define gcd 0 0 = 0

Please make any comments on the libraries list by Tuesday 15th September 2009.

A suggestion to change 'gcd 0 0' from[[BR]] gcd 0 0 = error "Prelude.gcd: gcd 0 0 is undefined"[[BR]] to[[BR]] gcd 0 0 = 0

The proposal has been discussed on haskell-cafe@haskell.org http://www.haskell.org/pipermail/haskell-cafe/2009-May/060788.html and there was a lot of support for the suggested change.

Current code: libraries/base/GHC/Real.lhs

-- | @'gcd' x y@ is the greatest (positive) integer that divides both @x@
-- and @y@; for example @'gcd' (-3) 6@ = @3@, @'gcd' (-3) (-6)@ = @3@,
-- @'gcd' 0 4@ = @4@.  @'gcd' 0 0@ raises a runtime error.
gcd             :: (Integral a) => a -> a -> a
gcd 0 0         =  error "Prelude.gcd: gcd 0 0 is undefined"
gcd x y         =  gcd' (abs x) (abs y)
                   where gcd' a 0  =  a
                         gcd' a b  =  gcd' b (a `rem` b)

Suggested code:

-- | @'gcd' x y@ is the greatest (positive) integer that divides both @x@
-- and @y@; for example @'gcd' (-3) 6@ = @3@, @'gcd' (-3) (-6)@ = @3@,
-- @'gcd' 0 4@ = @4@.  @'gcd' 0 0@ is defined to be 0.
gcd             :: (Integral a) => a -> a -> a
gcd x y         =  gcd' (abs x) (abs y)
                   where gcd' a 0  =  a
                         gcd' a b  =  gcd' b (a `rem` b)

[Note: I tried following the instructions at http://www.haskell.org/haskellwiki/Library_submissions to create a darcs patch. I successfully downloaded a copy of HEAD, but was unable to create a patch:

$ darcs record libraries/base/GHC/Real.lhs Recording changes in "libraries/base/GHC/Real.lhs":

No changes in selected files or directories!

I would claim that the instructions are insufficient for a darcs beginner. ]

Trac metadata

Trac field	Value
Version	6.10.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries/base
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	libraries@haskell.org
Operating system
Architecture