing

>   where

>     n = nplusk - k

So that `NPlusKPatterns` like

< func (n+42) = expr

may be desugared to

< func (maySubtractIntegral 42 -> Just n) = expr

rather than

< func nplusk | n <- nplusk - 42, n >= 0 = expr

The name of this function is, of course, subject to bikeshedding. Note that the signature is correct; though (Ord a, Num a) is more general, it's too weak for the semantics of the language extension.

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