Why does the [removed]true == true == true) produce a syntax error?

Question

Ruby:

true == true == true

syntax error, unexpected tEQ

Answer 1

TL;DR The syntax implies that all 3 values are equal this is not what it does in javascript or C, so by ruby giving a syntax error the door is open for this to be implemented in the future.

If I understand the question correctly value_a == value_b == value_c should only return true if they are all equal using == as the comparison operater as shown in this method

# version 1
def compare_3_values(a, b, c)
  a == b && a == c && b == c
end

there is another possible expected outcome though. to implement this as shown in the previous answer:

#version 2
def compare_3_values(a, b, c)
  (a == b) == c
end

The results are worlds apart.

JavaScript always uses version 2 which is pretty useless as the 3rd item is always being compared against true or false (0 or 1 if the 3rd item is an integer) that's why false == false == true returns true.

The good news is that because ruby gives a syntax error it's the only language that can implement this without breaking everyone's code.

for any other language it would break so much code that even if it were implemented in a later major version there would need to be a flag/setting to turn this on or off for years to come, hence it will never be worthwhile.

Some interesting results in Ruby

false .== false == true
=> true

false .== true == false
=> true

true .== false == false
=> true

false .== false == false
=> false

true .== true == false
false

And in javascript

false == false == true
=> true

false == true == false
=> true

true == false == false
=> true

false == false == false
=> false

true == true == false
=> false

Edit tested in C as well, acts similar to JavaScript in that it compares the result of the first two values against the third value