[Haskell-beginners] define action getInt like getLine

[kane96 at gmx.de]

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 <stephen.tetley at gmail.com>
> An: 
> CC: beginners at 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,  <kane96 at gmx.de> 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]"
> <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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