Module interpreter 

Tree-walking interpreter — Sexp → Value.

Minimal but complete enough to evaluate real Lisp programs that produce typed Derivation values. Special forms handled inline (to respect evaluation order / lazy arms); everything else dispatches via apply over evaluated arguments.

Special forms:

• quote — return the literal Sexp as Value
• if — short-circuit branches
• let — non-recursive bindings
• letrec — recursive bindings via thunks
• lambda — closure capture
• define — bind in top-level env
• begin — sequential evaluation
• set! — mutate a top-level binding (restricted; see docs)

Structs

Interpreter