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

[Brandon S. Allbery KF8NH]

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).

-- 
brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery at kf8nh.com
system administrator [openafs,heimdal,too many hats] allbery at ece.cmu.edu
electrical and computer engineering, carnegie mellon university    KF8NH


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