[Haskell] Silent error

Benoit T benoit.triquet at gmail.com
Mon Jan 21 02:26:34 CET 2013

On Mon, Jan 21, 2013 at 01:17:17AM +0200, miphis at meta.ua wrote:
> The program looks like this:
> --***********************
> factorial :: Int->Int
> factorial n = product [1..n]
> main = do
>         print $ factorial 50
> --***********************
> And that yields "0" (no errors). Is it a bug or feature? :)

Int is fixed-size (*) and is expected to use modular arithmetics (**)
therefore there is no notion of overflow, it just computes modulo

You must have been using a 32-bit haskell, that uses full-word Int, such
as HUGS, and it turns out that using 32-bit signed integers, you
eventually get -2**31 (this happens for n==32) and then multiplying by
any even integer yields 0 (34!==0).

GHC uses 31-bit or 63-bit Int (on 32- or 64-bit arch) and does not
encounter this case before factorial 50 (however the answer is also not
what you may have been expecting because it still doesn't fit).


*: this is specified in the standard
**: this does not seem to be specified in the standard

Benoit Triquet <benoit.triquet at gmail.com>
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