[Haskell-cafe] type class design

[Jean-Marie Gaillourdet]

Hi,

sorry for answering to such an old thread.

David Menendez <dave at zednenem.com> writes:

> On Fri, Oct 29, 2010 at 8:33 AM, Tillmann Rendel
> <rendel at informatik.uni-marburg.de> wrote:
>> Hi,
>>
>> Uwe Schmidt wrote:
>>>
>>> In the standard Haskell classes we can find both cases,
>>> even within a single class.
>>>
>>> Eq with (==) as f and (/=) as g belongs to the 1. case
>>
>> Note that the case of (==) and (/=) is slightly different, because not only
>> can (/=) be defined in terms (==), but also the other way around. The
>> default definitions of (==) and (/=) are mutually recursive, and trivially
>> nonterminating. This leaves the choice to the instance writer to either
>> implement (==) or (/=). Or, for performance reasons, both.
>
> While I understand the argument in general, I've never understood why
> it applies to Eq. Are there any types where it is preferable to define
> (/=) instead of (==)?

Yes for infinite data structures.

Cheers,
  Jean-Marie