Re: [Haskell-cafe] How to understand `|` in this code snippet ?

On Sat, Feb 27, 2010 at 9:29 AM, Brandon S. Allbery KF8NH wrote:

On Feb 27, 2010, at 04:07 , zaxis wrote:

...

xxxMain = do   timeout <- getEnv "xxx_TIMEOUT"   case timeout of       Just str | [(t, _)] <- reads str -> do           addTimeout t (hPutStrLn stderr "*** TIMEOUT" >> _exit 1)           return ()       _ -> return () .......

What does the `|` mean in "Just str | [(t, _)] <- reads str" ? Is it a logical `or` ?

It's a guard.  Same as with function definitions (in fact, function definitions of that form are converted to case expressions).

In fact it seems to be a pattern guard, which (until recently) are (were) a non-standard extension: http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html#pa...