Fwd: Re: [Haskell-cafe] [Newbie question] -- Looping stdin until condition is met

[Martin Blais]

FYI.

On Sat, 31 May 2008 02:11:13 +0200, "Daniel Fischer"
<daniel.is.fischer at web.de> said:
> Am Samstag, 31. Mai 2008 02:28 schrieb Martin Blais:
> > Allright, this is a definitely a newbie question.
> >
> > I'm learning Haskell and going through the exercises in the
> > beautiful Hutton book, and one of them requires for me to
> > write a loop that queries a line from the user (stdin),
> > looping until the user enters a valid integer (at least
> > that's how I want to implement the interface to the
> > exercise). I have tried tons of variants and modifications
> > of code, and I can't find the way to implement this. Here is
> > what my emacs buffer is at now::
> >
> >   import Text.Read
> >   import System.IO
> >   import qualified Control.Exception as C
> >
> >   getNum :: IO Int
> >   getNum = do line <- (hGetLine stdin)
> >               x <- (C.catch (do return (read line :: Int)) (\e -> getNum))
> >               return x
> 
> All you need is a little strictness,
> 	x <- (C.catch (return $! read line :: Int) (\e -> getNum))
> works. Another option is using evaluate instead of return.
> The problem is that (read line :: Int) is not evaluated until it is
> needed, 
> that is when it's going to be printed, but then it's too late to catch
> the 
> exception.
> 
> Some general remarks:
> hGetLine stdin === getLine
> do x <- action
>    return x
> is the same as 
> action
> 
> >
> >   main = do x <- getNum
> >             putStr ((show x) ++ "\n")
> 
> 	print x
> >
> > Now, I've tried the Prelude's catch, the Control.Exception
> > catch, I've tried moving it at the top of getnum, I've tried
> > without catch, I've tried a ton of other shtuff, so much
> > that I'm starting to think that Emacs is going to run out of
> > electrons soon. I asked some half-newbie friends who are
> > insanely enthousiastic about Haskell and they can't do it
> > either (I'm starting to think that those enthousiastic
> > friends are dating a beautiful girl with 3 PhDs, but she has
> > a 2-inch thick green and gooey wart smack on her nose and
> > they're so blindly in love that they can't admit that she
> > does). I've asked some university profs and they sidetrack
> > my question by saying I shouldn't do I/O so early. Can
> > anyone here restore my faith in the Haskell section of
> > humanity?
> >
> > 1. How do I catch the exception that is raised from "read"?
> 
> By forcing the evaluation.
> >
> > 2. Where do I find the appropriate information I need in
> >    order to fix this? I'm probably just not searching in the
> >    right place. (Yes, I've seen the GHC docs, and it doesn't
> >    help, maybe I'm missing some background info.)
> 
> Get used to lazy evaluation, the Hutton book should contain a chapter
> about 
> that.
> 
> >
> > 3. Please do not tell me I should solve the problem
> >    differently. Here is the problem I'm trying to solve, and
> >    nothing else:
> >
> >      "Write a program that reads a line from the user,
> >      looping the query until the line contains a valid
> >      integer."
> >
> > It shouldn't be too hard i think. The best answer would be a
> > two-liner code example that'll make me feel even more stupid
> > than I already do.
> >
> > Thanks in advance.
> >
>