Sum of [Int] causes overflow

Question

I have a function that returns [Int] and I would like to take the sum of the list. However, while each individual element is smaller than maxBound::Int, the sum is definitely larger.

A (contrived) exmple:

ghci> sum ([1..10000000] :: [Int])
-2004260032

Is there any way to force sum to accumulate into an Integer instead of an Int? Am I thinking about this wrong?

Is `sum $ map fromIntegral` an appropriate way to do this? – Wilduck Oct 05 '12 at 01:06 — Wilduck, Oct 05 '12 at 01:06

score 11 · Accepted Answer · answered Oct 05 '12 at 01:06

11

sum returns the same type as its input list elements:

sum :: Num a => [a] -> a

so you need to pass it a [Integer] in order to return an Integer. If your input list is already of type [Int], you can use the function:

sum . map fromIntegral

instead:

ghci> sum . map fromIntegral $ ([1..10000000] :: [Int])
50000005000000

answered Oct 05 '12 at 01:06

Conrad Parker

854
10
23

Hahaha, I came to the same conclusion just a few seconds before you posted this. Wonderful. – Wilduck Oct 05 '12 at 01:07
I'm surprised this works, given that the type is `:: Num a => a`. Is this GHCi's type defaulting coming in to play? – Matt Fenwick Oct 05 '12 at 12:21
1

@Matt Fenwick: Yes, Integer is used by default for Num a => a. – Ørjan Johansen Oct 06 '12 at 01:18

Sum of [Int] causes overflow

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