[Haskell-cafe] Re: Haskell and C++ program
S. Doaitse Swierstra
doaitse at swierstra.net
Mon Jan 19 22:12:31 EST 2009
On 17 jan 2009, at 22:22, Derek Elkins wrote:
> On Thu, 2009-01-15 at 13:40 +0100, Apfelmus, Heinrich wrote:
>> Eugene Kirpichov wrote:
>>> Well, your program is not equivalent to the C++ version, since it
>>> doesn't bail on incorrect input.
>> Oops. That's because my assertion
>> show . read = id
>> is wrong. We only have
>> read . show = id
>> show . read <= id (in the "less defined than" sense)
> No, you only have
> read . show = id which often doesn't hold in practice.
> show . read </= id
You do not even have that; the read may remove surplus parentheses
which will not be reinserted by the show.
> Assuming the first identity holds, you do of course have show . read .
> show = show and this probably holds even in most cases where read .
> = id does not hold.
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