Real.toString ignores the sign of zero

[p-ouellette]

val _ = print (Real.toString ~0.0 ^ "\n")

This prints "0". I expected it to print "~0", as SML/NJ does.

[MatthewFluet]

It seems that it does so purposefully; the code for Real.toString (which uses the GEN 12 adaptive notation) is at:
https://github.com/MLton/mlton/blob/master/basis-library/real/real.sml#L561
The determination of the sign is via x < 0.0 and floating-point comparisons ignore the sign of zero. This code for the adaptive notation is taken straight from the Basis Library specification:
https://smlfamily.github.io/Basis/string-cvt.html#SIG:STRING_CVT.realfmt:TY
(See the code under Discussion at the bottom of the page.)

But, maybe there is precedent in other languages. And, I'm not sure what happens with the other real format specifiers.

[fweimer]

Java, Python, contemporary C implementations (and thus Lua), C++ implementations, Javascript all print -1e-308 / 1e308 as some variant of -0.0. I'm surprised about how uniform it is. (Not sure if it's part of the referenced numerics standards for C and C++, or if it's just wide-spread implementation practice.)