[Haskell-cafe] Language semantics

Stefan O'Rear stefanor at cox.net
Wed Jun 27 16:54:18 EDT 2007

On Wed, Jun 27, 2007 at 09:46:41PM +0100, 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?

According to http://haskell.org/onlinereport/exps.html#sect3.17.2

Top level patterns in case expressions and the set of top level patterns
in function or pattern bindings may have zero or more associated guards.
A guard is a boolean expression that is evaluated only after   all of
the arguments have been successfully matched, and it must be true for
the overall pattern match to succeed. The environment of the guard is
the same as the right-hand-side of the case-expression
alternative, function definition, or pattern binding to which it is

So, if guard1 and guard2 both fail, then pattern1 doesn't match (and
pattern matching continues).  As such, your "corner case" cannot
actually exist.


More information about the Haskell-Cafe mailing list