A compile-time error is definitely possible. Haskell has it, but it uses
the full power of type classes.

To catch it at compile time, we'd either need to treat equality like
comparable, which sucks for user-defined types, or do something that looks
more like Haskell's type classes, with a bunch of automatic deriving. I'd
advocate the latter, although I think it could be done in a way that hides
most of the typeclassy stuff from the user.

On Wed, Oct 12, 2016 at 12:59 PM, Kasey Speakman <kjspeak...@gmail.com>
wrote:

> For context for future readers, this has to do with animation easing
> functions from another thread
> <https://groups.google.com/d/topic/elm-discuss/bOAHwSnklLc/discussion>. I
> have not constructed such a library, so I won't comment.
>
> On Wednesday, October 12, 2016 at 2:43:58 PM UTC-5, Zinggi wrote:
>>
>> I also think that comparing functions should just compare them by
>> reference, this should cover the most common cases.
>>
>> Another possible solution: What about a compile time error? Is this
>> possible?
>>
>> On Wednesday, 12 October 2016 18:48:12 UTC+2, Mark Hamburg wrote:
>>>
>>> As discussed elsewhere, the runtime exception related to function
>>> equality comparisons is a lurking time bomb for code that is really only
>>> addressed on a multi-developer project by being paranoid both about
>>> functions in data structures and use of the equality operator. Both points
>>> of paranoia get in the way of writing "good" code. There are other
>>> arguments around things like serialization for avoiding using functions in
>>> some places, but there are lots of other places where it is extremely
>>> useful to put functions in data structures. (Imagine writing an effects
>>> manager if it couldn't store functions in its data structures.)
>>>
>>> Full equality tests are, as I understand it, undecidable. That doesn't
>>> mean, however, that some limited analysis can't prove equality in most of
>>> the conditions that matter. In fact, the existing code doesn't always throw
>>> an exception. It only throws an exception if an "allocation identity" test
>>> fails. One could extend this to looking at the "code pointer" and
>>> allocation identity for the closure values and probably get many of the
>>> remaining cases that mattered. It's just a matter of how far one wants to
>>> push when trying to prove or disprove equality and I'm not arguing for a
>>> particular limit here.
>>>
>>> Rather, I think the central issue is around what happens when we give
>>> up. At that point, there are two choices: throw a runtime exception as
>>> happens now or return false. As mentioned above and discussed at further
>>> length elsewhere, the runtime exception leads to paranoid coding. What are
>>> the issues with returning false? The big problem with returning false is
>>> that this means that some compiler optimizations might then change cases
>>> that returned false without the optimizations into cases that returned true
>>> and people become understandably uncomfortable with the possibility that
>>> compiler optimizations could change the meaning of programs. And with that,
>>> I've more or less wanted a "return false" answer but convinced myself that
>>> throwing an exception wasn't unjustified and that maybe what we needed
>>> instead was type system support to avoid the issue.
>>>
>>> But then this morning's discussion of the virtual DOM had me realizing
>>> that runtime optimizations around when the render-diff-patch algorithm gets
>>> run create significant potential for variable program behavior. We embrace
>>> these optimizations even though they introduce non-determinism. Now, its
>>> non-determinism at the level of the physical DOM rather than the values
>>> produced in Elm itself — i.e., runtime v linguistic non-determinism — but
>>> its non-determinism in one of the most fundamental parts about how Elm is
>>> generally used so that distinction seems somewhat arbitrary.
>>>
>>> I think Elm makes the right call on the DOM. Maybe it should log cases
>>> where the DOM logic can recognize that there might be a problem, but I
>>> think this is overall a tradeoff worth making.
>>>
>>> But by the same argument, I think Elm programs would be better if the
>>> runtime exception for function equality comparisons was replaced with a
>>> false result even though it might actually be true. And if it's really
>>> irksome, then it could generate a log statement as well.
>>>
>>> Mark
>>>
>>>
>>>
>>> --
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