Why does a method call need to be disambiguated when it can in principle be a constant?

Question

Method calls can usually omit the receiver and the parentheses for the arguments:

def foo; \"foo\" end
foo # => \"foo\"

In the case abov

Answer 1

The set of local variables which is in scope at any given point in the program is defined lexically and can thus be determined statically, even as early as parse time. So, Ruby knows even before runtime which local variables are in scope and can thus distinguish between a message send and a local variable dereference.

Constants are looked up first lexically, but then via inheritance, i.e. dynamically. It is not known which constants are in scope before runtime. Therefore, to disambiguate, Ruby always assumes it's a constant, unless obviously it isn't, i.e. it takes arguments or has a receiver or both.