/
Interpreter.swift
457 lines (392 loc) · 18.3 KB
/
Interpreter.swift
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//
// Interpreter.swift
// slox
//
// Created by Danielle Kefford on 2/26/24.
//
import Foundation
class Interpreter {
var environment: Environment = Environment()
init() {
setUpGlobals()
}
private func setUpGlobals() {
for nativeFunction in NativeFunction.allCases {
environment.define(name: String(describing: nativeFunction),
value: .nativeFunction(nativeFunction))
}
}
func interpret(statements: [ResolvedStatement]) throws {
for statement in statements {
try execute(statement: statement)
}
}
func interpretRepl(statements: [ResolvedStatement]) throws -> LoxValue? {
for (i, statement) in statements.enumerated() {
if i == statements.endIndex-1, case .expression(let expr) = statement {
return try evaluate(expr: expr)
} else {
try execute(statement: statement)
}
}
return nil
}
private func execute(statement: ResolvedStatement) throws {
switch statement {
case .expression(let expr):
let _ = try evaluate(expr: expr)
case .if(let testExpr, let consequentStmt, let alternativeStmt):
try handleIfStatement(testExpr: testExpr,
consequentStmt: consequentStmt,
alternativeStmt: alternativeStmt)
case .print(let expr):
try handlePrintStatement(expr: expr)
case .variableDeclaration(let name, let expr):
try handleVariableDeclaration(name: name, expr: expr)
case .block(let statements):
try handleBlock(statements: statements,
environment: Environment(enclosingEnvironment: environment))
case .while(let expr, let stmt):
try handleWhileStatement(expr: expr, stmt: stmt)
case .class(let nameToken, let superclassExpr, let methods, let staticMethods):
try handleClassDeclaration(nameToken: nameToken,
superclassExpr: superclassExpr,
methods: methods,
staticMethods: staticMethods)
case .function(let name, let lambda):
try handleFunctionDeclaration(name: name, lambda: lambda)
case .return(let returnToken, let expr):
try handleReturnStatement(returnToken: returnToken, expr: expr)
}
}
private func handleIfStatement(testExpr: ResolvedExpression,
consequentStmt: ResolvedStatement,
alternativeStmt: ResolvedStatement?) throws {
if isTruthy(value: try evaluate(expr: testExpr)) {
try execute(statement: consequentStmt)
} else if let alternativeStmt {
try execute(statement: alternativeStmt)
}
}
private func handlePrintStatement(expr: ResolvedExpression) throws {
let literal = try evaluate(expr: expr)
print(literal)
}
private func handleClassDeclaration(nameToken: Token,
superclassExpr: ResolvedExpression?,
methods: [ResolvedStatement],
staticMethods: [ResolvedStatement]) throws {
// NOTA BENE: We temporarily set the initial value associated with
// the class name to `.nil` so that, according to the book,
// "allows references to the class inside its own methods".
environment.define(name: nameToken.lexeme, value: .nil)
let superclass = try superclassExpr.map { superclassExpr in
guard case .instance(let superclass as LoxClass) = try evaluate(expr: superclassExpr) else {
throw RuntimeError.superclassMustBeAClass
}
environment = Environment(enclosingEnvironment: environment);
environment.define(name: "super", value: .instance(superclass));
return superclass
}
var methodImpls: [String: UserDefinedFunction] = [:]
for method in methods {
guard case .function(let nameToken, let lambdaExpr) = method else {
throw RuntimeError.notAFunctionDeclaration
}
guard case .lambda(let paramTokens, let methodBody) = lambdaExpr else {
throw RuntimeError.notALambda
}
let isInitializer = nameToken.lexeme == "init"
let methodImpl = UserDefinedFunction(name: nameToken.lexeme,
params: paramTokens,
enclosingEnvironment: environment,
body: methodBody,
isInitializer: isInitializer)
methodImpls[nameToken.lexeme] = methodImpl
}
var staticMethodImpls: [String: UserDefinedFunction] = [:]
for staticMethod in staticMethods {
guard case .function(let nameToken, let lambdaExpr) = staticMethod else {
throw RuntimeError.notAFunctionDeclaration
}
guard case .lambda(let paramTokens, let methodBody) = lambdaExpr else {
throw RuntimeError.notALambda
}
let staticMethodImpl = UserDefinedFunction(name: nameToken.lexeme,
params: paramTokens,
enclosingEnvironment: environment,
body: methodBody,
isInitializer: false)
staticMethodImpls[nameToken.lexeme] = staticMethodImpl
}
let newClass = LoxClass(name: nameToken.lexeme,
superclass: superclass,
methods: methodImpls)
if !staticMethodImpls.isEmpty {
// NOTA BENE: This assigns the static methods to the metaclass,
// which is lazily created in `LoxInstance`
newClass.klass.methods = staticMethodImpls
}
// Note that we can't accomplish this via a defer block because we need
// to assign the class to the _outermost_ environment, not the enclosing one.
if superclassExpr != nil {
environment = environment.enclosingEnvironment!
