Haskell - Passing argument to map function?

Question

Basic question... I wish to map over a list and apply a function to each element, however I want to execute this from another function:

functionOne :: Int -> Int -> Int --Add x to each element of the list
functionOne element x = element + x 

functionTwo :: [Int] -> (Int -> Int -> Int) -> [Int]
functionTwo list fOne = map list fOne ave --map list by applying functionOne with ave as x
   where 
     ave = ((sum list) / length list)

Why isn't this working?

chi · Accepted Answer · 2016-09-17 11:56:31Z

2

You probably want this:

functionTwo :: [Int] -> (Int -> Int -> Int) -> [Int]
functionTwo list fOne = map (\el -> fOne el ave) list
   where 
     ave = sum list `div` length list

Above, we use the anonymous function (\el -> fOne el ave), which is the function mapping each el to fOne el ave. In this way, we fix the second argument of fOne to the wanted value.

Alternatively,

functionTwo :: [Int] -> (Int -> Int -> Int) -> [Int]
functionTwo list fOne = map (flip fOne ave) list
   where 
     ave = sum list `div` length list

Note div for integer division, / only works on floating point numbers.

flip f is the same function as f, but with the first two arguments in the other order.

Also possible: map (`fOne` ave) list.

edited Sep 17, 2016 at 11:56

answered Sep 17, 2016 at 10:24

chi

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4 Comments

Babra Cunningham Over a year ago

Can you explain, from your first example, what '(\el -> fOne el ave)' is doing? then i can accept

melpomene Over a year ago

@barbrac That's the syntax for a function. It's \PARAMETER -> BODY, i.e. in this case it's a function taking a value (el) and returning fOne el ave.

amalloy Over a year ago

Why flip? Addition is commutative, so just map (fOne ave) list.

chi Over a year ago

@amalloy Addition is, but I think the OP wanted the general case, since she specified "with ave as x".

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