Let parse transform

So the problem of intermediate variable naming came up again on erlang questions.

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Subject:Versioned variable names
From: Attila Rajmund Nohl
Date: Tue, 9 Jun 2009 17:12:34 +0200

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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,

...

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Mikael Pettersson pointed out that this really has less to do with immutable local variables and more to do with the lack of a let expression. That was insightful, and since a let expression can be considered syntactic sugar for a lambda expression, I realized that a parse transform could provide let like functionality. Let is a reserved keyword in Erlang so I used lyet instead.

Essentially the parse transform rewrites

lyet:lyet (A = B, C)

as

(fun (A) -> C end) (B)

so the above code could be rewritten as

Result = 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),

You must provide at least one argument to lyet:lyet. All but the last argument to lyet:lyet must be an assignment, and the last argument has to be a single expression (but you can use begin and end for a block of expressions inside the lyet). As you can see above, you can reuse a variable name across the assignment arguments to lyet:lyet. You can even use lyet:lyet on the right hand side of the assignments, or as part of the expression argument. Some examples of usage are present in the unit test.

Update: per Ulf's suggestion, the parse transform also recognizes the local call let_ in addition to the remote call lyet:lyet. It definitely looks nicer with let_.

The software is available on Google code.