this is an exercice :

-- Implement the function sumRights which sums all Right and discard Left
-- Examples:
--   sumRights [Right 1, Left "bad value", Right 2]  ==>  3
--   sumRights [Left "bad!", Left "missing"]         ==>  0

My idea was, as a first step, to transform the list with map into a list of integers only. I wrote the following :

transformListToInt :: [Either a Int] -> Int
transformListToInt list = map f list
    where f (Right b) = b 
          f (Left  c) = 0
     

But it gives me the following error :

* Couldn't match expected type `Int' with actual type `[a]'
* In the expression: map f list
  In an equation for `sumRights':
      sumRights list
        = map f list
        where
            f (Right b) = b
            f (Left c) = c

Could someone help me to understand why I get this error ? (the initial problem is not so important, I could solve it differently)

CodePudding user response：

You forgot to sum the list in the end:

transformListToInt :: [Either a Int] -> Int
transformListToInt list = sum $ map f list
    where f (Right b) = b 
          f (Left  c) = 0

A shorter way to write the same would be:

transformListToInt = sum . map (fromRight 0)

or even shorter:

transformListToInt = sum . rights

(both ways require importing Data.Either)

CodePudding user response：

map turns a list [x0,x1,...] into another list [f x0, f x1, ...]. It can not produce an Int as your type claims, so the compiler complains.

Since your map generates a value of type [Int], you need to transform that list into the sum of its elements (use sum).