How To Get List Reduce To Throw Arithmetic Overflow

Question

So, yesterday while working through some F# code as part of a coding exercise, another developer pointed out something interesting. We were just doing a quick piece of code to demonstrate summing a list. If I do:

[1..100000] |> Seq.sum

I get the following error:

System.OverflowException: Arithmetic operation resulted in an overflow.
   at <StartupCode$FSI_0003>.$FSI_0003.main@()
Stopped due to error

However, if I do:

[1..100000] |> List.reduce (+)

I get:

val it : int = 705082704

I realize although these two pieces of code should accomplish the same purpose they are very different. I am just curious is there a way to get the List.reduce to throw the OverflowException rather than giving me a bad answer?

Answer 1

You can use a checked operator:

[1..100000] |> List.reduce (Checked.(+))

Answer 2

From the f# source code

[<CompiledName("Sum")>]
let inline sum (source: seq< (^a) >) : ^a = 
  use e = source.GetEnumerator() 
  let mutable acc = LanguagePrimitives.GenericZero< (^a) >
  while e.MoveNext() do
      acc <- Checked.(+) acc e.Current
  acc

Notice the Checked.(operator) this checks for arithmetic overflows…

http://msdn.microsoft.com/en-us/library/vstudio/ee340296.aspx