[Haskell-beginners] define action getInt like getLine
kane96 at gmx.de
kane96 at gmx.de
Tue Feb 9 16:41:11 EST 2010
ahhh, I didn't know that it's so simple with read :)
-------- Original-Nachricht --------
> Datum: Mon, 8 Feb 2010 22:26:59 +0000
> Von: Stephen Tetley <stephen.tetley at gmail.com>
> An: kane96 at gmx.de
> Betreff: Re: [Haskell-beginners] define action getInt like getLine
> Upthread Daniel Fisher posted this one
> getInt :: IO Int
> getInt = fmap read getLine
> If you check the type of read
> *GHCi> :t read
> read :: (Read a) => String -> a
> 'reads' is polymorphic it will read anything from a String - well
> within reason, anything that is an instance of the Read type class.
> The "(Read a) => " part of the type signature indicates this
> constraint - that "a" ( the return type of the function read) must be
> an instance of Read.
> As I wrote, you can easily derive an instance of Read by adding Read
> to the deriving clause of your data type, so the next thing to do is
> write a function swapping as few parts from Daniel's example as
> -- Here you have to make a change as you want a Month rather than an
> Int, so swap the ??? for what you want...
> getMonth :: IO ???
> -- The function definition can stay the same - none of the components
> in Daniel's definition actually depended on reading an Int:
> getMonth = fmap read getLine
> On 8 February 2010 21:43, <kane96 at gmx.de> wrote:
> > but I have to write the action
> > readMonth :: IO Month
> > on my own and still don't have an idea how to do it
GRATIS für alle GMX-Mitglieder: Die maxdome Movie-FLAT!
Jetzt freischalten unter http://portal.gmx.net/de/go/maxdome01
More information about the Beginners