Trying to sum from a generated list

Question

Hello I recently started studying Haskell and I am trying to make a little program that given a function it creates a list and then I want to take the sum of the list:

f a b c = a+b+c

my_sum [] = 0
my_sum (x:xs) = x + my_sum xs

my_list f a b c = [f a b x |x <- [1..c]]

I am trying to take the sum of the list like this but I always get errors

  *Main> my_sum [my_list f 1 1 4]

<interactive>:13:1:
    No instance for (Num [t0]) arising from a use of `my_sum'
    Possible fix: add an instance declaration for (Num [t0])
    In the expression: my_sum [my_list f 1 1 4]
    In an equation for `it': it = my_sum [my_list f 1 1 4]

<interactive>:13:9:
    No instance for (Num t0) arising from a use of `my_list'
    The type variable `t0' is ambiguous
    Possible fix: add a type signature that fixes these type variabl
    Note: there are several potential instances:
      instance Num Double -- Defined in `GHC.Float'
      instance Num Float -- Defined in `GHC.Float'
      instance Integral a => Num (GHC.Real.Ratio a)
        -- Defined in `GHC.Real'
      ...plus three others
    In the expression: my_list f 1 1 4
    In the first argument of `my_sum', namely `[my_list f 1 1 4]'
    In the expression: my_sum [my_list f 1 1 4]

can you help me?

@Khaelid I edited the question. I had copied the wrong version of my code. — JmRag
– JmRag, Commented Mar 12, 2014 at 13:11
You say "given a function it creates a list", but you're not giving it (my_list) a function (but you did before the edit). — raymonad
– raymonad, Commented Mar 12, 2014 at 13:17

András Kovács · Accepted Answer · 2014-03-13 14:13:43Z

3

my_sum takes one argument, a list of numbers. Since my_list returns a list, wrapping its result in a list results in a list of lists (mismatching my_sum):

my_sum [my_list f 1 1 5] -- argument has type Num a => [[a]]
my_sum (my_list f 1 1 5) -- this is right

edited Mar 13, 2014 at 14:13

answered Mar 12, 2014 at 13:11

András Kovács

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2 Comments

Yuriosity Over a year ago

Original myList takes only four arguments.

András Kovács Over a year ago

@Yuuri Edited to match.

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