How to change AST before Elixir compilation

XY Notice

I know about macros, __before_compile__ and other attributes, compiler tracers and such stuff. The question is exactly about implementing wrapper for Elixir compiler which be compatible with Mix.Task.Compiler, nothing else.

The Question

How do I write the Mix.Task.Compiler which will do some global changes to the AST before calling Elixir’s compiler to generate .beam files.

Current solution

Right now my compiler

  1. Searches for .ex files in the path specified by Mix.Project.config()[:elixirc_paths]
  2. Calls Code.string_to_quoted() on each file
  3. Changes the AST
  4. Calls Code.compile_quoted with File.write!() to the build_path of the binaries returned by compile_quoted

Current solution drawbacks

  1. I think it is possible to compile straight to file (without File.write!)
  2. I think this kind of compilation does not call compiler tracers
  3. I think it can have some problems with protocol consolidation
  4. It needs to read/write elixir’s compiler manifest, which is some kind of a private structure.
  5. It is difficult to change the AST because modules can have compile-time code in their bodies, which I need to preevaluate, to change the code inside functions.
1 Like

Not possible. By design. We want to avoid anything that goes and globally changes code, without a trace. You can hand roll, as in your current solution, but it will have pitfalls such as losing incremental compilation.

I would suggest taking a step back and discuss what you want to solve and try to find a way that would be considered idiomatic Elixir.

Sorry if this is not the answer you were looking for. :slight_smile:


That’s sad to hear, but I kinda had a feeling that these limitations are intentional. I’ll think about other solutions then, thanks!