[Haskell-cafe] Language semantics

[Jon Cast]

On Wednesday 27 June 2007, Andrew Coppin wrote:
> I have a tricky little question...
>
> Suppose I write a function like this:
>
>   foo pattern1
>
>     | gard1 = ...
>     | gard2 = ...
>
>   foo pattern2
>
>     | gard3 = ...
>     | gard4 = ...
>
> According to one tutorial I read, if pattern1 matches, pattern2 will
> never be tried, even if both guard1 and guard2 fail.
>
> And according to another tutorial I read, if pattern1 matches but all
> guards fail, pattern2 *will* be tried.
>
> Can somebody comfirm which one is actually correct?

You could just try it, you know . . .

Anyway, the second tutorial is correct: if all guards on pattern1 fail, 
pattern2 will be tried next.  See Section 3.17.3 in the Haskell Report [1].

Btw., which was the first tutorial?

Sincerely,
Jonathan Cast
http://sourceforge.net/projects/fid-core
http://sourceforge.net/projects/fid-emacs

[1] http://haskell.org/onlinereport/exps.html#case-semantics