+1 On 9 May 2011 19:11, Jacques Carette wrote:

+1 Jacques

On 09/05/2011 6:49 PM, Daniel Fischer wrote:

...

I would like to propose the elimination of the special error case

gcd 0 0 = error "Prelude.gcd: gcd 0 0 is undefined"

to replace it with

gcd 0 0 = 0

(which would be an automatic consequence of removing the above line).

Rationale:

1. It makes gcd a total function. 2. It makes gcd associative. 3. It makes folding gcd over a list safe. 4. It conforms to common mathematical practice:

In a commutative ring, a greatest common divisor of two elements a, b is a common divisor g of a and b such that every common divisor d of a and b also divides g (if R is a commutative ring, a \in R, d \in R is a divisor of a iff there exists c \in R with a = d*c [leaving out noncommutative rings for simplicity]). Since every ring element divides 0, the above definition entails gcd 0 0 = 0.

Further, in a principal ideal ring, the greatest common divisors of two elements a and b are the generators of the ideal (a,b), again that characterisation of greatest common divisors says gcd 0 0 = 0: (0,0) = {0} = (0).

Pros: see above.

Cons: ? I'm not aware of any, the change shouldn't break existing code, since that would have to check for the special case anyway.

Daniel

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On Tuesday 10 May 2011 08:02:50, Judah Jacobson wrote:

On Mon, May 9, 2011 at 3:49 PM, Daniel Fischer

wrote:

...

I would like to propose the elimination of the special error case

gcd 0 0 = error "Prelude.gcd: gcd 0 0 is undefined"

to replace it with

gcd 0 0 = 0

(which would be an automatic consequence of removing the above line).

In my email archives, I found two previous threads on the subject. Both ended up in favor of the proposal: http://www.haskell.org/pipermail/haskell/2001-December/008560.html http://www.haskell.org/pipermail/haskell-cafe/2009-May/060880.html

As a result of the latter discussion, a ticket was opened on GHC's trac: http://hackage.haskell.org/trac/ghc/ticket/3304 But it was eventually closed for lack of attention.

So if someone actually follows through with this as an official libraries submission (with a patch, deadline, etc.), the odds seem in favor of it.

I'll try to see it through, although the process seems rather daunting. It has annoyed me for too long. Patches for GHC are at http://hackage.haskell.org/trac/ghc/ticket/3304 (plural, because the error needs to be in the haskell98 and haskell2010 packages, since the report demands it). Although this time there has not been much discussion yet, I refer to the previous discussions linked by Judah, to conclude that after some discussion (albeit spread over many years), the proposal seems plausible. Following http://hackage.haskell.org/trac/haskell-prime/wiki/Process#Proposals I hereby volunteer to become the proposal owner. Cheers, Daniel

On Wed, May 25, 2011 at 08:24:52PM +0200, Daniel Fischer wrote:

If it's considered to be a small enough change so a libraries proposal would be sufficient, all the better, but if not, I'd like to pursue the haskell-prime process further.

My understanding is that for changes to libraries mentioned in the language report, we use the libraries@ process, not the H' process. Thanks Ian