Re: [Haskell-beginners] define action getInt like getLine

thanks so far. I used "getInt = fmap read getLine", because I think it's enough for me I have to do it for a more complex case of reading an own data type (myDatatype) with is deriving Show and return an error otherwise. I tried the following which didn't work: readMyDatatype :: IO myDatatype readMyDatatype = do if readLn == (show readLn) then return readLn else do error "input error" -------- Original-Nachricht --------

Datum: Tue, 2 Feb 2010 22:39:07 +0100 Von: Daniel Fischer An: beginners@haskell.org CC: kane96@gmx.de Betreff: Re: [Haskell-beginners] define action getInt like getLine

Am Dienstag 02 Februar 2010 22:20:03 schrieb kane96@gmx.de:

...

Hi, how can I write an action getInt :: IO Int that works like getLine :: IO String but for Int instead of String. I know that I can read the input with getLine and then convert it by using read, but don't know how to write it as an action. I tried getInt :: IO Int getInt read <- getLine but that doesn't work.

There are many possibilities.

The shortest is

getInt :: IO Int getInt = readLn

another short and sweet is

getInt :: IO Int getInt = fmap read getLine -- or liftM read getLine

But neither of these deals well with malformed input, if that's a possibility to reckon with, use e.g. the reads function

getInt :: IO Int getInt = do inp <- getLine case reads inp of ((a,tl):_) | all isSpace tl -> return a _ -> handle malformed input

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