In-process scripting

Alongside the compiled Rust path, Faber supports in-process interpreted execution through the MIR stepper.

Usage

faber run --interpret script.fab

This runs Faber source in-process after the normal front half of the compiler (parse through typecheck + MIR lowering), without invoking rustc or spawning a build process.

How it works

The compiler produces analysed HIR, validated MIR, and a resolved runtime-intrinsic table. The MIR stepper dispatches MIR blocks straight to a host, skipping the wasm emit/instantiate round-trip:

Source → Lex → Parse → Collect → Resolve → Lower → Typecheck
                                                      ↓
                                                 MIR lowering
                                                      ↓
                                              MIR stepper + Host

Latency

The scripting path runs the same linear frontend as the compiled path, plus stepper time proportional to what the script actually executes:

The stepper never invokes rustc or spawns a process, so startup is fast enough to feel like a shell script.

Limitations

• The MIR stepper does not support all host I/O routes that the compiled

path does — some norma:* wrappers remain compiled-only

• The stepper is a MIR-native diagnostic/reference executor, not a

production runtime for deployed applications

• Package compilation through Cargo remains the primary product path