Question

Hi everyone, I need help in Haskell Please

1) Write a function that takes a two-operand integer function and a list of Ints, and applies the function to each in the list twice. it is OK to call the map in the function.

Answer 1

According to the question, a function that takes a two-operand integer function(here function 'mulList') and a list of 'Ints',

and applies the function to each in the list twice.

mulList is given as:

mulList :: Num a => [a] -> a -> [a]
mulList [] y = []
mulList (x:[]) y = (x * y):[]
mulList (x:xs) y = (x * y): (mulList xs y)

Map returns a list constructed by appling a function (the first argument) to all items in a list passed as the second argument.

Lets first create a function that applies to the list once.

import Data.Char  
import Prelude hiding (map)
map :: (a -> b) -> [a] -> [b]
map _ [] = []
map f xs = [f x | x <- xs] //function f= mulList in our case.
This function takes two arguments:

1.a function f which maps as to bs, and

2.a list xs of as. It returns a list of bs which are the results of applying f to every member of xs.

for example, map square [1,1,2,3,5,8] would yield the list [1,1,4,9,25,64].

To apply the function to each in the list element twice

just do the following

f w = concat (map mulList (mulList w))
OR

import Control.Monad; f = mulList >=> mulList

Hope this may help you

Thank you ??