Deprecate Foldable for Either

Mon Mar 20 00:29:23 UTC 2017

The criteria for determining this apparent danger, in all cases I have
seen, is arbitrary nonsense. Try defining it in a coherent way.

"But the inconsistency with my intuition for the function named
l-e-n-g-t-h that I learned in C programming school in the 1980s" is the
best criteria that I have seen, and which commands outright dismissal. I
am sceptical of anything better, and therefore, worth considering.

Other contenders:
* feels bad in my bones
* don't like it
* there's lots of us!

All these arguments have been, by moral imperative, and in practice,
dismissed.

The length of ((,) a) is very definitely 1. That this is not bleedingly
obvious, busted intuitions aside, is becoming more laborious as this
not-really-debate drags on.

We have a kind system, we have a type system, we have parametricity. Use
them. If you choose not to, fine, but don't try to take superior tools
away from me for no benefit. I will stop you.

----

An anecdote on danger. A twin piston engine aircraft recently crashed
into a shopping centre on take-off from an airport in southern
Australia. All onboard were killed. Nobody on the ground was injured.
The cause is yet unknown and is under investigation. It is an unusual
circumstance. A friend of mine declared to me that the airport should be
closed. When I asked why, I was told, "I used to work in the same type
of shopping centre, at a different airport, and I felt like I was in
danger when aeroplanes flew overhead." It is extremely fortunate that we
don't have such ridiculous assessments of danger dictating the policies
of aviation safety. Indeed, flying aeroplanes is one very effective way
to get away from that nonsense. Imagine if that "sense of danger" were
to dictate aviation safety regulations. What a dangerous world we would
be living in!

----

Ask yourself where the danger is and be reasonable about it. Hope that
helps.

On 20/03/17 10:30, amindfv at gmail.com wrote:
>
>
> El 19 mar 2017, a las 17:19, Tony Morris <tmorris at tmorris.net
> <mailto:tmorris at tmorris.net>> escribió:
>
>> That you can't should be a hint that length for ((,) a) is very
>> definitely 1.
>>
>
> To be frank, this seems like a non-sequitur. Many of us find e.g.:
>
> length (_, _) = 1
> maximum (_, b) = b
>
> to be dangerous instances. We don't want to accidentally call them.
> We'd like a way to opt out or warn on use.
>
> The inability to opt out or be warned does not seem to me to be an
> argument that the above instances are good instances.
>
> Feel free to clarify if I haven't understood your point.
>
> Thanks!
> Tom
>
>
>> Simply, use a different function, not length, which is well-defined
>> for ((,) a) and other instances.
>>
>> On Mon, Mar 20, 2017 at 3:15 AM, <amindfv at gmail.com
>> <mailto:amindfv at gmail.com>> wrote:
>>
>>     Is there a clear way to implement this instance warning? I.e. given:
>>
>>     f x = 2 * length x
>>
>>     Can we guarantee at compile time that "f" will never be passed a
>>     2-tuple?
>>
>>     Tom
>>
>>     El 18 mar 2017, a las 20:04, Adam Bergmark <adam at bergmark.nl
>>     <mailto:adam at bergmark.nl>> escribió:
>>
>>>     I'm on the fence about the instance existing. I'm +1 for a
>>>     warning, and thus would be +1 on keeping the instance. +1 on
>>>     making the warning opt-in and +1 keeping it out of -Wall.
>>>
>>>
>>>
>>>     On Sun, 19 Mar 2017 at 00:51 <amindfv at gmail.com
>>>     <mailto:amindfv at gmail.com>> wrote:
>>>
>>>
>>>
>>>         > El 18 mar 2017, a las 16:01, Lana Black <lanablack at amok.cc
>>>         <mailto:lanablack at amok.cc>> escribió:
>>>         >
>>>         >> On 18/03/17 19:49, Henning Thielemann wrote:
>>>         >>
>>>         >>> On Sat, 18 Mar 2017, Carter Schonwald wrote:
>>>         >>>
>>>         >>> for what?
>>>         >>
>>>         >> A warning if someone e.g. calls 'length (a,b)', or more
>>>         generally, if
>>>         >> certain instances are used.
>>>         >> _______________________________________________
>>>         >> Libraries mailing list
>>>         >> Libraries at haskell.org <mailto:Libraries at haskell.org>
>>>         >>
>>>         http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries
>>>         <http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries>
>>>         >
>>>         > Please no. Many of us like our code Wall-clean while still
>>>         being able to
>>>         > write polymorphic functions. Adding more warnings that are
>>>         often
>>>         > triggered by correct code (redundant constraints, anyone?)
>>>         only leads to
>>>         > more headache.
>>>         >
>>>         > You could make that an hlint rule on the other hand.
>>>
>>>         Can it be a hlint rule? It seems quite difficult to predict
>>>         that "length" will not ever be passed e.g. a 2-tuple in the
>>>         general case, within hlint.
>>>
>>>         I would also favor a warning, and happily have -Wall not
>>>         include it (though I'd prefer inclusion).
>>>
>>>         Tom
>>>
>>>
>>>         > _______________________________________________
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>>>         > http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries
>>>         <http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries>
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>>>
>>
>>     _______________________________________________
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>>
>>

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