[Haskell-cafe] type class design

David Menendez dave at zednenem.com
Fri Oct 29 17:30:14 EDT 2010

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 (==)?

Dave Menendez <dave at zednenem.com>

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