[Haskell-cafe] How to understand `|` in this code snippet ?
haskell at benmachine.co.uk
Sat Feb 27 11:51:09 EST 2010
On Sat, Feb 27, 2010 at 9:29 AM, Brandon S. Allbery KF8NH
<allbery at ece.cmu.edu> 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:
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