Why does the expression (true == true == true) produce a syntax error?

Association direction, which controls the order of operators having their arguments evaluated, is not defined for the == method, same as for ===, !=, =~ and <=> methods as well (all of which have the same precedence and form a separate precedence group exclusively).

Documentation

Thus evaluation order in case of multiple operators from the list mentioned above being chained in a row should be set explicitly via either

• parenthesis ():

  (true == true) == true # => true
  true == (true == true) # => true
  

• or dot operator . (can be omitted for the last equality check in a row):

  true .== true == true # => true
  

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

The first answer is excellent, but just in case it's not completely clear (and people asking why), here are few more examples.

In C, the == operator is left-to-right associative and boolean is represented as 1 (true) and 0 (false), so the first 1 == 1 evaluates to 1 (true) and then you are evaluating the result of first expression with the second. You can try:

2 == 2 == 2 // => 0

Which in C, is evaluated as:

(2 == 2) == 2
1 == 2 // => 0

In Javascript, similarly to C, == is left to right associative. Let's try with 0 this time (although the same example from C would work as well):

0 == 0 == 0
false

Again:

0 == 0 == 0
true == 0 // => false

In Ruby == does not have associative properties, ie. it can't be used multiple times in single expression, so that expression can't be evaluated. Why that decision was made is a question for the author of the language. Further, Ruby doesn't define numeric 1 as a boolean, so 1 == true evaluates to false.

The second answer states there are some "weird" cases in Ruby, but they all evaluate as expected:

(1 == 1) == 1
true == 1 # => false

1 == (1 == 1)
1 == true # => false

1 .== 1 == 1
(1 == 1) == 1
true == 1 # => false

false .== false == true
(false == false) == true
true == true # => true

false .== true == false
(false == true) == false
false == false # => true

true .== false == false
(true == false) == false
false == false # => true

false .== false == false
(false == false) == false
true == false # => false

true .== true == false
(true == true) == false
true == false # => false