6. Shared Format Module with Symbol Tables

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Date: 2026-03-31

Status

Accepted

Context

TLX has 6 emitters (TLA+, PlusCalC, PlusCalP, Elixir, Symbols, Config) that all need to format the same AST structures. Initially, each emitter had its own formatting logic, leading to ~400 lines of duplication and inconsistent handling of edge cases.

The emitters differ only in surface syntax:

  • TLA+ uses /\ for conjunction, ~ for negation, bare atoms
  • PlusCal uses the same but quotes atom strings
  • Elixir uses and, not, colon-prefixed atoms
  • Symbols uses ∧, ¬, Unicode math notation

Decision

Extract all AST formatting into TLX.Emitter.Format, parameterized by symbol tables. Each emitter provides a map of symbols (%{and: "/\\", not: "~", atom: :unquoted, ...}) and delegates all formatting to shared functions.

Four symbol tables are defined: tla_symbols/0, pluscal_symbols/0, unicode_symbols/0, elixir_symbols/0.

# Each emitter does:
@symbols Format.tla_symbols()
defp format_ast(ast), do: Format.format_ast(ast, @symbols)

Consequences

Positive:

  • One place to add support for new AST constructs — all emitters get it automatically
  • Adding a new emitter requires only a new symbol table
  • Bug fixes in formatting apply to all emitters
  • The Symbols emitter was trivial to implement (new symbol table, done)

Negative:

  • The Format module is large (~430 lines) with many clause patterns
  • Pattern matching order matters: guarded when is_list(meta) clauses must come before unguarded 3-element tuple catch-alls
  • Some emitters need formatting that doesn't fit the symbol table model (e.g., PlusCal assignment syntax x := expr;)