}
try environment.assignAtDepth(name: nameToken.lexeme, value: .instance(newClass), depth: 0)
}
private func handleFunctionDeclaration(name: Token, lambda: ResolvedExpression) throws {
guard case .lambda(let params, let body) = lambda else {
throw RuntimeError.notALambda
}
let environmentWhenDeclared = self.environment
let function = UserDefinedFunction(name: name.lexeme,
params: params,
enclosingEnvironment: environmentWhenDeclared,
body: body,
isInitializer: false)
environment.define(name: name.lexeme, value: .userDefinedFunction(function))
}
private func handleReturnStatement(returnToken: Token, expr: ResolvedExpression?) throws {
var value: LoxValue = .nil
if let expr {
value = try evaluate(expr: expr)
}
throw Return.return(value)
}
private func handleVariableDeclaration(name: Token, expr: ResolvedExpression?) throws {
var value: LoxValue = .nil
if let expr = expr {
value = try evaluate(expr: expr)
}
environment.define(name: name.lexeme, value: value)
}
func handleBlock(statements: [ResolvedStatement], environment: Environment) throws {
let environmentBeforeBlock = self.environment
self.environment = environment
// This ensures that the previous environment is restored
// if the try below throws, which is what will happen if
// there is a return statement.
defer {
self.environment = environmentBeforeBlock
}
for statement in statements {
try execute(statement: statement)
}
}
private func handleWhileStatement(expr: ResolvedExpression, stmt: ResolvedStatement) throws {
while isTruthy(value: try evaluate(expr: expr)) {
try execute(statement: stmt)
}
}
private func evaluate(expr: ResolvedExpression) throws -> LoxValue {
switch expr {
case .literal(let literal):
return literal
case .grouping(let expr):
return try evaluate(expr: expr)
case .unary(let oper, let expr):
return try handleUnaryExpression(oper: oper, expr: expr)
case .binary(let leftExpr, let oper, let rightExpr):
return try handleBinaryExpression(leftExpr: leftExpr, oper: oper, rightExpr: rightExpr)
case .variable(let varToken, let depth):
return try handleVariableExpression(varToken: varToken, depth: depth)
case .assignment(let varToken, let valueExpr, let depth):
return try handleAssignmentExpression(name: varToken, expr: valueExpr, depth: depth)
case .logical(let leftExpr, let oper, let rightExpr):
return try handleLogicalExpression(leftExpr: leftExpr, oper: oper, rightExpr: rightExpr)
case .call(let calleeExpr, let rightParen, let args):
return try handleCallExpression(calleeExpr: calleeExpr, rightParen: rightParen, args: args)
case .get(let instanceExpr, let propertyNameToken):
return try handleGetExpression(instanceExpr: instanceExpr, propertyNameToken: propertyNameToken)
case .set(let instanceExpr, let propertyNameToken, let valueExpr):
return try handleSetExpression(instanceExpr: instanceExpr,
propertyNameToken: propertyNameToken,
valueExpr: valueExpr)
case .this(let thisToken, let depth):
return try handleThis(thisToken: thisToken, depth: depth)
case .lambda(let params, let statements):
return try handleLambdaExpression(params: params, statements: statements)
case .super(let superToken, let methodToken, let depth):
return try handleSuperExpression(superToken: superToken, methodToken: methodToken, depth: depth)
}
}
private func handleUnaryExpression(oper: Token, expr: ResolvedExpression) throws -> LoxValue {
let value = try evaluate(expr: expr)
switch oper.type {
case .minus:
guard case .number(let number) = value else {
throw RuntimeError.unaryOperandMustBeNumber
}
return .number(-number)
case .bang:
return .boolean(!isTruthy(value: value))
default:
throw RuntimeError.unsupportedUnaryOperator
}
}
private func handleBinaryExpression(leftExpr: ResolvedExpression,
oper: Token,
rightExpr: ResolvedExpression) throws -> LoxValue {
let leftValue = try evaluate(expr: leftExpr)
let rightValue = try evaluate(expr: rightExpr)
if case .number(let leftNumber) = leftValue,
case .number(let rightNumber) = rightValue {
switch oper.type {
case .plus:
return .number(leftNumber + rightNumber)
case .minus:
return .number(leftNumber - rightNumber)
case .star:
return .number(leftNumber * rightNumber)
case .slash:
return .number(leftNumber / rightNumber)
case .greater:
return .boolean(leftNumber > rightNumber)
case .greaterEqual:
return .boolean(leftNumber >= rightNumber)
case .less:
return .boolean(leftNumber < rightNumber)
case .lessEqual:
return .boolean(leftNumber <= rightNumber)
default:
break
}
}
if case .string(let leftString) = leftValue,
case .string(let rightString) = rightValue,
case .plus = oper.type {
return .string(leftString + rightString)
