merlin

Parse transform library to make writing those a breeze. It builds on top of merl, and takes inspiration of parse_trans.

The latter might have support for carrying a state during transform, but it's not documented. The one function that is only supports simple transforms, and require intimate knowledge about the Erlang AST. While I do think knowing the AST is good, merl makes it much easier to work with, and it's a shame it doesn't fit into parse_trans so well.

Merlin rectifies that by embracing merl and provides a small parse transform to extend its case matching to function heads.

-include_lib("merlin/include/merlin_quote.hrl").

parse_transform(Forms, CompileOptions) ->
    {Forms1, _State1} = merlin:transform(Forms, fun transform/3, #{
        options => CompileOptions
    }),
    merlin:return(Forms1).

transform(enter, ?Q("_@Var"), State) when erl_syntax:type(Var) =:= variable ->
    ?Q("_@Var = 123");
transform(_, _, _) ->
    continue.

This example also demonstrate how to use merlin:transform/3. The idea is to thread the given transformer function through all AST nodes while carrying a user provided state. Depending on the return value, the current node is either replaced, kept as is or deleted. In fact, you can return more then one node, which is nigh impossible with a plain transform.

In all of the above cases, you can also return an updated state. Thus you can easily keep track of the current set of variables, or how many times a certain functions is called etc.

merlin_lib provide some helpers for common tasks, like finding the -module attribute, or generating new, unique, variables to avoid accidental badmatch errors.

Developing

This uses rebar3, and contains a Makefile for some common tasks. It also uses logger, with a sample config. Once copied you can tweak it to log what's happening under the hood.

$ make examples