On Fri, May 30, 2008 at 5:28 PM, Martin Blais wrote:

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

main = do x <- getNum putStr ((show x) ++ "\n")

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"?

I think you want readIO, which yields a computation in the IO monad, so it can be caught.

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.)

In this particular case, I am not sure where you'd find this information. It's not very intuitive to a beginner why "read" doesn't work in this case.

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.

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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).

1. How do I catch the exception that is raised from "read"?

The best thing to do is bypass read and use 'reads' to define your own safe read. maybeRead :: Read a => String -> Maybe a maybeRead s = case reads s of [(x, "")] -> Just x _ -> Nothing For example, yielding: import System.IO main = do x <- getNum print x getNum :: IO Integer getNum = do n <- getLine case maybeRead n of Nothing -> getNum Just n -> return n

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.)

I think Control.Exception.catch should be fine here.

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.

Of course, it's easy. You can have fun now abstracting out the loop form in getNum using say, MaybeT or friends. But a loop is simple and easy. If you want to write it with explict exception handling of the read parse failure, it's more tedious, as you need to ensure the read exception is thrown within the body of the enclosing catch. For example, import System.IO import qualified Control.Exception as C main = do x <- getNum print x getNum :: IO Integer getNum = do y <- maybeRead case y of Nothing -> getNum Just n -> return n maybeRead :: Read a => IO (Maybe a) maybeRead = C.catch (do x <- getLine let n = read x n `seq` return (Just n)) -- ensure any exception is thrown here (const (return Nothing)) -- Don