Re: [Haskell] return?

30 Apr 2004

      This actually does do the work, although I don't understand why an extra
indentation is not needed.
 The "do" gives me a big surprise.

                      Hal Daume III                                                                                                    
                      <hdaume@ISI.EDU>         To:       Ben_Yu@asc.aon.com                                                            
                                               cc:       haskell@haskell.org                                                           
                      04/30/2004 10:54         Subject:  Re: [Haskell] return?                                                         
                      AM                                                                                                               

You can also do something like:

myfunction = do
  ...
  if somecondition then return 5 else do
  more stuff here
  if someothercondition then return 6 else do
  more stuff here
  return 7

which will do what you want

On Fri, 30 Apr 2004 Ben_Yu@asc.aon.com wrote:
...
Thanks you guys. I really love this mail list. Can always learn nice tips
from experienced people here. :-)
I guess I'll go with the guard suggestion. ContT is nice, but don't want
to
...
introduce too much extras to the program just for syntax reason.
I do agree with you, Graham. Actually
do
  do
    earlyreturn 1
    return 2
return 3
is equivalent to:
do
  earlyreturn 1
  return 2
  return 3
While the first should be 3 by intuition. the second should be 1.
I guess that's the reason why imperative return is not possible in monad.

...
Graham Klyne

...
<GK@ninebynine.or        To:
Ben_Yu@asc.aon.com, haskell@haskell.org
...
g>                       cc:

...
Subject:  Re: [Haskell]
return?
...
04/30/2004 04:45

...
AM

...

...
...
Is this possible at all?
I don't think so, in the form that you suggest.
Ultimately, it all comes down to function applications, for which there
is
...
no such "bail out".  Rather, I think something like this is required:
do
     { ...
     ; if cond then return 1
       else do
         (the rest)
     }
Here's an example from some real (tested) code:
[[
-- Open and read file, returning its handle and content, or Nothing
-- WARNING:  the handle must not be closed until input is fully evaluated
repOpenFile :: String -> RepStateIO (Maybe (Handle,String))
repOpenFile fnam =
     do  { (hnd,hop) <- lift $
             if null fnam then
                 return (stdin,True)
             else
             do  { o <- try (openFile fnam ReadMode)
                 ; case o of
                     Left  e -> return (stdin,False)
                     Right h -> return (h,True)
                 }
         ; hrd <- lift $ hIsReadable hnd
         ; res <- if hop && hrd then
             do  {
                 ; fc <- lift $ hGetContents hnd
                 ; return $ Just (hnd,fc)
                 }
             else
             do  { lift $ hClose hnd
                 ; repError ("Cannot read file: "++fnam) 3
                 ; return Nothing
                 }
         ; return res
         }
]]
#g
--
...
Hi,
While writing monad programs, I sometimes want to do a return as it is
in
imperative program. i.e.,
do{return 1; return 2} is same as return 1
This seems useful to me when I need to do something like
do
  mwhen cond $ return 1
  ...... -- subsequent actions
I know I can do
if cond then return 1 else (
  ...--subsequent actions
  )
However, that syntax does not look very pleasant to me due to this
extra
indentation and the pair of parens.
Is this possible at all?
Ben.
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--
 Hal Daume III                                   | hdaume@isi.edu
 "Arrest this man, he talks in maths."           | www.isi.edu/~hdaume

Re: [Haskell] return?

Ben_Yu＠asc.aon.com