[Haskell-cafe] constant functions

[Donald Bruce Stewart]

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