[Haskell] return?

[Hal Daume III]

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 at 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 at ninebynine.or        To:       Ben_Yu at asc.aon.com, haskell at 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.
> >
> >_______________________________________________
> >Haskell mailing list
> >Haskell at haskell.org
> >http://www.haskell.org/mailman/listinfo/haskell
> 
> ------------
> Graham Klyne
> For email:
> http://www.ninebynine.org/#Contact
> 
> 
> 
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> 

-- 
 Hal Daume III                                   | hdaume at isi.edu
 "Arrest this man, he talks in maths."           | www.isi.edu/~hdaume