BEAM/Elixir AST Processing Gotchas

Problem

When building tools that process Elixir/Erlang ASTs (like Grafema's beam-analyzer), several non-obvious AST structural issues cause silent failures or crashes.

Context / Trigger Conditions

Gotcha 1: Elixir AST Body Double-Nesting

• Symptom: MODULE node created correctly, but 0 FUNCTION/CALL/VARIABLE nodes
• Trigger: Any Elixir file with functions inside a module
• Root cause: {:defmodule, meta, [alias | body]} produces body = [[do: ...]], NOT body = [do: ...]. The body is wrapped in an extra list layer.
• Same issue for functions: {:def, meta, [{name, _, args} | body]} also gives body = [[do: ...]]

Fix: Always unwrap with List.first(body) || [] before pattern matching on [do: ...]:

# WRONG — body is [[do: ...]], not [do: ...]
walk_module_body(body, ctx)

# RIGHT
keyword_body = List.first(body) || []
walk_module_body(keyword_body, ctx)

Gotcha 2: OTP 26+ Erlang AST Location Format

• Symptom: (ArgumentError) cannot convert the given list to a string
• Trigger: Processing Erlang .erl files with OTP 26 or newer
• Root cause: OTP 26 changed Erlang abstract forms from {:attribute, LineNumber, ...} to {:attribute, [text: ~c"...", location: N], ...}. The second element is now a keyword list, not an integer.

Fix: Add a location extractor helper:

defp extract_line(loc) when is_integer(loc), do: loc
defp extract_line(loc) when is_list(loc), do: Keyword.get(loc, :location, 0)
defp extract_line(_), do: 0

Apply to ALL Erlang form handlers: :module, :export, :function, :type, :spec, :import, :call, :match, :case, etc.

Gotcha 3: Pipe Operator Clause Ordering

• Symptom: CALL nodes with name |> instead of desugared function names
• Trigger: Elixir pipe chains like data |> String.trim() |> Enum.map(...)
• Root cause: {:|>, meta, [left, right]} matches the general clause {name, meta, args} when is_atom(name) and is_list(args) because :|> is an atom. If the general clause is defined before the pipe-specific clause, it catches pipes first.

Fix: Always define the {:|>, ...} clause BEFORE {name, meta, args}:

# Pipe clause MUST come first
defp walk_pipe_arg({:|>, _meta, [left, right]}, ctx) do ...
defp walk_pipe_arg({{:., _, _}, meta, args}, ctx) do ...    # dot calls
defp walk_pipe_arg({name, meta, args}, ctx) when is_atom(name) do ...  # general - LAST

Gotcha 4: Multi-Module Files

• Symptom: Only first module processed, or 0 nodes if top-level is __block__
• Trigger: Elixir files with multiple defmodule at the top level
• Root cause: Code.string_to_quoted returns {:__block__, _, [defmodule1, defmodule2, ...]} for multi-module files, but {:defmodule, ...} for single-module files.

Fix: Handle both in the walker:

defp walk_elixir({:__block__, _, statements}, ctx) do
  Enum.reduce(statements, ctx, fn stmt, ctx -> Rules.Modules.process(stmt, ctx) end)
end
defp walk_elixir(ast, ctx), do: Rules.Modules.process(ast, ctx)

Gotcha 5: ::: Atom Quoting Warning

• Symptom: Elixir 1.16+ warning: atom ::: must be written between quotes
• Trigger: Pattern matching on typespec AST {:::, _, [...]}
• Fix: Use {:"::", _, [...]} instead of {:::, _, [...]}

Verification

• Run mix run verify.exs on comprehensive fixture files
• Check that all expected node types appear (MODULE, FUNCTION, CALL, VARIABLE, etc.)
• Verify Erlang files produce nodes without crashes
• Verify pipe chains produce individual CALL nodes per function

Notes

• The Elixir AST with columns: true, token_metadata: true options adds extra metadata to every node — be aware when pattern matching
• OTP version detection: check :erlang.system_info(:otp_release) if you need conditional handling
• For debugging AST structure, use Code.string_to_quoted(source) |> IO.inspect(pretty: true)

Installing Erlang/Elixir on macOS with Outdated Clang

If brew install erlang fails with a clang crash on PCRE2 (Clang 14.x):

1. Install kerl: brew install kerl
2. Build with problematic deps disabled:

   KERL_CONFIGURE_OPTIONS="--without-wx --without-odbc --without-javac --disable-jit" \
     kerl build 26.2.5 26.2.5
   kerl install 26.2.5 ~/.kerl/installs/26.2.5
   source ~/.kerl/installs/26.2.5/activate
   

3. Install Elixir from pre-built binary (match OTP version):

   curl -fsSL https://github.com/elixir-lang/elixir/releases/download/v1.16.3/elixir-otp-26.zip \
     -o /tmp/elixir.zip
   unzip -q /tmp/elixir.zip -d ~/.elixir
   export PATH="~/.kerl/installs/26.2.5/bin:~/.elixir/bin:$PATH"