| |
[Haskell-beginners] define action getInt like getLine  | |
02-02-10, 09:21 PM
| | |  [Haskell-beginners] define action getInt like getLine
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.
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02-02-10, 09:39 PM
| | | Re: [Haskell-beginners] define action getInt like getLine
> 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
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02-02-10, 09:41 PM
| | | Re: [Haskell-beginners] define action getInt like getLine
Am Dienstag 02 Februar 2010 22:20:03 schrieb Code: Content visible to registered users only.
:
> 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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 |
02-02-10, 10:01 PM
| | | Re: [Haskell-beginners] define action getInt like getLine
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
> Code: Content visible to registered users only.
> Code: Content visible to registered users only.
>
>
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|
02-02-10, 10:18 PM
| | | Re: [Haskell-beginners] define action getInt like getLine
Am Dienstag 02 Februar 2010 23:06:41 schrieb legajid:
> -- Définition de la fonction de calcul
> f Nothing = -9999
> f (Just x) = 2 * x
That's already in the standard libs:
ghci> :t maybe
maybe :: b -> (a -> b) -> Maybe a -> b
ghci> maybe (-9999) (*2) (Just $ fromEnum 'k')
214
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07-02-10, 08:03 PM
| | | 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: Code: Content visible to registered users only.
> CC: Code: Content visible to registered users only.
> Betreff: Re: [Haskell-beginners] define action getInt like getLine
> Am Dienstag 02 Februar 2010 22:20:03 schrieb Code: Content visible to registered users only.
:
> > 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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|
07-02-10, 08:25 PM
| | | 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 Code: Content visible to registered users only.
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: Code: Content visible to registered users only.
>> CC: Code: Content visible to registered users only.
>> Betreff: Re: [Haskell-beginners] define action getInt like getLine
>
>> Am Dienstag 02 Februar 2010 22:20:03 schrieb Code: Content visible to registered users only.
:
>>> 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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|
08-02-10, 09:19 PM
| | | Re: [Haskell-beginners] define action getInt like getLine
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: Code: Content visible to registered users only.
> CC: Code: Content visible to registered users only.
> 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
>
> Code: Content visible to registered users only.
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: Code: Content visible to registered users only.
> >> CC: Code: Content visible to registered users only.
> >> Betreff: Re: [Haskell-beginners] define action getInt like getLine
> >
> >> Am Dienstag 02 Februar 2010 22:20:03 schrieb Code: Content visible to registered users only.
:
> >>> 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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08-02-10, 09:43 PM
| | | Re: [Haskell-beginners] define action getInt like getLine
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: Code: Content visible to registered users only.
> 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:
>
> Code: Content visible to registered users only.
>
>
> 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: Code: Content visible to registered users only.
> >> CC: Code: Content visible to registered users only.
> >> 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
> >>
> >> Code: Content visible to registered users only.
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: Code: Content visible to registered users only.
> >> >> CC: Code: Content visible to registered users only.
> >> >> Betreff: Re: [Haskell-beginners] define action getInt like getLine
> >> >
> >> >> Am Dienstag 02 Februar 2010 22:20:03 schrieb Code: Content visible to registered users only.
:
> >> >>> 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 Code: Content visible to registered users only.
> > _______________________________________________
> > Beginners mailing list
> > Code: Content visible to registered users only.
> > Code: Content visible to registered users only.
> >
> _______________________________________________
> Beginners mailing list
> Code: Content visible to registered users only.
> Code: Content visible to registered users only.
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|
08-02-10, 10:01 PM
| | | Re: [Haskell-beginners] define action getInt like getLine
Am Montag 08 Februar 2010 22:43:01 schrieb Code: Content visible to registered users only.
:
> but I have to write the action
> readMonth :: IO Month
> on my own and still don't have an idea how to do it
Go take a look at Code: Content visible to registered users only.
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