Using surfex as a library

Some projects use surfex for one thing: a surface golden, a committed document generated from a source scan, listing what the code declares, which CI regenerates and byte-compares so it can't disagree with the code. They call two modules directly and keep no relation log. Both are supported library API (spec §23).

The two modules

• Surfex.SourceScan reads Elixir source without compiling it:

  • lib_sources/1: the project's own lib/**/*.ex, skipping deps/, _build/, test/, tmp/ and hidden directories;
  • defmodules/1: the modules in a quoted file;
  • defs/1 and types/1: the public definitions and types of a module, each with its version;
  • module_hash/1: a module's public surface as one version;
  • definition_hash/1: the version of any node or snippet: a SHA-256 over its AST with metadata removed, so a blank line or a comment above it doesn't change it;
  • project_root/1,2, line_range/1 and hidden_module?/1.

• Surfex.Golden renders a golden from a plain data spec (Surfex.Golden.spec/0):

  • render/1;
  • stat/2 and stat_line/2 for its stats lines;
  • natural_key/1 for ordering rows.

  The renderer owns the notation, so every project's goldens read the same way, and it never emits a timestamp, so a golden is a pure function of source.

A surface golden in a few lines

rows =
  for file <- Surfex.SourceScan.lib_sources(root),
      {:defmodule, _, [name, _]} = mod <-
        file |> File.read!() |> Code.string_to_quoted!() |> Surfex.SourceScan.defmodules(),
      d <- Surfex.SourceScan.defs(mod),
      do: %{
        "Function" => {:code, "#{Macro.to_string(name)}.#{d.name}/#{d.arity}"},
        "Version" => {:version, d.hash}
      }

Surfex.Golden.render(%{
  name: "API_SURFACE.md",
  purpose: "Every public function, with its version.",
  task: "my_app.surface",
  gate: "surface",
  hardness: :hard,
  columns: ["Function", "Version"],
  rows: rows
})

CI runs the task with --check, regenerating and comparing. A difference fails the build, and the diff shows the reviewer exactly which definitions changed.

The stability promise

• Signatures: the documented functions and types of both modules keep their names, arities and return shapes. They have since v0.1.0. A test in surfex pins the documented surface, so a change is deliberate.
• Hashes: definition_hash/1 gives the same version for the same code from release to release, so a golden doesn't churn on an upgrade.
• Changes: a change to either module never comes in a patch release. It comes in a minor release, listed in the CHANGELOG under "Changed" and marked for library users. One such change already: lib_sources/1 started skipping test/, tmp/ and hidden directories.

This isn't adoption

A surface golden says what the code declares. Surfex's purpose is keeping a spec, its tests and its code aligned, which needs the relation log. When a project depends on surfex but has no .surfex/, mix surfex.info says so. mix surfex.info adoption covers starting the log on an existing project.