[Haskell] specified or not

Malcolm Wallace Malcolm.Wallace at cs.york.ac.uk
Thu Mar 3 09:45:14 EST 2005

Scott Turner <p.turner at computer.org> writes:

> Is the behavior of evaluating z unspecified?
>   z = f (0, z)
>   f x = case x of
>  (1,1) -> z
>  _ -> 0
> Hugs and GHC agree that z evaluates to 0. However, if the first line is 
> changed to
>   z = f (z,0)
> then both implementations loop. In other words, the behavior depends on
> order  of evaluation, which AFAIK is not specified.

Patterns are matched left-to-right, which fully explains the behaviour
you see.

   Haskell Report section 3.17.2,  Informal Semantics of Pattern Matching


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