From: Joel VanderWerf <vjoel@...>
Date: 2006-02-08T00:16:20+09:00
Subject: Re: lazy evaluation?

Martin DeMello wrote:
> Could someone explain why this code works:
> 
>     def repeat(condition)
>       puts "condition: #{condition}"
>       yield
>       retry if not condition
>     end
> 
>     j=0
>     repeat (j >= 10) do
>      puts j
>      j+=1
>     end
> 
>     puts "after loop, j = #{j}"
> 
> I'd have expected repeat (j >= 10) to pass "false" into the method,
> which would then yield repeatedly to the do/end block, never seeing the
> (condition) part again.
> 
> martin

What confused me about this is that retry has two meanings, one in the
context of a loop or block, and another in the context of begin...end
(inside a rescue clause). From PickAxe v2:

> break terminates the immediately enclosing loop--control resumes at
> the statement following the block. redo repeats the loop from the
> start, but without reevaluating the condition or fetching the next
> element (in an iterator). The next keyword skips to the end of the
> loop, effectively starting the next iteration. retry restarts the
> loop, reevaluating the condition. [p.330]
        ^^^^^^^^^^^^^^^^^^^^^^^^^^
        Apparently, this includes the arguments of the method, in
        the case of an iterator method called with a block.

> The retry statement can be used within a rescue clause to restart the
> enclosing begin/end block from the beginning. [p. 347]

Which interpretation is used can depend on whether a block is present:

$ cat thrice.rb
def thrice(x)
  @count = 0 if !@count || @count >= 3
  @count += 1
  retry if @count < 3
end

thrice puts("foo") do end
thrice puts("foo")

$ ruby thrice.rb
foo
foo
foo
foo
thrice.rb:4:in `thrice': retry outside of rescue clause (LocalJumpError)
        from thrice.rb:8


So there's no need to worry about object.foo(ary.pop) reevaluating its
argument. That can only happen if a block is present, in which case the
caller should be aware of any "control structure" semantics that #foo
might have.

-- 
      vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407