Unary minus operation with variables

[vitoyucepi]

Describe the bug
Liquidsoap doesn't support unary minus with variables.

To Reproduce

a = 1
b = -a
if a == -b then
  print(a+-b)
end

Expected behavior
Unary minus supported.

Version details

• OS: Debian in docker
• Version: 2.2.1, b2a6924

Install method
savonet/liquidsoap:rolling-release-v2.2.x

Common issues
N/A

[vitoyucepi]

Also -nan and -infinity.

[toots]

Thanks for reporting this. I revisited the way we do it in #3399 and it's not satisfactory.

The problem we have is that there is ambiguity in the language. We accept implicit term separation, so for instance you can write:

x + 1
(1, 2, 3)

Without a ; separating them like most language do.

(It is worth noting that we also support:

x + 1;
(1, 2, 3)

but, I guess, nobody every uses that..)

Morally, what we really should mean is that the line return is the term separator. However, for technical reasons, this is not how it's implemented: our current parser does not see line returns at all.

This creates ambiguity, for instance:

# Just x alone
x
# The number -3.14
-3.14

is seen the same way as:

# x minus 3.14
x - 3.14

The current work around is to distinguish between:

1. <integer, float, variable name or an expression terminated by )> - <anything>
2. <anything else> - <integer or float or (expression)>.

The meaning for each is then:

1. <integer, float or an expression terminated by )> <minus> <anything>
2. <anything else> <negative integer or float or (expression)>.

Examples:

# 1 minus 2
1 - 2

# x minus 2
x - 2

# application function minus 2
f(x) - 2

# y equals the number -1
y = -1

# This fails because we don't know how to parse it. There is an ambiguity in the grammar
y = -x

# This is the workaround:
y = -(x)

This is clearly a hack and we need to do better. We could keep adjusting the workaround but, the best is to add support for line return as term separator, which I will have a look at soon I hope.