Compilation error - An implementation limit was reached

[danielres]

Hi,

Thank you for this library.

I'm getting this error after adding md to my project, which uses unicode_guards as a dependency.

Any suggestion would be greatly appreciated 🙏

== Compilation error in file lib/unicode_guards.ex ==
** (CompileError) Elixir.Unicode.Guards:1: function 'MACRO-is_graph'/2+7:
  An implementation limit was reached.
  Try reducing the complexity of this function.

  Instruction: {move,{x,1},{y,1709}}

    (stdlib 6.2) lists.erl:2310: :lists.foreach_1/2
could not compile dependency :unicode_guards, "mix compile" failed. Errors may have been logged above. You can recompile this dependency with "mix deps.compile unicode_guards --force", update it with "mix deps.update unicode_guards" or clean it with "mix deps.clean unicode_guards"

I'm using;

elixir 1.18.2-otp-27
erlang 27.2.2

defp deps do
    [
      {:phoenix, "~> 1.7.19"},
      {:phoenix_ecto, "~> 4.5"},
      {:ecto_sql, "~> 3.10"},
      {:ecto_sqlite3, ">= 0.0.0"},
      {:phoenix_html, "~> 4.1"},
      {:phoenix_live_reload, "~> 1.2", only: :dev},
      {:phoenix_live_view, "~> 1.0.4", override: true},
      {:floki, ">= 0.30.0"},
      {:phoenix_live_dashboard, "~> 0.8.3"},
      {:esbuild, "~> 0.8", runtime: Mix.env() == :dev},
      {:tailwind, "~> 0.2", runtime: Mix.env() == :dev},
      {:heroicons,
       github: "tailwindlabs/heroicons",
       tag: "v2.1.1",
       sparse: "optimized",
       app: false,
       compile: false,
       depth: 1},
      {:swoosh, "~> 1.5"},
      {:finch, "~> 0.13"},
      {:telemetry_metrics, "~> 1.0"},
      {:telemetry_poller, "~> 1.0"},
      {:gettext, "~> 0.20"},
      {:jason, "~> 1.2"},
      {:dns_cluster, "~> 0.1.1"},
      {:bandit, "~> 1.5"},
      {:earmark, "~> 1.4"},
      {:yaml_elixir, "~> 2.11"},
      {:memoize, "~> 1.4"},
      {:mix_test_watch, "~> 1.0", only: [:dev, :test], runtime: false},
      {:html_sanitize_ex, "~> 1.4"},
      {:credo, "~> 1.7", only: [:dev, :test], runtime: false},
      {:md, "~> 0.10.6"}
    ]
  end

[danielres]

I'm fairly new to elixir.
But I thought of asking o3-mini for some hints, here's the result:

---

The error message indicates that one of the guard macros expands to an extremely large function body (i.e. too many instructions), and in this case the log mentioned "MACRO-is_graph", so it’s very likely that the problematic code is this guard:

defguard is_graph(codepoint)
           when is_integer(codepoint) and match?(codepoint, "[[:graph:]]")

When you call match?/2 with a pattern like "[[:graph:]]", the macro expands the Unicode character set corresponding to that property. Because Unicode’s definition of “graph” (visible, non-space characters) is very broad, the expansion can generate a huge number of instructions—more than the BEAM’s implementation limit allows.

In summary, the guard definition for is_graph/1 (and possibly similar ones that use complex Unicode sets) is likely causing the issue due to the complexity of the pattern passed to match?/2.

[kipcole9]

Confirmed, and thank you for the report. This is probably the most challenging guard since the idea of what is "graphic" results in a very disjoined set of code points. I'm not sure if this is even resolvable. But it's possible that the inverse set of code points (ie non-graphic) might be smaller.

I'll see what optimisations might be possible.

[kipcole9]

It's very strange.

This works fine, using the match?/2 macro:

def my_fun(codepoint) when is_integer(codepoint) and Unicode.Set.match?(codepoint, "[[:graph:]]") do
  codepoint
end

This does not (defining a guard):

defguard is_graph(codepoint)
  when is_integer(codepoint) and Unicode.Set.match?(codepoint, "[[:graph:]]")

I'll suggest upstream that the md lib uses the more explciit form. BTW, I've not seen this issue on earlier OTP releases so there's a change somewhere that surfaces this.

[vlemann]

I also added a Github Action to this PR so that it shows the tests ran successfully right in github interface. If you don’t want it, use #8 instead.