Bifunctor instance for (,) is not strictly law-abiding

[David Feuer]

This is part of what makes lazy State confusing. I haven't really messed
with Writer too much. The extra laziness can be very bad for performance in
some cases. Since the laws lead to a different performance intuition, any
such variance should be documented in Haddock. Note also that fmap and
second have *different* strictness for pairs, which seems very surprising
as well.
On Feb 17, 2016 4:24 PM, "Edward Kmett" <ekmett at gmail.com> wrote:

> As a general rule we let the results of many operations be more defined
> than the laws require.
>
> Stream fusion runs into this exact same "issue".
>
> This sort of thing is necessary to makes a lot of things like "lazy state"
> and "lazy writer" possible at all.
>
> -Edward
>
> On Wed, Feb 17, 2016 at 2:25 PM, David Feuer <david.feuer at gmail.com>
> wrote:
>
>> I don't think we should fix this, because it would break code, but I do
>> think we should document it.
>>
>> Bifunctor law:
>>
>> bimap id id = id
>>
>> But actually,
>>
>> bimap id id _|_ = (_|_, _|_)
>>
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>>
>
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