Question

This code does not perform as I expect:

case    
when -> { false } then "why?"        
else "This is what I expect"        
end      
# => "why?"

Neither does this

case
when ->(x) {false} then "why?"  
else "This is what I expect"
end  
# => "why?"

The first then clause is executed in both cases, which must mean that the lambda I supply to the when clause is not being called. I understand that the case equality operator === should be called on whatever the subject of the when clause is. I am wondering what goes on the other side of the === when there is no argument supplied to case. I was thinking it might be nil, but it can’t be:

-> {false} === nil
# => ArgumentError: wrong number of arguments (1 for 0)

->(x) {false} === nil
# => false

This performs as expected and, if it were being executed, would lead to my expected case results or an exception. Can someone explain the results above? It seems that the case equality operator isn’t being used at all, and yet the first when clause is evaluating to true. BTW, I am doing this because the output of a case can be used for variable assignment and it is less wordy then having several elsif clauses. I would like to be able to use arbitrary Procs in a case statement with no argument.

Answer 1

case    
when -> { false } then puts "why?"        
else puts "This is what I expect"        
end

case    
when 'cat' then puts "why?"        
else puts "This is what I expect"        
end

case    
when -> { false }.call then puts "why?"        
else puts "This is what I expect"        
end

outputs:

why?
why?
This is what I expect

As The Pickaxe ( http://pragprog.com/book/ruby3/programming-ruby-1-9 ) says, there are two forms of case statement.

The first allows a series of conditions to be evaluated, executing
code corresponding to the first condition that is true: case when ‹
boolean-expression ›+ ‹ then › …

The second form of a case expression takes a target expression
following the case keyword. case target when ‹ comparison ›+ ‹ then ›
…

In your case (case without target) any expression that is not false or nil (such as a Proc or the string ‘cat’) evaluates to true. The Proc is not executed, unless you call it.