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

2010-02-27 Thread zaxis 

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` ? 

Sincerely!



-
fac n = let {  f = foldr (*) 1 [1..n] } in f 
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Re: [Haskell-cafe] How to understand `|` in this code snippet ?

2010-02-27 Thread Lee Duhem 
On Sat, Feb 27, 2010 at 5:07 PM, zaxis z_a...@163.com 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 part of a case expression, see
http://www.haskell.org/onlinereport/exps.html#sect3.13

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

2010-02-27 Thread 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] allb...@kf8nh.com
system administrator [openafs,heimdal,too many hats] allb...@ece.cmu.edu
electrical and computer engineering, carnegie mellon universityKF8NH




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

2010-02-27 Thread Ben Millwood 
On Sat, Feb 27, 2010 at 9:29 AM, Brandon S. Allbery KF8NH
allb...@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:
http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html#pattern-guards
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Re: [Haskell-cafe] How to understand `|` in this code snippet ?

2010-02-27 Thread zaxis 

Then can i change it to :
case timeout of
Just str - do
[(t, _)] - reads str
addTimeout t (hPutStrLn stderr *** TIMEOUT  _exit 1)
return ()
_ - return () 

Sincerely!


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).
 
 -- 
 brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allb...@kf8nh.com
 system administrator [openafs,heimdal,too many hats] allb...@ece.cmu.edu
 electrical and computer engineering, carnegie mellon universityKF8NH
 
 
 
  
 ___
 Haskell-Cafe mailing list
 Haskell-Cafe@haskell.org
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Re: [Haskell-cafe] How to understand `|` in this code snippet ?

2010-02-27 Thread Daniel Fischer 
Am Sonntag 28 Februar 2010 02:08:18 schrieb zaxis:
 Then can i change it to :
 case timeout of
 Just str - do
 [(t, _)] - reads str
 addTimeout t (hPutStrLn stderr *** TIMEOUT  _exit 1)
 return ()
 _ - return ()

 Sincerely!

No. The | [(t,_)] - reads str in

case timeout of
  Just str | [(t,_)] - reads str - ...

is a pattern guard, not a monadic bind (and where p - reads str  is a 
monadic bind, it's in the list monad).
You can change it to

case timeout of
  Just str -
case reads str of
  [(t,_)] - addtimeout (hPutStrLn stderr *** TIMEOUT  _exit 1)
  _ - return ()
  _ - return ()

but why would you?


 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).
 
  --
  brandon s. allbery [solaris,freebsd,perl,pugs,haskell]
  allb...@kf8nh.com system administrator [openafs,heimdal,too many hats]
  allb...@ece.cmu.edu electrical and computer engineering, carnegie
  mellon universityKF8NH

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

2010-02-27 Thread Felipe Lessa 
On Sat, Feb 27, 2010 at 05:08:18PM -0800, zaxis wrote:
 Then can i change it to :
 case timeout of
 Just str - do
 [(t, _)] - reads str
 addTimeout t (hPutStrLn stderr *** TIMEOUT  _exit 1)
 return ()
 _ - return ()

No, that's different.  You could change it to:

case timeout of
  Just str - case reads str of
[(t, _)] - do addTimeout t (hPutStrLn stderr *** TIMEOUT  _exit 1)
   return ()
_- other -- (1)
  _- other -- (2)
where other = return ()

The cases (1) and (2) are the same and simulate the fact that
when the pattern guard fails, then execution falls to the next
case.

Of course you could just write

case fmap (reads str) timeout of
  Just [(t, _)] - ...
  _ - ...

HTH,

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

2010-02-27 Thread zaxis 

thanks! 

case timeout of
  Just str -
case reads str of
  [(t,_)] - addtimeout (hPutStrLn stderr *** TIMEOUT  _exit 1)
  _ - return ()
  _ - return () 

is VERY clear!


Daniel Fischer-4 wrote:
 
 Am Sonntag 28 Februar 2010 02:08:18 schrieb zaxis:
 Then can i change it to :
 case timeout of
 Just str - do
 [(t, _)] - reads str
 addTimeout t (hPutStrLn stderr *** TIMEOUT  _exit 1)
 return ()
 _ - return ()

 Sincerely!
 
 No. The | [(t,_)] - reads str in
 
 case timeout of
   Just str | [(t,_)] - reads str - ...
 
 is a pattern guard, not a monadic bind (and where p - reads str  is a 
 monadic bind, it's in the list monad).
 You can change it to
 
 case timeout of
   Just str -
 case reads str of
   [(t,_)] - addtimeout (hPutStrLn stderr *** TIMEOUT  _exit 1)
   _ - return ()
   _ - return ()
 
 but why would you?
 

 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).
 
  --
  brandon s. allbery [solaris,freebsd,perl,pugs,haskell]
  allb...@kf8nh.com system administrator [openafs,heimdal,too many hats]
  allb...@ece.cmu.edu electrical and computer engineering, carnegie
  mellon universityKF8NH
 
 ___
 Haskell-Cafe mailing list
 Haskell-Cafe@haskell.org
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