GHC needs to be more careful about pattern match order

The language report specifies pattern match order to be sequential, left-to-right, out-in, top-down. It explicitly allows for different implementations, as long as the differences are not observable and certain rules remain valid. One such rule is:

    case e of {p1->e1;p2->e2} = 
    case e of {p1->e1;_->case e of {p2->e2;_->error "No match"}}

GHC invalidates this rule:

    newtype N = N Int deriving (Show,Eq)
    
    instance Num N where
      fromInteger 0 = error "0"
      fromInteger 1 = N 0
      fromInteger _ = N 1
    
    f x = case x of
            1 -> False
            0 -> True
    
    g x = case x of
            1 -> False
            _ -> case x of
                  0 -> True
                  _ -> error "No match"
    
    main = do
      print $ g (N 0)
      print $ f (N 0)

    -- ghc
    $ ghc -w -e main PMOrderSpec.hs
    False
    <interactive>: 0

    -- hugs
    Main> main
    False
    False

The same effect can be achieved via Data.String.IsString (see attached test case).

See also:

• compilation of pattern matching?
• Wrongpat-matchorderforrecords

Trac metadata

Trac field	Value
Version	6.11
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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