[Haskell-beginners] define action getInt like getLine

[kane96 at gmx.de]

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]" <mutilating.cauliflowers.stephen at blacksapphire.com>
> An: kane96 at gmx.de
> CC: beginners at 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 at 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 <daniel.is.fischer at web.de>
> >> An: beginners at haskell.org
> >> CC: kane96 at gmx.de
> >> Betreff: Re: [Haskell-beginners] define action getInt like getLine
> > 
> >> Am Dienstag 02 Februar 2010 22:20:03 schrieb kane96 at 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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