haskell , apply a transformation with function inline


apply a transformation with function inline

Question:

Tag: haskell

Starting from a simple case of "fold" (I used (+) but can be anything else):

Prelude.foldl (+) 0 [10,20,30]

is it possible apply an inline transformation similar to (that doesn't work):

Prelude.foldl ((+) . (\x -> read x :: Int)) 0 ["10","20","30"]

In case not, is there an alternative to fold, to apply a generic function and inline transformation (apart from using specific functions like 'sum', 'max' etc)?


Answer:

The read lambda applies to the first argument and the first argument to the function given to foldl is the accumulator. Those two arguments are the opposite for foldr. So, expanded, it looks like this:

foldl (\acc element -> (read acc :: Int) + element) 0 ["10", "20", "30"]

Since acc is an Int, this doesn't work.

So, with this information in hand, you can do this with foldr since it has the opposite argument order:

foldr ((+) . (read :: String -> Int)) 0 ["10","20","30"]

If you want an inline foldl version, you can use flip to achieve this.

You could also use map first (everything else being equal, this would be the solution I'd prefer):

foldl (+) 0 $ map (read :: String -> Int) ["10","20","30"]

Also, you probably want foldl' instead of foldl.


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