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

-- NEU: Mit GMX DSL über 1000,- ¿ sparen! http://portal.gmx.net/de/go/dsl02

I want to read a "Month" from input and if this month issn't declared in my data type "Month" I want to throw an error message. data Month = Jan | Feb | Mar | Apr | May | Jun | Jul | Ago | Sep | Oct | Nov | Dec deriving (Eq,Enum,Show) -------- Original-Nachricht --------

Datum: Mon, 08 Feb 2010 09:25:13 +1300 Von: "Stephen Blackheath [to Haskell-Beginners]" An: kane96@gmx.de CC: beginners@haskell.org Betreff: Re: [Haskell-beginners] define action getInt like getLine

Hi there,

Data types are required to start with a capital letter, so you'll have to call it MyDatatype. I can't see where readLn is defined. Can you paste a bit more of the code? I don't quite understand how your reading of your own data type is meant to work - normally you would use read or reads from the Read type class.

A more subtle point - because of the way lazy evaluation works, it is generally better to use 'fail' rather than 'error' when in a monad. In some monads it's possible that 'error' may do nothing.

Steve

kane96@gmx.de wrote:

...

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

-- GRATIS für alle GMX-Mitglieder: Die maxdome Movie-FLAT! Jetzt freischalten unter http://portal.gmx.net/de/go/maxdome01

but I have to write the action readMonth :: IO Month on my own and still don't have an idea how to do it -------- Original-Nachricht --------

Datum: Mon, 8 Feb 2010 21:27:42 +0000 Von: Stephen Tetley An: CC: beginners@haskell.org Betreff: Re: [Haskell-beginners] define action getInt like getLine

Hello

You can derive Read:

...

data Month = Jan | Feb | Mar | Apr | May | Jun | Jul | Ago | Sep | Oct | Nov | Dec deriving (Eq,Enum,Show,Read)

This will certainly throw an error on failure (might not be an error you would want to transmit to a user though), and it will only read "Jan" not "jan" or "January"...

Plenty of good suggestions in this thread:

http://www.haskell.org/pipermail/haskell-cafe/2010-January/072177.html

Best wishes

Stephen

On 8 February 2010 21:19, wrote:

...

I want to read a "Month" from input and if this month issn't declared in my data type "Month" I want to throw an error message.

data Month = Jan | Feb | Mar | Apr | May | Jun | Jul | Ago | Sep | Oct | Nov | Dec deriving (Eq,Enum,Show)

-------- Original-Nachricht --------

...

Datum: Mon, 08 Feb 2010 09:25:13 +1300 Von: "Stephen Blackheath [to Haskell-Beginners]" An: kane96@gmx.de CC: beginners@haskell.org Betreff: Re: [Haskell-beginners] define action getInt like getLine

...

Hi there,

Data types are required to start with a capital letter, so you'll have to call it MyDatatype.  I can't see where readLn is defined.  Can you paste a bit more of the code?  I don't quite understand how your reading of your own data type is meant to work - normally you would use read or reads from the Read type class.

A more subtle point - because of the way lazy evaluation works, it is generally better to use 'fail' rather than 'error' when in a monad.  In some monads it's possible that 'error' may do nothing.

Steve

kane96@gmx.de wrote:

...

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

-- GRATIS für alle GMX-Mitglieder: Die maxdome Movie-FLAT! Jetzt freischalten unter http://portal.gmx.net/de/go/maxdome01 _______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners

_______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners

-- GRATIS für alle GMX-Mitglieder: Die maxdome Movie-FLAT! Jetzt freischalten unter http://portal.gmx.net/de/go/maxdome01

kane96@gmx.de schrieb:

but I have to write the action readMonth :: IO Month on my own and still don't have an idea how to do it

Your idea (below) to use show on all possible values and compare it to the input line isn't that bad.

...

On 8 February 2010 21:19, wrote:

...

I want to read a "Month" from input and if this month issn't declared in my data type "Month" I want to throw an error message. data Month = Jan | Feb | Mar | Apr | May | Jun | Jul | Ago | Sep | Oct | Nov | Dec deriving (Eq,Enum,Show)

The Enum class allows you to create the list of all possible values [Jan .. Dec] This list can be turn into a lookup-list (for Data.List.lookup) by map (\ a -> (show a, a))

...

...

...

...

readMyDatatype = do if readLn == (show readLn) then return readLn else do error "input error"

This code is unfortunate, because "readLn" is an IO-Action (that cannot be compared or shown). Even if you insert as first line: readLn <- getLine readLn is a String that will never be equal to "show readLn", because show would add the double quotes. Also this shadowing of "readLn" is no good practise and should be avoided, so better use: str <- getLine and use the string "str" to look it up in the list above. Later on you may generalize readMonth to: readMyDatatype :: (Show a, Enum a, Bounded a) => IO a and also trim leading and trailing white space and case differences. Cheers Christian P.S. The functions read, readIO and readLn all have disadvantages in case of errors. In the spirit of readIO it is possible to program readMaybe, readEither or readM that are far more useful and missed very often. Many such variants are somewhere, but one of readMaybe, readEither or readM should be in a standard library. readM would not be a generalization of readIO! But readIO could be expressed using readM.

Hi, here's another idea : saisie_choix :: IO Int saisie_choix=do putStrLn "Type in 1 digit :" xchoix <- getLine if (length xchoix) /= 1 || head xchoix < '0' || head xchoix > '9' then saisie_choix else do let nchoix=read xchoix::Int return nchoix or, with maybe : {- -------------- MAYBE ------------------ -} -- Fonction principale mainmaybe=do x <- lire if isNothing x then do putStrLn "Donnee invalide" else do putStr " \b" putStrLn ("Resultat : " ++ show (f x)) -- Définition de la fonction de calcul f Nothing = -9999 f (Just x) = 2 * x -- Fonction d'IO qui valide la saisie lire :: IO (Maybe Integer) lire = do -- saisie xn <- getLine let x=read xn ::Integer -- validation if x < 5 then do return (Just x) else do return Nothing Didier. Rahul Kapoor a écrit :

...

Hi, how can I write an action getInt :: IO Int that works like getLine :: IO String

The most literal way to write getInt would be:

getInt :: IO Int getInt = do s <- getLine return (read s)

which is just a more verbose version of:

getInt' = read `fmap` getLine

The above versions don't do any error checking, so you might prefer a getInt :: IO Maybe Int which returns Nothing in case the input is not an integer.

You should read up on Monadic IO in Haskell. The Real World Haskell book is probably a good starting point.

Rahul _______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners