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

Question

Ruby:

true == true == true

syntax error, unexpected tEQ

Answer 1

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

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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

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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

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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