[Haskell-cafe] Language semantics
andrewcoppin at btinternet.com
Wed Jun 27 17:28:05 EDT 2007
Stefan O'Rear wrote:
> 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.
Wow, wait a sec - case expressions are allowed to have guards too??
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