Hal Daume III
<[EMAIL PROTECTED]> To: [EMAIL PROTECTED]
cc: [EMAIL PROTECTED]
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 [EMAIL PROTECTED] 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
> <[EMAIL PROTECTED] To:
[EMAIL PROTECTED], [EMAIL PROTECTED]
> 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
> >[EMAIL PROTECTED]
> >http://www.haskell.org/mailman/listinfo/haskell
>
> ------------
> Graham Klyne
> For email:
> http://www.ninebynine.org/#Contact
>
>
>
> _______________________________________________
> Haskell mailing list
> [EMAIL PROTECTED]
> http://www.haskell.org/mailman/listinfo/haskell
>
--
Hal Daume III | [EMAIL PROTECTED]
"Arrest this man, he talks in maths." | www.isi.edu/~hdaume
_______________________________________________
Haskell mailing list
[EMAIL PROTECTED]
http://www.haskell.org/mailman/listinfo/haskell
