Comment by kragen | Hacker News Reader

Comment by kragen | Hacker News Reader

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i think the belief is rather that the compiler needs to reliably distinguish between -0 and +0 except where it explicitly invokes arithmetic comparison, and so the semantics of both =:= and unification should change to distinguish -0 from +0

i don't know what it would mean to not unify cases where the ieee floating point number had the same representation, assuming that by 'unify' you mean 'paattern-match) (erlang doesn't have full bidirectional unification where run-time data can contain variables which could be instantiated with constants from a pattern). what would the semantics of unification be then? no pattern could ever match a term containing a floating point number?

or perhaps you aren't aware that 'unify' means 'pattern-match' (prolog style) and we are discussing a pattern-matching bug, and by 'unify cases' you just mean 'remove one of two cases'. i think it's very likely that the compiler needs to remove redundant cases that fall out of its optimizer in order to get decent performance, and if the user puts them in their code it's probably a bug

> no pattern could ever match a term containing a floating point number?

No equivalence matching on floating point numbers seems like a much healthier approach, yes. I don't expect people to have huge problems with "x greater than 0.0" or "z less than 16.35" but "p is exactly 8.9" suggests muddled thinking.

The floats are very strange. While machine integers are already a bit weird compared to the natural numbers we learned in school, the floats are much stranger so I expect code which is trying to test equivalence on these values is going to keep running into trouble.

We know that humans, including human programmers, do not appreciate how weird floats really are, so I think telling programmers they just can't perform these matches will have a better outcome than confusing them by allowing this and then it rarely has totally unexpected consequences because a+epsilon was equivalent to b even though mathematically a+epsilon != b

as documented in https://news.ycombinator.com/item?id=35889463 this would prevent you from translating lapack into your ideal language

you have to get rid of order comparisons too, though, or people will just replace a == b with a >= b && a <= b

also it seems like a compiler attempting to determine whether two pieces of code are equivalent (so it can throw one away) needs to be able to test whether two constants in them could ever produce different computational results; this is important for code-movement optimizations and for reunifying the profligate results of c++ template expansion

similarly, a dataflow framework like observablehq needs to be able to tell if an observable-variable update needs to propagate (because it could change downstream results) or not; for that purpose it needs to even be able to distinguish different nans. like the compiler, it needs the exact-bitwise-equality relation rather than ordinary arithmetic equality

where i think we agree is that floating-point arithmetic is probably a bad default for most people most of the time because it brings in all kinds of complexity most programmers don't even suspect

your comment reads to me as 'i don't understand floating point equality and therefore no human does so no human should have access to it'

but there are more things in heaven and earth than are dreamed of in your philosophy, tialaramex

My fear isn't about floating point equality but about equivalence. As we discussed repeatedly, nothing changed for equality, -0.0 = 0.0 just as -0 = 0 in integers. Equality works fine, though it may be surprising in some cases for floating point values because sometimes a != a and b + 1 == b.

"Exact-bitwise-equality" is yet a further different thing from equivalence. Is this what Erlang's =:= operator does? Erlang's documentation described it as "Exactly equal to" which is a very silly description (implying maybe == is approximately equal to), presumably there's formal documentation somewhere which explains what they actually meant but I didn't find it.

Presumably Exact-bitwise-equality is always defined in Erlang? In a language like Rust or C++ that's Undefined in lots of cases so this would be a terrible idea. I still think it's better not to prod this particular dragon, but you do you.

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