What do you assume when you see fromListN in a library?

Sat May 23 23:25:00 UTC 2020

I'm advocating that fromListN should be able to do basically anything if
you violate its contract. It is in GHC.Exts, it isn't in a place marked
Safe or Trustworthy, and it is designed to make _fast_ literals. Anything
that compromises that mission to do other things, seems ill considered to
me, as then you'd have to make the fast thing anyways under another and
would lose the ability to get the notation which was the very reason why
the class was created in the first place. When used by GHC for desugaring
overloaded list literals fromListN *never* has its contract violated.

If you want something safer for manual use where the length is a mere hint,
build something safer, then put it in a subclass of IsList. There there are
multiple semantics you might want. Do you truncate the list if it is longer
than the number of elements supplied? Do you give fewer elements than the
number specified if not enough are given to you? Do you just throw an error
on length disagreement? Either way you are counting down twice, once on the
list, once on the counter, doing potentially twice as much work as you go,
possibly walking the list twice like how fromList can delegate to fromListN
by computing a length.

Giving the hint-at-best, crash-at-worst version that at least doesn't
segfault its own method name means fromListN and overloaded list literals
do not pay for extra protection they do not need, and you can grab your
safer version, which unlike fromListN could actually be (re)exported from a
Trustworthy module along with the safe parts of IsList. The nice thing
about this is that by importing that module the user could use your safer
version, and use of OverloadedLists syntax doesn't care about fromListN
being in scope.

The other caveat I had was just that I don't like the fact that IsList
mashes toList and fromList together in one class, preventing its use in the
case where a list is just one case of several, but this is completely
orthogonal to the issue at hand, and if it were ever to be solved would
need an issue/proposal in its own right.

-Edward

On Thu, May 21, 2020 at 6:53 PM Carter Schonwald <carter.schonwald at gmail.com>
wrote:

> Ed, could you clarify what perspective you’re advocating for or against?
>
> On Thu, May 21, 2020 at 3:38 PM Edward Kmett <ekmett at gmail.com> wrote:
>
>> If you want to make a subclass that exists just to claim those stronger
>> guarantees, fine, but asking existing consumers of this API to give up
>> performance and the invariant that the length of the list matches the
>> number given, is something I'm very strongly against.
>>
>> It seems that the thing that is requesting the extra functionality should
>> pay for it via subclassing and either providing another method, or
>> incurring extra laws for the superclass method. I have a general preference
>> for making the operation with the extra laws have another name and be an
>> inhabitant of the subclass, though, because then users aren't hoist on the
>> horns of the dilemma of which semantics they want their fromListN to have.
>> They don't have to choose between speed and being able to implement the
>> subclass.
>>
>> Mind you IsList is a terrible class. I _really_ wish toList could be
>> allowed to fail, or could be moved to a separate class, then you could use
>> it in syntax trees where one of the cases was list like and what not. We
>> can use fromString in all sorts of places where fromList cannot be used
>> because of this self-correcting embedding/projection-like constraint.
>>
>> -Edward
>>
>> On Mon, May 18, 2020 at 2:59 PM Carter Schonwald <
>> carter.schonwald at gmail.com> wrote:
>>
>>> The problem is that some implementations will silently truncate data,
>>> and while the original reason for the interface was desugaring list
>>> notation to a different data structure, it’s now used for other things.
>>>
>>> Frankly, the main performance win was meant to be with respect to single
>>> round of data structure allocation/ associated fragmentation.  For a
>>> statically known list.
>>>
>>> On Sun, May 17, 2020 at 7:45 AM Andreas Abel <andreas.abel at ifi.lmu.de>
>>> wrote:
>>>
>>>> A question would be how to validate the given length without
>>>> performance
>>>> penalty.  The existence of fromListN in addition to fromList is
>>>> **only**
>>>> justified by performance consideration, namely that the length of the
>>>> list is known and does not have to be computed.
>>>>
>>>> In general, it seems that the given length has to be trusted, however,
>>>> there might be implementations that can validate the length parameter
>>>> as-you-go without additional cost (in the good case that the length is
>>>> correctly given).
>>>>
>>>> On 2020-05-15 21:37, Carter Schonwald wrote:
>>>> > validating would *prevent inconsistent data*.
>>>> >
>>>> > it is precisely the issue that current semantics are *not* consistent
>>>> > across that needs to be addressed!
>>>> >
>>>> > On Thu, May 14, 2020 at 6:38 PM Joseph C. Sible <
>>>> josephcsible at gmail.com
>>>> > <mailto:josephcsible at gmail.com>> wrote:
>>>> >
>>>> >     On Thu, May 14, 2020 at 10:41 AM Carter Schonwald
>>>> >     <carter.schonwald at gmail.com <mailto:carter.schonwald at gmail.com>>
>>>> wrote:
>>>> >      > My inclination is we change the semantics of fromListN to be
>>>> >     strictly validating with an error when the length is wrong. This
>>>> is
>>>> >     the most consistent and humane of options.
>>>> >
>>>> >     I disagree that validating would be consistent. Look how common
>>>> the
>>>> >     phrases "the precondition is not checked" and "violation of this
>>>> >     condition is not detected" are in the containers library and so
>>>> many
>>>> >     others on Hackage.
>>>> >
>>>> >     Joseph C. Sible
>>>> >
>>>> >
>>>> > _______________________________________________
>>>> > Libraries mailing list
>>>> > Libraries at haskell.org
>>>> > http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries
>>>> >
>>>> _______________________________________________
>>>> Libraries mailing list
>>>> Libraries at haskell.org
>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries
>>>>
>>> _______________________________________________
>>> Libraries mailing list
>>> Libraries at haskell.org
>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries
>>>
>>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/libraries/attachments/20200523/db211a79/attachment.html>