Schooner

An embeddable, sandboxed Scheme interpreter for the BEAM, targeting the r7rs-small language minus its mutable operations. Schooner is intended as a scripting layer for Elixir applications: hosts hand a script source to one of the entry points below, get back an Elixir term, and resource-bound the work with the standard process tools (:max_heap_size, Task.shutdown/2).

Quick example

iex> alias Schooner.Host
iex> env =
...>   Schooner.Environment.new(
...>     pre_imports: [["scheme", "base"]],
...>     libraries: [
...>       Host.library(
...>         primitives: [
...>           {"shout", 1, fn [msg] ->
...>              text = Host.to_string!(msg, op: "shout")
...>              Host.string(String.upcase(text) <> "!")
...>            end}
...>         ]
...>       )
...>     ]
...>   )
iex> Schooner.eval(~s|(shout (string-append "hello, " "world"))|, env)
{:ok, "HELLO, WORLD!"}
iex> {:ok, double} = Schooner.eval("(lambda (x) (* x 2))", env)
iex> Schooner.apply(double, [21])
{:ok, 42}

What's happening:

• Schooner.Environment.new/1 builds a sandbox surface — (scheme base) is pre-imported (so string-append, lambda, * are in scope without an explicit (import ...)), plus an anonymous host library exposing shout as a Scheme procedure backed by an Elixir function.
• Schooner.eval/2 returns {:ok, value} on success. Schooner.Host.to_string!/2 extracts the underlying binary; Schooner.Host.string/1 constructs a Scheme string for the return.
• Schooner.apply/2 invokes any Scheme procedure value (closure, primitive, or parameter) from Elixir.

See the Embedding and Host Functions guides for the full story.

Installation

Add schooner to your list of dependencies in mix.exs:

def deps do
  [
    {:schooner, "~> 0.1.0"}
  ]
end

Documentation is published at https://hexdocs.pm/schooner.

Choosing an entry point

Schooner has two top-level evaluation entry points. They differ in what's in scope when the script starts, and picking the wrong one for untrusted input is a sandbox hole.

For untrusted input, use Schooner.eval/2. A script that omits (import ...) cannot reach any primitive, so the embedder sets the surface deliberately.

Both functions also have raising bang variants (run!/1, eval!/2,3) and a richer environment-construction path via Schooner.Environment.new/1 — see the Embedding guide.

Deviations from r7rs-small

Schooner targets r7rs-small but deliberately ships a smaller surface. The table below summarises every intentional gap; conformance tests under test/conformance/ cover the surface that is shipped, with each excluded upstream case annotated inline. The Deviations guide expands every row with a runnable example and a workaround.