Re: Odd side effect of using chr

I think I see what's going on. Prelude> :t f where f n = snd( chr n , n^25 `mod` 18721 ) let {...} in f :: Int -> Int Prelude> :t g where g n = n^25 `mod` 18721 let {...} in g :: Integral a => a -> a In the case of f, the use of chr forces n to be an Int, so it's calculating n^25 as an Int, which is 0 (because of arithmetic overflow). In the case of g, it's presumably using Integer, so gets the correct answer. Writing Prelude> let f n = snd( chr n , fromInt n^25 `mod` 18721 ) in f 4 gives the right answer. Gavin

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From gavinl Mon Jan 6 15:34:20 2003 Date: Mon, 6 Jan 2003 15:34:19 GMT X-Authentication-Warning: client104.comlab: gavinl set sender to gavin.lowe@comlab.ox.ac.uk using -f From: Gavin Lowe CC: Gavin.Lowe@comlab.ox.ac.uk Reply-to: gavin.lowe@comlab.ox.ac.uk

Hi

I think this is a hugs bug.

If I calculate

Prelude> let g n = n^25 `mod` 18721 in g 4

I get

17173

But then if I calculate

Prelude> let f n = snd( chr n, n^25 `mod` 18721 ) in f 4

I get

0

Which is different! I think the former is correct, FWIW. Replacing the "chr n" by something not involving chr and n gives 17173.

This is with Hugs Version December 2001, running on a SunBlade 100 with Solaris 9.

Gavin

-- Gavin Lowe

Oxford University Computing Laboratory, Parks Road, Oxford, OX1 3QD. Phone: +44 1865 273841. Fax: +44 1865 273839. E-mail: gavin.lowe@comlab.ox.ac.uk

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