Re: ? assignments are not expressions ?

[Reuben Thomas <rrt@...>]

> Personally, I think all expressions should return values. To me, Lua feels a
> lot like scheme, except that expressions don't return values. Another thing
                               ^^^^^^^^^^^
Surely "assignments"?

> you could do if assignments returned values is:
>
> x = y = z = 0

I agree that it's a pity that not everything returns a value (but it's only
worth doing if all statements return a value e.g. the value of an
if...then...else...end should be the value of whichever branch of the if is
executed; the value of a block of statements is the value of the last
statement; the value of a while or for block is the value of the last
iteration &c.

> > I don't really agree. You just have to use some self-discipline. And if
> > you don't like the costruct, you're not forced to use it... But you
> > can't deny that
> >
> > while ( X = read() )
> > do
> > print ( X )
> > end
> >
> > is by far nicer and CLEARER than
> >
> > X = read()
> > while X
> > do
> > print ( X )
> > X = read()
> > end

But I'm still worried about allowing assignments everywhere, because it can
indeed introduce subtle bugs (although using := for assignment and = for
equality would make life a lot better, because then the bugs become much
less obscure (and are often just syntax errors)).

The nastiness of the code above is really an artefact of not having loops in
which you can test the condition anywhere. You could rewrite it:

while 1 do
  X = read()
  if not(X) then break end
  print (X)
end

which would look nicer as:

repeat
  X = read()
  while X
  print (X)
end

...but of the languages I know, only Forth allows this sort of construct
(and it's forced to, because of postfix evaluation). This to my mind would
be a much nicer addition to the language: you can write while...endwhile
loops with it:

repeat
  while foo
  ...
end

repeat...until loops:

repeat
  ...
  while foo
end

and in-between loops, as above. You could even have multiple whiles (because
the semantics of while is just "while cond" = "if not (cond) then break
end"). You can of course have unending loops by having no while or break.
And the current while syntax could be retained as syntactic sugar:

while foo do --> repeat while foo

although there is a subtle issue that the current syntax is effectively
while foo <block>, whereas it translates to something that pushes code
inside the <block>. Fortunately, you can't current write while foo
<statement>, so there's no real problem (also, blocks of statements are not
first class, so you can't have while foo b, where b is a variable whose
value is a block).

Anyway, just my heap o' change.

-- 
http://sc3d.org/rrt/
L'art des vers est de transformer en beautés les faiblesses (Aragon)