[erlang-questions] Versioned variable names

[Paul Mineiro]

fwiw, i cleaned this up so it could handle an arbitrary number of
assignments, and released to google code.

http://code.google.com/p/fwtemplates/downloads/list?can=2&q=lyet&colspec=Filename+Summary+Uploaded+Size+DownloadCount

the original example from the poster now looks like:

------

MO5 = lyet:lyet(
  % Take away underscore and replace with hyphen
  MO = re:replace(MO, "_", "-", [{return, list}, global]),
  MO = toupper(MO),
  % Replace zeros
  MO = re:replace(MO, "RX0", "RXO", [{return, list}, global]),
  % Insert hyphen if missing
  case re:run(MO, "-", [{capture, none}]) of
    nomatch ->
      insert_hyphen(MO);
    match ->
      MO
  end
),

------

cheers,

-- p

On Tue, 9 Jun 2009, Paul Mineiro wrote:

> I took Ulf's ct_expand parse transform and modified it to provide let like
> behaviour.  Something like
>
> --------
> -module (testlyt).
> -compile ({ parse_transform, lyt }).
> -compile (export_all).
>
> g (X) -> X + 1.
> h (X) -> X + 2.
> l (X) -> X + 3.
> m (X) -> X + 4.
>
> f (X) ->  lyt:lyt (X = g (X), begin
>     lyt:lyt (X = h (X), begin
>       lyt:lyt (X = l (X), begin
>         m (X)
>       end)
>     end)
>   end).
> --------
>
> should work (let is a reserved keyword so i used lyt).  It produces
> output like:
>
> % erl -noshell -noinput -eval 'io:format ("~p~n", [ testlyt:f (1) ]).' -s
> erlang halt
> 11
>
> The transform is attached.
>
> Cheers,
>
> -- p
>
>
> On Tue, 9 Jun 2009, Mikael Pettersson wrote:
>
> > Attila Rajmund Nohl writes:
> >  > Hello!
> >  >
> >  > I think there wasn't any grumbling this month about the immutable
> >  > local variables in Erlang, so here's real world code I've found just
> >  > today:
> >  >
> >  >     % Take away underscore and replace with hyphen
> >  >     MO1 = re:replace(MO, "_", "-", [{return, list}, global]),
> >  >     MO2 = toupper(MO1),
> >  >     % Replace zeros
> >  >     MO3 = re:replace(MO2, "RX0", "RXO", [{return, list}, global]),
> >  >     % Insert hyphen if missing
> >  >     MO5 = case re:run(MO3, "-", [{capture, none}]) of
> >  > 	      nomatch ->
> >  > 		  insert_hyphen(MO3);
> >  > 	      match ->
> >  > 		  MO3
> >  > 	  end,
> >  >
> >  > I think it's fairly clumsy to use MOx (MO for managed object) in the
> >  > code. MO4 was removed during the regexp->re refactoring step. How to
> >  > eliminate the versioned variable names? The
> >  > MOAfterUnderscoresWereReplaced, UpperCaseMO, MOAfterRX0WasReplaced,
> >  > etc. variablenames are really ugly. It used to use regexp, so at that
> >  > point it wasn't possible to easily nest the whole into one call, but
> >  > that would be still ugly. So any other ideas?
> >
> > This has nothing to do with mutable variables (values or
> > bindings), and everything to do with Erlang's unorthodox
> > (for a functional language) scoping rules and semantics
> > for "Var = Expr".
> >
> > Languages using LET for local bindings tend to allow you
> > to re-bind already-bound variables in nested scopes. For
> > instance, in Standard ML I would often write:
> >
> > fun f (..) =
> >   let state = init()
> >       state = stage1(state)
> >       state = finish(state)
> >   in
> >     ... state ...
> >   end
> >
> > and similarly in Scheme using LET*.
> >
> > In Erlang you can't do this, so you're stuck with:
> > - numbered variables (ugly but often reasonably practical)
> > - function composition like f(x) -> a(b(c(x))), but that
> >   quickly gets unreadable
> > - using a separate function for each stage and tailcall
> >   between the stages:
> > 	  f() -> g(init()).
> > 	  g(X) -> h(update(X)).
> > 	  h(X) ->
> > 	    NewX = if ... -> update2(X); true -> X end,
> > 	    i(NewX).
> > 	  i(X) ->
> >   this doesn't entirely eliminate the variable naming problem,
> >   but it does limit it to say two versions per function body,
> >   which is manageable (I use this approach quite a lot in the
> >   HiPE compiler backends when translating generic intermediate
> >   code to architecture specific code)
> > - foldl (as another poster suggested), which is essentially
> >   equivalent to function composition or using tailcalls between
> >   per-stage functions, except it's expressed in different syntax
> >
> > ________________________________________________________________
> > erlang-questions mailing list. See http://www.erlang.org/faq.html
> > erlang-questions (at) erlang.org
> >
> >
>
> Well for these men if they succeed; well also, though not so well, if
> they fail, given only that they have nobly ventured, and have put forth
> all their heart and strength.
>
>         -- Theodore Roosevelt