CODEWITHSUNDEEP
haskell
Lazare Rossillon
Lazare Rossillon

Reputation: 53

Apply a function to every element in a list to every element in another list

I'd like a more elegant way to write the following two functions, ideally in one :

applyOperatorBetweenVariableLists:: [Variable] -> String -> [Variable] -> [Variable]
applyOperatorBetweenVariableLists firstList operator secondList = concat $ map (test firstList operator) secondList

test:: [Variable] -> String -> Variable -> [Variable]
test firstVariables operator secondVariable = concat $ map (applyOperatorBetweenVariables secondVariable operator) firstVariables

The declaration of applyOperatorBetweenVariables is :

applyOperatorBetweenVariables:: Variable -> String -> Variable -> [Variable]

I'm quite sure there must be a Prelude Function that does exactly this, or a very elegant way to write it.

Upvotes: 1

Views: 137

Answers (1)

Joseph Sible-Reinstate Monica
Joseph Sible-Reinstate Monica

Reputation: 48572

This can be done concisely with a do block:

applyOperatorBetweenVariableLists firstList operator secondList = do
  secondVariable <- secondList
  firstVariable <- firstList
  applyOperatorBetweenVariables secondVariable operator firstVariable

If you wanted to be even more concise, you could reorder the arguments to both applyOperatorBetweenVariableLists and applyOperatorBetweenVariables, and then use either liftJoin2 or bind2 to implement it (like my final sentence below but with it in place of liftA2).

My original answer was wrong, as it left a layer of nesting (i.e., should have done an extra concat or join):

That's almost just liftA2, but your arguments are in a weird order. Here's how you'd implement what you wrote in terms of that:

import Control.Applicative (liftA2)
applyOperatorBetweenVariableLists firstList operator secondList = liftA2 (flip applyOperatorBetweenVariables operator) secondList firstList

From that definition, it should be clear how you could change and simplify that to applyOperatorBetweenVariableLists = liftA2 . applyOperatorBetweenVariables just by reordering the arguments to it and to applyOperatorBetweenVariables.

Upvotes: 1

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