What distinguishes a flip-flop from a range?

The syntax is the same: expression ‘..’ expression (end-inclusive range) or expression ‘…’ expression (end-exclusive range). A range is considered a flip-flop operator if any of its bound is not an integer literal and if it is used as a condition of the following expressions:

• condition ‘?’ expression ‘:’ expression
• statement ‘if’ condition
• statement ‘unless’ condition
• statement ‘while’ condition
• statement ‘until’ condition
• ‘if’ condition then statements if-tail ‘end’
• ‘unless’ condition then statements else-opt ‘end’
• ‘while’ condition ‘do’ statements ‘end’
• ‘until’ condition ‘do’ statements ‘end’

For example:

irb(main):001:0> 1 if TRUE..FALSE 
=> 1

In addition, parenthesized ranges used in a condition are also flip-flops, e.g.:

irb(main):001:0> 1 if (TRUE..FALSE) 
=> 1

‘(‘ statements ‘)’ is a block expression with a single statement expression TRUE..FALSE. If there are more statements in the block in front of the range it is no longer a flip-flop operator:

irb(main):008:0> 1 if (puts;TRUE..FALSE) 
ArgumentError: bad value for range 

The exception is raised since a range bounds cannot be Booleans.

Any number of nested block expressions works:

irb(main):001:0> 1 if ((((TRUE..FALSE)))) 
=> 1 

And begin-end blocks work as well:

irb(main):032:0> 1 if begin TRUE..FALSE end 
=> 1 

As do any combinations of the two (but only in Ruby 1.9, in Ruby 1.8.6 some combinations don’t work, which I consider a bug):

irb(main):001:0> 1 if (begin (((begin begin TRUE..FALSE end end))) end) 
=> 1 

Also, ranges used in Boolean expressions used as conditions are considered flip-flops:

irb(main):018:0> 1 if t(1)..f(2) and not t(3)..f(4) or t(5)..f(6) 
123456=> 1 
irb(main):018:0> 1 if (t(1)..f(2)) && !(t(3)..f(4)) || (t(5)..f(6)) 
123456=> 1 

where t and f are functions that print the argument and return true and false respectively. The second example also doesn’t work in Ruby 1.8.6, oups.

Let’s summarize what have we just found out: block expressions (parenthesized or begin-end) containing a single range and Boolean expressions propagate the “in-condition” property to their children AST nodes. The “in-condition” property then turns ranges into flip-flop operators. Other nodes don’t propagate it. A method call doesn’t, for example:

irb(main):018:0> 1 if puts(t(1)..f(2)) 
ArgumentError: bad value for range 

Side note: this property is also applied on regular expressions. If a regex is “in-condition” it is compiled as a match against $_ variable:

irb(main):029:0> $_ = 'xz' 
=> "xz" 
irb(main):030:0> 1 if /y/ or ((begin((/x/ and /(z)/))end)) 
=> 1 
irb(main):031:0> $1 
=> "z" 

How does flip-flop work?

We know from the books that flip-flop would probably define some state variable that changes values as the expressions of the operator are evaluated. To figure out how exactly it works I wrote a simple script:

F = false
T = true
x = X = '!'

B = [F,T,x,x,x,T,x,F,F]
E = [x,x,F,F,T,x,T,x,x]
       
def b
  step('b',B)
end

def e
  step('e',E)
end

def step name,value
  r = value[$j]
  puts "#{$j}: #{name} -> #{r.inspect}"
  
  $j += 1
  
  $continue = !r.nil?  
  r == X ? raise : r  
end

$j = 0
$continue = true
while $continue
  if b..e 
    puts "#{$j}: TRUE" 
  else
    puts "#{$j}: FALSE" 
  end
end

In this code we evaluate an end-inclusive flip-flop in a loop. Each time the flip-flop operator is evaluated ‘b’ or ‘e’ method is called, or both methods are called. Global variable $j is an index into B and E arrays defined at the top. The arrays define a sequence of values the methods b and e should return. If x value (‘!’) is to be returned an exception is thrown. This way we can track what the automaton behind the flip-flop operator does. Using the output of the script we can easily deduce how the automaton could look like:

0: b -> false
1: FALSE
1: b -> true
2: TRUE
2: e -> false
3: TRUE
3: e -> false
4: TRUE
4: e -> true
5: TRUE
5: b -> true
6: TRUE
6: e -> true
7: TRUE
7: b -> false
8: FALSE
8: b -> false
9: FALSE
9: b -> nil
10: FALSE.

It has four states: BEFORE, INSIDE, AFTER and a lambda state. In each state b or e expression is evaluated and the automaton transitions to another state based upon the result: true (T) or false (F). If it transitions to INSIDE or AFTER state the flip-flop operator returns true. If it transitions to BEFORE state the operator returns false. If it transitions to the lambda state the operator evaluates b and does one more transition.

The previous automaton works for an end-inclusive flip-flop operator. The one bellow does for an end-exclusive operator – the lambda state is gone:

What code does IronRuby emit?

We emit the following code for end-inclusive flip-flop operator {begin}..{end}:

if state || IsTrue({begin})
  state = IsFalse({end})
  true
else  
  false
end  

The state variable is a Boolean and it’s true iff the automaton is in INSIDE or lambda state. In both states this means evaluate {end} expression, transition to the state given by negated result and return true. If the state variable is false we are either in AFTER or in BEFORE state. In any case {begin} is evaluated. If it returns false we go to/stay in BEFORE state and the result of the flip-flop operator is false. Otherwise we go to the lambda state and evaluate end.

Similarly, the end-exclusive flip-flop operator {begin}…{end} is implemented as

if state
  state = IsFalse({end}) 
  true
else
  state = IsTrue({begin})
end

The state variable is allocated in the inner-most method scope. A flip-flop operator used in a block preserves the state across multiple calls to the block:

def y *a; yield *a; end

def test
  $p = proc { |b,e|
    puts b..e ? TRUE : FALSE
  }
  
  y false, &$p  
  y true, true, &$p
  y false, &$p
  y true, false, &$p
end

test

Output:

false 
true 
false 
true 

The method that transforms IronRuby’s RangeExpression node into DLR AST in the case the range is a flip-flop operator looks like:

private MSA.Expression/*!*/ TransformReadCondition(AstGenerator/*!*/ gen) {
    // Define state variable in the inner most method scope.
    var stateVariable = gen.CurrentMethod.Builder.DefineHiddenVariable("#in_range", typeof(bool));

    var begin = Ast.Box(_begin.TransformRead(gen));
    var end = Ast.Box(_end.TransformRead(gen));

    if (_isExclusive) {
        return Ast.Condition(
            stateVariable,
            Ast.Comma(Ast.Assign(stateVariable, Methods.IsFalse.OpCall(end)), Ast.True()),
            Ast.Assign(stateVariable, Methods.IsTrue.OpCall(begin))
        );  
    } else {
        return Ast.Condition(
            Ast.OrElse(stateVariable, Methods.IsTrue.OpCall(begin)),
            Ast.Comma(Ast.Assign(stateVariable, Methods.IsFalse.OpCall(end)), Ast.True()),
            Ast.False()
        );
                          
    }
}

This code can be found in Ruby\Compiler\Ast\Expressions\RangeExpression.cs. Propagation of “in-condition” property to AST nodes is performed by ToCondition virtual method overridden by RangeExpression, RegularExpression, AndExpression, OrExpression and BodyExpression and called from parser (Ruby\Compiler\Parser\Parser.y).