Skip to content

GitLab

GHC
GHC
Open
Opened by Guillaume Bouchard@guibouReporter

Pattern exhaustivity check for simple arithmetic

Motivation

The following:

foo :: Int -> Bool
foo i
  | i < 0 = True
  | i == 0 = True
  | i > 0 = True

results on the following warning:

 Pattern match(es) are non-exhaustive
    In an equation for ‘foo’: Patterns not matched: _
  |
2 | foo i
  | ^^^^^...

When the guard is indeed exhaustive. One solution is to replace the latest case by otherwise, but more often than never it leads to subtle bug when a case was indeed missing.

Proposal

I'd like the exhaustivity checker to not complain in this case.

However I realize that it is not a trivial task. I know that it is exhaustive because I know the mathematical rules of comparison for integer, but GHC has no reason to think that it is exhaustive. Actually, someone could implement really weird Eq and Ord instances which makes this guard not exhaustive.

May it be possible to extend the COMPLETE pragma for some instances and using variables? For example (imaginary syntax):

{-# COMPLETE forall x :: Int. (<x) (>x) (==x) #-} 
-- For symmetry
{-# COMPLETE forall x :: Int. (x<) (x>) (x==) #-}
Edited by Guillaume Bouchard