Order-of-operations confusion when using augmented assignment and length operator

[Grissess]

Hello!

I was just bitten by a curious little code generation bug: when using an augmented assignment with the length operator, Lua is generated that doesn't follow the order of operations. It's a bit odd, because the generated Lua does follow the order of operations if the length operator isn't involved. Here's a minimal reproduction:

count, minuend = 3, 2
count -= minuend + 1
print count

count, datum = 3, "ab"
count -= #datum + 1
print count

When using MoonScript version 0.5.0, moonc generates this:

local count, minuend = 3, 2
count = count - (minuend + 1)
print(count)
local datum
count, datum = 3, "ab"
count = count - #datum + 1
return print(count)

which, in Lua 5.4.4 Copyright (C) 1994-2022 Lua.org, PUC-Rio, prints this when interpreted:

0
2

This behavior is a bit unexpected, as assignment (including augmented assignment) is supposed to have the lowest precedence in ANSI C (apologies for the C++ ref, but the same table is in my book), and I assume the operators here introduced are supposed to be semantically compatible with that interpretation. Indeed, if that's the case, the count = count - #datum + 1 is incorrect by the standard, and it's in general safe to always convert LHS $= RHS to LHS = LHS $ (RHS) for all operators $.

[arch1t3cht]

The same happens with other operators like *=, i.e. a *= #b + 1 compiles to local a = a * #b + 1 rather than local a = a * (#b + 1) on 0.6.0.

The last time I looked into this the issue was that #b + 1 actually parses as #(b + 1), which makes the update assignment operator not parenthesize the second operand:

> ./moonc -T /dev/stdin /dev/stdout
a *= #b + 1
{[1] "update", {[1] "ref", "a"}, "*=", {[5] "length", {"exp", {[7] "ref", "b"}, "+", {[10] "number", "1"}}}}

[pigpigyyy]

If you are using the cli tools to compile MoonScript. You can get the fix in YueScript as a MoonScript successor.

[Grissess]

local count, minuend = 3, 2
count = count - (minuend + 1)
print(count)
local datum
count, datum = 3, "ab"
count = count - (#datum + 1)
return print(count)

0
0

Aye, Yuescript does seem to get this one right! Thanks, @pigpigyyy :)

Of course there's no reason not to backport this to Moonscript, too...