brangi / merl

Metaprogramming in Erlang

Merl - Metaprograming in Erlang

NOTE: as of Erlang/OTP 18.0, Merl is included in the syntax_tools application! This repository remains here for use by those unable to upgrade to version 18.0 or later.

Merl is a more user friendly interface to the erl_syntax module in the standard library syntax_tools application, making it easy both to build new ASTs (abstract syntax trees) from scratch and to match and decompose existing ASTs.

To enable the full power of Merl, your module needs to include the Merl header file:

-include_lib("merl/include/merl.hrl").

Then, you can use ?Q(Text) macros in your code to create ASTs or match on existing ASTs. For example:

Tuple = ?Q("{foo, 42}"),
?Q("{foo, _@Number}") = Tuple,
Call = ?Q("foo:bar(_@Number)")

Calling merl:print(Call) will then print the following code:

foo:bar(42)

The ?Q macros turn the quoted code fragments into ASTs, and lifts metavariables such as _@Tuple and _@Number to the level of your Erlang code, so you can use the corresponding Erlang variables Tuple and Number directly. This is the most straightforward way to use Merl, and in many cases it's all you need.

You can even write case switches using ?Q macros as patterns. For example:

case AST of
    ?Q("{foo, _@Foo}") -> handle(Foo);
    ?Q("{bar, _@Bar}") when erl_syntax:is_integer(Bar) -> handle(Bar);
    _ -> handle_default()
end

For the full documentation, run make docs and open doc/index.html in a browser. For a quick look at the user guide without generating the HTML docs, see the file doc/overview.edoc.