[Haskell-cafe] constant functions
michael rice
nowgate at yahoo.com
Wed Dec 27 23:22:28 EST 2006
Hi Donald,
I think you misunderstood what I was asking. There's
not two cases. Maybe I'm not saying it sufficiently
well but the function ALWAYS just returns a function
that always returns the original argument to ALWAYS no
matter what else you give the resulting function.
when one is define as follows
one = always 1
then
> one 4
1
> one "abc"
1
> one (2,3)
1
> one [0,4,8,2]
1
> map one ["one","two","three"]
[1,1,1]
The mapping example is just an alternative way of
illustrating the functionality. No matter what the
defined function is given it always gives back the
original value give to ALWAYS.
Michael
--- Donald Bruce Stewart <dons at cse.unsw.edu.au> wrote:
> nowgate:
> > I'm trying to learn Haskell and translating some
> Lisp
> > functions as exercises.
> >
> > How would I write a Haskell function named ALWAYS
> that
> > behaves like this:
> >
> > one = always 1
> > bozo = always "clown"
> >
> > > map one [2,3,4,5,6]
> > [1,1,1,1,1]
> >
> > > one 62
> > 1
> >
> > > map bozo [2,3,4,5,6]
> > ["clown","clown" ,"clown", "clown"," clown"]
> >
> > > bozo 62
> > "clown"
> >
> > i.e. ALWAYS returns a function with a single
> parameter
> > that is ignored, returning instead the value given
> to
> > ALWAYS when the function was created.
> >
> > This is what I've been trying:
> >
> > always :: (a -> a) -> a -> a
> > always x = (\y -> x)
> >
> > one = always 1
>
>
> So there are two cases you want to handle:
> * return a list, if the argument is a list
> * return a single value, if the argument is a
> single value.
> We can write thes functions separately:
>
> alwayList :: a -> [b] -> [a]
> alwayList n xs = map (const n) xs
>
>
> *Main> let one = always 1
> *Main> one "foo"
> [1,1,1]
>
>
> *Main> let bozo = always "clown"
> *Main> bozo "haskell"
>
>
["clown","clown","clown","clown","clown","clown","clown"]
>
> Now handle the non-list case:
>
>
> alwaysAtom :: a -> b -> a
> alwaysAtom a b = a
>
> *Main> let one = alwaysAtom 1
> *Main> one 'x'
> 1
>
> Unifying these two under a common type class is left
> as an exercise ;)
>
> I guess the type signature for such a class would be
> something like:
>
> class Const a b a' | b -> a', a' -> a where
>
> Something like that.
>
> -- Don
>
__________________________________________________
Do You Yahoo!?
Tired of spam? Yahoo! Mail has the best spam protection around
http://mail.yahoo.com
More information about the Haskell-Cafe
mailing list