Re: Named loops and blocks for use with break

[Rici Lake <lua@...>]

On 6-Oct-05, at 5:59 AM, Doug Rogers wrote:

> Thank you for sharing your patch. I like it, though I think some of the
> iteration controls are hard to read - especially the ones that perform 
> a
> 'continue' operation. With time I'm sure I would grow more comfortable
> with them. I'd prefer 'break when' and 'continue when'.

Well, you're used to 'break' and 'continue'. But those are very 
difficult to explain to novice programmers, particularly 'continue'. 
Indeed, I think many programmers feel just a little uncomfortable with 
'continue' because they don't really understand its full semantics, and 
hence avoid using it.

Shivers' vocabulary strikes me as well thought-out, at least for 
English language speakers.

Here's the little example, again:

function command_shell(stream, handlers, endword)
  for line in stream:lines() do
    until line == endword
    unless line = line:gsub("^%s*", "")
      satisfies line == "" or line:find("^#")
    local cmd, rest = line:match("(%S+)%s*(.*)")
    if h = handlers[cmd] satisfies h then
      h(argsplit(cmd, rest))
    else
      print("Unrecognized command "..cmd)
    end
  end
end

Here, "until" is pretty clear: stop looping if the condition is true. I 
think "unless" is also pretty clear: don't do the rest of this loop if 
the condition is true. (I'm not sure that adding the satisfies clause 
is any improvement to the original, but it was free so I thought I'd 
try it.)

Since the guards are written in strict linear order, they are easy to 
explain. The loop always starts at the top and works down to the end. 
Contrast that with the confusion over whether 'continue' does or does 
not execute the tailguard in "repeat ... continue ... until cond"

But maybe that's just my perspective.

On the other hand, they don't read well if you want to terminate a 
surrounding loop.


>  The 'when'
> keyword could then be used to support a 'case' statement 
> implementation.
> And now we're off on another grand exploration!