// L4-eval-box.ts // L4 with mutation (set!) and env-box model // Direct evaluation of letrec with mutation, define supports mutual recursion. import { map, repeat, zipWith } from "ramda"; import { isBoolExp, isCExp, isLitExp, isNumExp, isPrimOp, isStrExp, isVarRef, isSetExp, isAppExp, isDefineExp, isIfExp, isLetrecExp, isLetExp, isProcExp, Binding, VarDecl, CExp, Exp, IfExp, LetrecExp, LetExp, ProcExp, Program, SetExp, parseL4Exp, DefineExp} from "./L4-ast"; import { applyEnv, applyEnvBdg, globalEnvAddBinding, makeExtEnv, setFBinding, theGlobalEnv, Env, FBinding } from "./L4-env-box"; import { isClosure, makeClosure, Closure, Value } from "./L4-value-box"; import { applyPrimitive } from "./evalPrimitive-box"; import { first, rest, isEmpty } from "../shared/list"; import { Result, bind, mapv, mapResult, makeFailure, makeOk } from "../shared/result"; import { parse as p } from "../shared/parser"; // ======================================================== // Eval functions const applicativeEval = (exp: CExp, env: Env): Result<Value> => isNumExp(exp) ? makeOk(exp.val) : isBoolExp(exp) ? makeOk(exp.val) : isStrExp(exp) ? makeOk(exp.val) : isPrimOp(exp) ? makeOk(exp) : isVarRef(exp) ? applyEnv(env, exp.var) : isLitExp(exp) ? makeOk(exp.val as Value) : isIfExp(exp) ? evalIf(exp, env) : isProcExp(exp) ? evalProc(exp, env) : isLetExp(exp) ? evalLet(exp, env) : isLetrecExp(exp) ? evalLetrec(exp, env) : isSetExp(exp) ? evalSet(exp, env) : isAppExp(exp) ? bind(applicativeEval(exp.rator, env), (proc: Value) => bind(mapResult((rand: CExp) => applicativeEval(rand, env), exp.rands), (args: Value[]) => applyProcedure(proc, args))) : exp; export const isTrueValue = (x: Value): boolean => ! (x === false); const evalIf = (exp: IfExp, env: Env): Result<Value> => bind(applicativeEval(exp.test, env), (test: Value) => isTrueValue(test) ? applicativeEval(exp.then, env) : applicativeEval(exp.alt, env)); const evalProc = (exp: ProcExp, env: Env): Result<Closure> => makeOk(makeClosure(exp.args, exp.body, env)); // KEY: This procedure does NOT have an env parameter. // Instead we use the env of the closure. const applyProcedure = (proc: Value, args: Value[]): Result<Value> => isPrimOp(proc) ? applyPrimitive(proc, args) : isClosure(proc) ? applyClosure(proc, args) : makeFailure(`Bad procedure ${JSON.stringify(proc, null, 2)}`); const applyClosure = (proc: Closure, args: Value[]): Result<Value> => { const vars = map((v: VarDecl) => v.var, proc.params); return evalSequence(proc.body, makeExtEnv(vars, args, proc.env)); } // Evaluate a sequence of expressions (in a program) export const evalSequence = (seq: Exp[], env: Env): Result<Value> => isEmpty(seq) ? makeFailure("Empty program") : evalCExps(first(seq), rest(seq), env); const evalCExps = (first: Exp, rest: Exp[], env: Env): Result<Value> => isDefineExp(first) ? evalDefineExps(first, rest) : isCExp(first) && isEmpty(rest) ? applicativeEval(first, env) : isCExp(first) ? bind(applicativeEval(first, env), _ => evalSequence(rest, env)) : first; // Eval a sequence of expressions when the first exp is a Define. // Compute the rhs of the define, extend the env with the new binding // then compute the rest of the exps in the new env. // L4-BOX @@ // define always updates theGlobalEnv // We also only expect defineExps at the top level. const evalDefineExps = (def: DefineExp, exps: Exp[]): Result<Value> => bind(applicativeEval(def.val, theGlobalEnv), (rhs: Value) => { globalEnvAddBinding(def.var.var, rhs); return evalSequence(exps, theGlobalEnv); }); // Main program // L4-BOX @@ Use GE instead of empty-env export const evalProgram = (program: Program): Result<Value> => evalSequence(program.exps, theGlobalEnv); export const evalParse = (s: string): Result<Value> => bind(p(s), (x) => bind(parseL4Exp(x), (exp: Exp) => evalSequence([exp], theGlobalEnv))); // LET: Direct evaluation rule without syntax expansion // compute the values, extend the env, eval the body. const evalLet = (exp: LetExp, env: Env): Result<Value> => { const vals = mapResult((v: CExp) => applicativeEval(v, env), map((b: Binding) => b.val, exp.bindings)); const vars = map((b: Binding) => b.var.var, exp.bindings); return bind(vals, (vals: Value[]) => evalSequence(exp.body, makeExtEnv(vars, vals, env))); } // @@ L4-EVAL-BOX // LETREC: Direct evaluation rule without syntax expansion // 1. extend the env with vars initialized to void (temporary value) // 2. compute the vals in the new extended env // 3. update the bindings of the vars to the computed vals // 4. compute body in extended env const evalLetrec = (exp: LetrecExp, env: Env): Result<Value> => { const vars = map((b: Binding) => b.var.var, exp.bindings); const vals = map((b: Binding) => b.val, exp.bindings); const extEnv = makeExtEnv(vars, repeat(undefined, vars.length), env); // @@ Compute the vals in the extended env const cvalsResult = mapResult((v: CExp) => applicativeEval(v, extEnv), vals); const result = mapv(cvalsResult, (cvals: Value[]) => zipWith((bdg, cval) => setFBinding(bdg, cval), extEnv.frame.fbindings, cvals)); return bind(result, _ => evalSequence(exp.body, extEnv)); }; // L4-eval-box: Handling of mutation with set! const evalSet = (exp: SetExp, env: Env): Result<void> => bind(applicativeEval(exp.val, env), (val: Value) => mapv(applyEnvBdg(env, exp.var.var), (bdg: FBinding) => setFBinding(bdg, val)));