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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