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. 
> Mark 

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