Engines

A magusfile is written in Buzz and runs on the embedded Buzz VM through a small internal seam. A magusfile.buzz exposes the magus.* API and composes spells, targets, and charms. This page covers the seam and how a new language would plug in.

The engine interface

The backend implements one small interface, engine.Engine, which is a factory for engine.Session:

// internal/interp/engine/engine.go
type Engine interface {
    ID() string
    NewSession(ctx context.Context) (Session, error)
}

type Session interface {
    Close() error
    SetGlobal(name string, v Value)
    GetGlobal(name string) Value
    NewTable() Table
    LoadString(code string) (Value, error)
    DoString(code string) error
    Call(p CallParams, args ...Value) error
}

Value and Table are engine-neutral handles, so host code reads and writes script values without knowing the concrete VM. The backend registers itself at init() time:

// internal/interp/engine/buzz/buzz.go
func init() { engine.Register("buzz", engineImpl{}) }

and is found by name through engine.Lookup(name). The registered backend in a stock magus binary is buzz.

The backend is pulled into the binary by blank import in cmd/magus/packs_interp.go:

_ ".../internal/interp/engine/buzz"

The spell contract

A spell exports a fixed set of mgs_-prefixed functions (see spells). The list of optional functions and the decoder keys they map to is single-sourced in internal/spellruntime/contract.go as OptionalContract, and the Buzz resolver (internal/spellruntime/resolve.go) iterates that one list. A spell's mgs_ functions decode to a Spec for every scalar and list contribution (needs, provides, claims, version_cmd, opaque) and for record-shaped ops ({bin, args, charms}).

An op is a command or a service; a function-valued op is fun(Target) > Command or fun(Target) > Service, called once at load to record the declarative value it returns (a Command's {bin, args, charms} or a Service's {command, readiness?, stop?}, a long-running process magus run blocks on). The op's kind is inferred from that value, so one spell mixes both. A remote cache backend is not an op: it is a spell that exports the backend functions (enabled/get_artifact/put_artifact/prune), detected by name and wired with magus\cache.remote (see Remote caching).

Doc-comment capture. Buzz captures a handler's doc comment at compile time (the parser binds the comment to the function node; FunDoc reads it back), and magus doctor enforces one on each function-handler target. Note Buzz's Chunk.Doc is in-memory only and not serialized to bytecode, so Buzz captures docs only for freshly-compiled workspace .buzz spells, never the embedded built-ins.

Host modules are a superset of Buzz's stdlib

magus layers its host methods onto Buzz's own stdlib modules under the same bare names: import "os" carries both Buzz's os.* (sleep, env, execute) and magus's additions (os\exec, os\which, ...); import "fs" carries Buzz's fs plus fs\glob/readFile; and magus adds whole modules Buzz lacks (vcs, archive, http, charm, ...). One import per domain covers the union, with no separate extra namespace to remember which side a call lives on.

Where a method overlaps a Buzz stdlib call, the magus form is sandbox-aware while the bare stdlib is not. For example, env\get/lookup honor the env allowlist, whereas Buzz's os\env is raw. Those overlaps are noted per-method in the module reference (either works); the cross-reference lives in std/buzz_stdlib.go.

A few entries are not treated as duplicates because the magus behavior the stdlib can't reproduce: magus's os\exit raises a lifecycle error (Buzz's hard-exits the process), magus's os\sleep is cancellable (Buzz's blocks), and magus's crypto.*_file hashes a file (Buzz's hash only takes a string). These stay on the magus surface.

A workspace spell lives at spells/<name>/spell.buzz (or flat spells/<name>.buzz).

"Built-in spell" vs language "builtins"

A built-in spell is a spell whose bytecode is compiled from spells/<name>/spell.buzz and embedded in the magus binary (go, typescript, docker, ...; see spells). This is a magus concept and is unrelated to Buzz's language builtins (spawn, list/map methods, etc.), which are part of the Buzz language itself. The docs always write "built-in spell" when they mean the former.

Adding a new language

The engine interface is the stable, clean part of the seam. Plugging in a second language today, however, also touches a handful of hard-coded dispatch spots above the interface. This is the current state, not the end state:

1. Implement engine.Engine/Session for the VM and engine.Register it (the clean part).
2. Map the file extension to the engine in engineForExt (internal/interp/source.go) and add the glob to scriptExts / the magusfile.<ext> lists.
3. Branch the runtime where it special-cases an engine by name (src.Engine == "buzz" in internal/interp/runtime.go).
4. Provide the per-engine host bindings (the magus.* surface), as internal/interp/bindings/buzz.go does today.

Future direction: registry-driven discovery. The intent is to derive extensions, magusfile filenames, and dispatch from the engine registry itself, so adding a language means registering a backend (with its extensions and binding installer) and nothing else, with no edits to source.go, runtime.go, or switch statements. That refactor is deliberately out of scope for now. The hard-coded spots above are the seam's known leaks, documented so they are visible rather than surprising.

See also

• spells: the mgs_ spell contract and how spells compose.
• targets: the runnable unit and its CLI grammar.
• modules/index: the magus.* host module reference.