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
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.
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