Constraints on definition of `length` should be strengthened

[Jon Fairbairn]

Sven Panne <svenpanne at gmail.com> writes:

> 2017-04-06 9:33 GMT+02:00 Jon Fairbairn <jon.fairbairn at cl.cam.ac.uk>:
>
>> Sven Panne <svenpanne at gmail.com> writes:
>> [...] Although something like Error/OK would have been better than
>> > Left/Right, a slight majority preferred to give a bias to Either. The
>> > reasoning was that using "Right" for a "wrong" outcome (i.e. failure)
>> would
>> > be a bit obscure, and there was already quite some code using it in the
>> way
>> > we still do today. The bias is even explicitly documented in the Haddock
>> > docs for Data.Either for ages, so it would not be very wise to change the
>> > meaning here after roughly 2 decades.
>>
>> I guess this means that Haskell has failed to sufficiently avoid
>> success. If a mistake in library design is bad enough (not
>> necessarily the case for Either, but arguably so), it should be
>> corrected even after 20 years.
>>
>
> Just to clarify my POV: I didn't want to criticize anything here, I just
> wanted to point to some previous discussion. In my POV, Either *is*
> intended as a biased sum type (there are tons of more or less standard
> libraries which use it that way), while pairs/tuples are intended as
> unbiased product types.

I agree with that, although somewhat reluctantly about Either.
The original intention (as I remember it, but how reliable is my
memory of meetings nearly thirty years ago?) was to pick a name
for an unbiased sum type and its constructors. We chose Either
and Left and Right (the latter two being names that had been
used in prior languages). It was then noticed that Either
provided what was needed for results that could either be a
success or an error, and Right was the obvious choice for the
success. (,) never had that: it was unbiased, apart from the
inevitable syntactic and evaluation order biases.

I would like to be able to say that (excluding questions of
evaluation order) if one systematically replaced (a,b) with
(b,a) throughout the source of a programme, it would still be
the same programme. I would also like to think that instances
provided in base were “the only sensible instance” (for some
value of sensible). This is the case for Foldable Maybe, maybe
for Either (but not for an unbiased sum) and definitely not for
(,).

-- 
Jón Fairbairn                                 Jon.Fairbairn at cl.cam.ac.uk