[Haskell-beginners] apply a function to all params of another function

[Brent Yorgey]

On Mon, May 26, 2014 at 04:46:29PM +0200, Kees Bleijenberg wrote:
> In have a lot of functions in a program with params that are alle Int's.  
> f :: Int -> Int -> Int
> g :: Int -> Int -> Int
> h :: Int -> Int -> Int -> Int
> ...
> and convertParam :: Int -> Int -- say (+1) 
> I want to call functions like f, g and h, but apply convertParam to all
> params first.
> f2 a b c = f (convertParam a) (convertParam b) (convertParam c)
> 
> I tried:
> run1p f  conv = \a  -> f (conv a)     		-- for functions with one
> param
> run2p f  conv = \a b ->  f (conv a) (conv b) 	-- for functions with two
> params
> .
> Can you do this in a more generalized way (using currying?)
> Any ideas?

class Convertible t where
  convert :: t -> t

instance Convertible Int where
  ...

instance Convertible t => Convertible (Int -> t) where
  ...

I'll let you fill in the ... ! =) Note this only works well because
the base case is Int.  You could also add some extra base cases for
other concrete return types.  However, it is quite difficult to give a
Convertible instance which applies to "all types which are not
functions", so this will only work if you are willing to make one
instance for each concrete result type your functions use.

-Brent