Hidden Pipe

Erlang parse_transform that changes all _ in expressions to the result of the previous expression in the same expressions block.

It's possible to write code that behaves similarly to the |> (pipe) operator.

In detail:

This parse transforms searchs for a specific variable name in expressions (_ by default, because it's unbound in expressions). If it finds it anywhere but in the 1st expression in an expression block, it's switched with a new, automatic variable that matches the previous expression.

Thus, the following (incorrect pure erlang) code

foo() ->
    bar(),
    case _ of
      baz ->
        a(),
        b(_)
    end,
    baz(_).

gets transformed internally into (the automatic vars have stranger names, though)

foo() ->
    _Auto1 = bar(),
    _AUto3 = case _Auto1 of
      baz ->
        _Auto2 = a(),
        b(_Auto2)
    end,
    baz(_Auto3).

And it allows methods to be 'magically' chained with the use of _ (Akin to |>, but without the operator and with magic variables because I did not want to deal with the tokenizer):

pipe_example() ->
    "Erlang parse_transforms are really flexible", string:uppercase(_), string:split(_, " ", all),
    ["ERLANG", "PARSE_TRANSFORMS" | _] = _.

pipe_block_example() ->
    ["Erlang", "parse_transforms"] = begin "Erlang parse_transforms", string:split(_, " ") end.

-define(pipe, begin).
pipe_block_example2() ->
    ["Erlang", "parse_transforms"] = ?pipe "Erlang parse_transforms", string:split(_, " ") end,
    ok.

Keep in mind that the Patterns are unchanged, so you can still do things like

{_, B} = {a, b},
<<0 || _ <- lists:seq(0,256)>>,
[A || << A, _>> <= <<0,1,2,3,4,5>>],

More examples can be found in the tests dir

How to use:

Include it as a dependency and add the parse_transform compile option to each module or globally:

-compile({parse_transform, hidden_pipe}).

The -hidden_pipe({to_substitute, atom()}). and -hidden_pipe({verbose, boolean()}). attributes allow you to control the behaviour of this module.

Disclaimer:

I've yet to decide if this parse_transform is a good or a terrible idea.

Run tests:

rebar3 ct

Authors

• Jose M Perez Ramos - Kuroneer

License

MIT License