}
switch oper.type {
case .bangEqual:
return .boolean(!isEqual(leftValue: leftValue, rightValue: rightValue))
case .equalEqual:
return .boolean(isEqual(leftValue: leftValue, rightValue: rightValue))
case .plus:
throw RuntimeError.binaryOperandsMustBeNumbersOrStrings
case .minus, .star, .slash, .greater, .greaterEqual, .less, .lessEqual:
throw RuntimeError.binaryOperandsMustBeNumbers
default:
throw RuntimeError.unsupportedBinaryOperator
}
}
private func handleVariableExpression(varToken: Token, depth: Int) throws -> LoxValue {
return try environment.getValueAtDepth(name: varToken.lexeme, depth: depth)
}
private func handleAssignmentExpression(name: Token,
expr: ResolvedExpression,
depth: Int) throws -> LoxValue {
let value = try evaluate(expr: expr)
try environment.assignAtDepth(name: name.lexeme, value: value, depth: depth)
return value
}
private func handleLogicalExpression(leftExpr: ResolvedExpression,
oper: Token,
rightExpr: ResolvedExpression) throws -> LoxValue {
let leftValue = try evaluate(expr: leftExpr)
if case .and = oper.type {
if !isTruthy(value: leftValue) {
return leftValue
} else {
return try evaluate(expr: rightExpr)
}
} else {
if isTruthy(value: leftValue) {
return leftValue
} else {
return try evaluate(expr: rightExpr)
}
}
}
private func handleCallExpression(calleeExpr: ResolvedExpression,
rightParen: Token,
args: [ResolvedExpression]) throws -> LoxValue {
let callee = try evaluate(expr: calleeExpr)
let actualCallable: LoxCallable = switch callee {
case .userDefinedFunction(let userDefinedFunction):
userDefinedFunction
case .nativeFunction(let nativeFunction):
nativeFunction
case .instance(let klass as LoxClass):
klass
default:
throw RuntimeError.notACallableObject
}
guard args.count == actualCallable.arity else {
throw RuntimeError.wrongArity(actualCallable.arity, args.count)
}
var argValues: [LoxValue] = []
for arg in args {
let argValue = try evaluate(expr: arg)
argValues.append(argValue)
}
return try actualCallable.call(interpreter: self, args: argValues)
}
private func handleGetExpression(instanceExpr: ResolvedExpression,
propertyNameToken: Token) throws -> LoxValue {
guard case .instance(let instance) = try evaluate(expr: instanceExpr) else {
throw RuntimeError.onlyInstancesHaveProperties
}
return try instance.get(propertyName: propertyNameToken.lexeme)
}
private func handleSetExpression(instanceExpr: ResolvedExpression,
propertyNameToken: Token,
valueExpr: ResolvedExpression) throws -> LoxValue {
guard case .instance(let instance) = try evaluate(expr: instanceExpr) else {
throw RuntimeError.onlyInstancesHaveProperties
}
let propertyValue = try evaluate(expr: valueExpr)
instance.set(propertyName: propertyNameToken.lexeme, propertyValue: propertyValue)
return propertyValue
}
private func handleThis(thisToken: Token, depth: Int) throws -> LoxValue {
return try environment.getValueAtDepth(name: thisToken.lexeme, depth: depth)
}
private func handleLambdaExpression(params: [Token], statements: [ResolvedStatement]) throws -> LoxValue {
let environmentWhenDeclared = self.environment
let function = UserDefinedFunction(name: "lambda",
params: params,
enclosingEnvironment: environmentWhenDeclared,
body: statements,
isInitializer: false)
return .userDefinedFunction(function)
}
private func handleSuperExpression(superToken: Token, methodToken: Token, depth: Int) throws -> LoxValue {
guard case .instance(let superclass as LoxClass) = try environment.getValueAtDepth(name: "super", depth: depth) else {
throw RuntimeError.superclassMustBeAClass
}
guard case .instance(let thisInstance) = try environment.getValueAtDepth(name: "this", depth: depth - 1) else {
throw RuntimeError.notAnInstance
}
if let method = superclass.findMethod(name: methodToken.lexeme) {
return .userDefinedFunction(method.bind(instance: thisInstance))
}
throw RuntimeError.undefinedProperty(methodToken.lexeme)
}
// Utility functions below
private func isEqual(leftValue: LoxValue, rightValue: LoxValue) -> Bool {
switch (leftValue, rightValue) {
case (.nil, .nil):
return true
case (.number(let leftNumber), .number(let rightNumber)):
return leftNumber == rightNumber
case (.string(let leftString), .string(let rightString)):
return leftString == rightString
case (.boolean(let leftBoolean), .boolean(let rightBoolean)):
return leftBoolean == rightBoolean
default:
return false
}
}
// In Lox, `false` and `nil` are false; everything else is true
private func isTruthy(value: LoxValue) -> Bool {
switch value {
case .nil:
return false
case .boolean(let boolean):
return boolean
default:
return true
}
}
}