[Haskell-beginners] function multiple args

[mike h]

Thanks Stuart, Ovidiu. Yes, partially applied functions is the way.

Mike

> On 18 Dec 2017, at 16:00, Stuart Dootson <stuart.dootson at gmail.com> wrote:
> 
> Something like the code below - populate a list with partially applied functions, then use foldl to apply the functions in turn to the input list?
> 
> v :: Int -> [a] -> [a]
> v = drop
> 
> w :: Int -> Int -> [a] -> [a]
> w keep reject = (take keep).(drop reject)
> 
> -- Apply the list of functions (in turn) to the list, yielding a list
> apply_ops :: [ [a] -> [a] ] -> [a] -> [a]
> apply_ops fns l = foldl (flip ($)) l fns
> 
> -- Partially apply functions with non-list arguments to get list of functions of type [a] -> [a]
> ops :: [ [a] -> [a] ]
> ops = [v 3, w 2 1]
> 
> -- result = (w 2 1) ((v 3)[1,2,3,4,5,6,7,8])
> --        = w [4,5,6,7,8]
> --        = [5,6]
> result :: [Int]
> result = apply_ops ops [1,2,3,4,5,6,7,8]
> 
> main =
>   do
>     print result
> 
> On 17 December 2017 at 09:32, mike h <mike_k_houghton at yahoo.co.uk <mailto:mike_k_houghton at yahoo.co.uk>> wrote:
> Hi,
> 
> I have a number of functions like these:
> 
> v :: Int -> [a] -> [a]
> w :: Int -> Int -> [a] -> [a]
> x :: a -> a -> [a] -> [a]
> 
> y :: a -> b ->…  <other args>... ->  [a] -> [a]
> z…
> etc.
> 
> where there are any number of args of different types but the last two are common to all the functions i.e. they all have  [a] -> [a]
> 
> What I’m trying to do is build a collection (ordered) of such functions and then apply each in turn to a [a] to get the final [a]. What would be the best approach to this?
> 
> Thanks
> 
> Mike
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