Ruby:
true == true == true syntax error, unexpected tEQ
vs. JavaScript:
true == true == true // => true vs. C:
1 == 1 == 1 // => 1 
3 Answers
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).
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) # => trueor dot operator
.(can be omitted for the last equality check in a row):true .== true == true # => true
5 Comments
Cary Swoveland
I would be interested in knowing why, but I think that's beyond the scope of the question and your answer.
Stefan
@CarySwoveland maybe because
1 == 1 == 1 would evaluate to false in Ruby if == was left-associative, which would probably result in several bugs. And it wouldn't make much sense either – the only reasonable values for the 3rd operand are true and false, i.e. a == b == true and a == b == false which can be expressed as a == b and a != b.
souslov
@Stefan: there is only one situation when it can be used like this in a way that it would make sense, though I must say that the example is kinda far-fetched:
== can be overriden in a custom class and thus equality check can return something other than just true or false (for example, nil)
Arye Eidelman
javascript-truthiness-in-boolean-to-numbers-comparison why we should all stop using 0's or 1's in javascript examples comparing integers with integers or booleans
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
Comments
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 
==,===, and!=) that do this. Even<and>parse correctly and then produce a runtime error like you would expect. Plus, the only sources I can find which claim to have a complete grammar for Ruby would seem to indicate that this syntax is allowable.true .== true .== true==is listed as not-associative (A=N) in the answer, meaning such aX==Y==Zproduction is invalid (associativity is what "adds the implicit parenthesis around operators of the same precedence"). There are many links in the question that might go back to a more "Official Source" that could be cleanly cited. (This question is more of less about a specific subset/application of grammar rules in that question/answer.)<and friends are left-associative.. so should parse (and "work", given valid runtime inputs). I'm not sure what the grammar rational about why<would be associative while==would not be, although changing that might break lots of expectations..