Question

I’m stuck on this problem and I’m very new to Haskell, I was trying to complete the first Euler problem with the following code:

main = putStrLn . show . sum $ [3,6..1000]:[5,10..1000]

And here’s the first part of the error:

euler/1.hs:1:19:
    No instance for (Show t0) arising from a use of `show'
    The type variable `t0' is ambiguous
    Possible fix: add a type signature that fixes these type variable(s)
    Note: there are several potential instances:
      instance Show Double -- Defined in `GHC.Float'
      instance Show Float -- Defined in `GHC.Float'
      instance (Integral a, Show a) => Show (GHC.Real.Ratio a)
        -- Defined in `GHC.Real'
      ...plus 23 others
    In the first argument of `(.)', namely `show'
    In the second argument of `(.)', namely `show . sum'
    In the expression: putStrLn . show . sum

Answer 1

What do you expect [3,6..1000]:[5,10..1000] to do? x : xs preppeds an object to a list of objects. Here both arguments are lists of integers. Did you want ++ instead (concatenation).

It needs to be said that your approach is not right, no matter what you expected : to do. I’ll not expand on this in case you want to figure it out yourself.