On Wed, Jun 27, 2007 at 10:28:05PM +0100, Andrew Coppin wrote: > 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 > >attached. > > > >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??
[EMAIL PROTECTED]:~$ ghci Loading package base ... linking ... done. ___ ___ _ / _ \ /\ /\/ __(_) / /_\// /_/ / / | | GHC Interactive, version 6.7.20070612, for Haskell 98. / /_\\/ __ / /___| | http://www.haskell.org/ghc/ \____/\/ /_/\____/|_| Type :? for help. Prelude> case () of { () | True -> "x" } "x" Prelude> Stefan _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
