CODEWITHSUNDEEP
haskelltuples
user4709293
user4709293

Reputation:

Dealing with tuples and bindings

I'm writing a function that sums the elements of a list of tuples like this:

sumAll [(2,4,11), (3,1,-5), (10,-3,6)] = (15,2,12)

I do have this:

sumAll :: (Num a, Num b, Num c) => [(a,b,c)] -> (a,b,c)
sumAll l = (foldr (+) 0 as, foldr (+) 0 bs, foldr (+) 0 cs) 
               where trd (a,b,c) = c
                     as = (map (fst) l) 
                     bs = (map (snd) l)
                     cs = (map (trd) l)

However the compiler complains:

Couldn't match type `(a, b, c)' with `(b1, b0)'
    Expected type: [(b1, b0)]
      Actual type: [(a, b, c)]
    Relevant bindings include
      as :: [b1] (bound at ficha3.hs:22:22)
      cs :: [c] (bound at ficha3.hs:24:22)
      l :: [(a, b, c)] (bound at ficha3.hs:20:12)
      sumAll :: [(a, b, c)] -> (a, b, c) (bound at ficha3.hs:20:1)
    In the second argument of `map', namely `l'
    In the expression: (map (fst) l)

And the same for the expression (map (snd) l).

If I delete 'c' parameters from function definition it Works. Like this:

sumAll :: (Num a, Num b) => [(a,b)] -> (a,b)
sumAll l = (foldr (+) 0 as, foldr (+) 0 bs) 
               where as = (map (fst) l) 
                     bs = (map (snd) l)

My questions, if possible:

  1. Why is 'c' "screwing" the output tuple?
  2. How can I work this around?

Upvotes: 0

Views: 109

Answers (1)

ohw
ohw

Reputation: 1639

Consider how your fst, snd, and trd are defined. If you are using fst from Prelude, then it has a type of fst :: (a, b) -> a, which does not work for 3-tuples.

Upvotes: 4

Related Questions