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
– Wilduck

2012-10-05 01:06:12 +00:00
Commented Oct 5, 2012 at 1:06 — Wilduck
– Wilduck, Commented Oct 5, 2012 at 1:06

Conrad Parker · Accepted Answer · 2012-10-05 01:06:39Z

10

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 5, 2012 at 1:06

Conrad Parker

84410 silver badges23 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Wilduck Over a year ago

Hahaha, I came to the same conclusion just a few seconds before you posted this. Wonderful.

Matt Fenwick Over a year ago

I'm surprised this works, given that the type is :: Num a => a. Is this GHCi's type defaulting coming in to play?

Ørjan Johansen Over a year ago

@Matt Fenwick: Yes, Integer is used by default for Num a => a.

Collectives™ on Stack Overflow

Sum of [Int] causes overflow

1 Answer 1

3 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related