[Haskell-cafe] Re: instance Eq (a -> b)

John Meacham john at repetae.net
Wed Apr 14 14:12:21 EDT 2010

On Wed, Apr 14, 2010 at 02:07:52AM -0700, Ashley Yakeley wrote:
>> So the facts that
>> (1) f == g
>> (2) f undefined = 6
>> (3) g undefined = undefined
>> is not a problem?
> This is not a problem. f and g represent the same moral function, they  
> are just implemented differently. f is smart enough to know that its  
> argument doesn't matter, so it doesn't need to evaluate it. g waits  
> forever trying to evaluate its function, not knowing it doesn't need it.

Hence they are distinct functions, and should not be determined to be
equal by an equality instance. A compiler will not transform g into f
because said distinction is important and part of the definition of a
function. Not considering 'bottom' a normal value leads to all sorts of
issues when trying to prove properties of a program. Just because they
don't occur at run time, you can't pretend they don't exist when
reasoning about the meaning of a program, any more than you can
reasonably reason about haskell without taking types into account simply
because types don't occur in the run-time representation.


John Meacham - ⑆repetae.net⑆john⑈ - http://notanumber.net/

More information about the Haskell-Cafe mailing list