/
Resolver.swift
539 lines (446 loc) · 21 KB
/
Resolver.swift
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//
// Resolver.swift
// slox
//
// Created by Danielle Kefford on 3/3/24.
//
struct Resolver {
private enum FunctionType {
case none
case function
case method
case lambda
case initializer
}
private enum ClassType {
case none
case `class`
case subclass
}
private enum LoopType {
case none
case loop
}
private var scopeStack: [[String: Bool]] = []
private var currentFunctionType: FunctionType = .none
private var currentClassType: ClassType = .none
private var currentLoopType: LoopType = .none
// Main point of entry
mutating func resolve(statements: [Statement]) throws -> [ResolvedStatement] {
let resolvedStatements = try statements.map { statement in
return try resolve(statement: statement)
}
return resolvedStatements
}
// Resolver for statements
private mutating func resolve(statement: Statement) throws -> ResolvedStatement {
switch statement {
case .block(let statements):
return try handleBlock(statements: statements)
case .variableDeclaration(let nameToken, let initializeExpr):
return try handleVariableDeclaration(nameToken: nameToken, initializeExpr: initializeExpr)
case .class(let nameToken, let superclassExpr, let methods, let staticMethods):
return try handleClassDeclaration(nameToken: nameToken,
superclassExpr: superclassExpr,
methods: methods,
staticMethods: staticMethods)
case .function(let nameToken, let lambdaExpr):
return try handleFunctionDeclaration(nameToken: nameToken,
lambdaExpr: lambdaExpr,
functionType: .function)
case .expression(let expr):
return try handleExpressionStatement(expr: expr)
case .if(let testExpr, let consequentStmt, let alternativeStmt):
return try handleIf(testExpr: testExpr, consequentStmt: consequentStmt, alternativeStmt: alternativeStmt)
case .print(let expr):
return try handlePrintStatement(expr: expr)
case .return(let returnToken, let expr):
return try handleReturnStatement(returnToken: returnToken, expr: expr)
case .while(let conditionExpr, let bodyStmt):
return try handleWhile(conditionExpr: conditionExpr, bodyStmt: bodyStmt)
case .for(let initializerStmt, let testExpr, let incrementExpr, let bodyStmt):
return try handleFor(initializerStmt: initializerStmt,
testExpr: testExpr,
incrementExpr: incrementExpr,
bodyStmt: bodyStmt)
case .break(let breakToken):
return try handleBreak(breakToken: breakToken)
case .continue(let continueToken):
return try handleContinue(continueToken: continueToken)
}
}
mutating private func handleBlock(statements: [Statement]) throws -> ResolvedStatement {
beginScope()
defer {
endScope()
}
let resolvedStatements = try resolve(statements: statements)
return .block(resolvedStatements)
}
mutating private func handleVariableDeclaration(nameToken: Token, initializeExpr: Expression?) throws -> ResolvedStatement {
try declareVariable(name: nameToken.lexeme)
var resolvedInitializerExpr: ResolvedExpression? = nil
if let initializeExpr {
resolvedInitializerExpr = try resolve(expression: initializeExpr)
}
defineVariable(name: nameToken.lexeme)
return .variableDeclaration(nameToken, resolvedInitializerExpr)
}
mutating private func handleClassDeclaration(nameToken: Token,
superclassExpr: Expression?,
methods: [Statement],
staticMethods: [Statement]) throws -> ResolvedStatement {
let previousClassType = currentClassType
let previousLoopType = currentLoopType
currentClassType = .class
currentLoopType = .none
defer {
currentClassType = previousClassType
currentLoopType = previousLoopType
}
try declareVariable(name: nameToken.lexeme)
defineVariable(name: nameToken.lexeme)
// ACHTUNG! We need to attmept to resolve the superclass _before_
// pushing `this` onto the stack, otherwise we won't find it!
var resolvedSuperclassExpr: ResolvedExpression? = nil
if case .variable(let superclassName) = superclassExpr {
currentClassType = .subclass
if superclassName.lexeme == nameToken.lexeme {
throw ResolverError.classCannotInheritFromItself
}
resolvedSuperclassExpr = try handleVariable(nameToken: superclassName)
}
if resolvedSuperclassExpr != nil {
beginScope()
scopeStack.lastMutable["super"] = true
}
defer {
if resolvedSuperclassExpr != nil {
endScope()
}
}
beginScope()
defer {
endScope()
}
// NOTA BENE: Note that the scope stack is never empty at this point
scopeStack.lastMutable["this"] = true
let resolvedMethods = try methods.map { method in
guard case .function(let nameToken, let lambdaExpr) = method else {
throw ResolverError.notAFunction
}
let functionType: FunctionType = if nameToken.lexeme == "init" {
.initializer
} else {
.method
}
return try handleFunctionDeclaration(nameToken: nameToken,
lambdaExpr: lambdaExpr,
functionType: functionType)
}
let resolvedStaticMethods = try staticMethods.map { method in
guard case .function(let nameToken, let lambdaExpr) = method else {
throw ResolverError.notAFunction
}
if nameToken.lexeme == "init" {
throw ResolverError.staticInitsNotAllowed
}
return try handleFunctionDeclaration(nameToken: nameToken,
lambdaExpr: lambdaExpr,
functionType: .method)
}
return .class(nameToken, resolvedSuperclassExpr, resolvedMethods, resolvedStaticMethods)
}
mutating private func handleFunctionDeclaration(nameToken: Token,
lambdaExpr: Expression,
functionType: FunctionType) throws -> ResolvedStatement {
guard case .lambda(let paramTokens, let statements) = lambdaExpr else {
throw ResolverError.notAFunction
}
try declareVariable(name: nameToken.lexeme)
defineVariable(name: nameToken.lexeme)
let resolvedLambda = try handleLambda(params: paramTokens,
statements: statements,
functionType: functionType)
return .function(nameToken, resolvedLambda)
}
mutating private func handleExpressionStatement(expr: Expression) throws -> ResolvedStatement {
let resolvedExpression = try resolve(expression: expr)
return .expression(resolvedExpression)
}
mutating private func handleIf(testExpr: Expression,
consequentStmt: Statement,
alternativeStmt: Statement?) throws -> ResolvedStatement {
let resolvedTestExpr = try resolve(expression: testExpr)
let resolvedConsequentStmt = try resolve(statement: consequentStmt)
var resolvedAlternativeStmt: ResolvedStatement? = nil
if let alternativeStmt {
resolvedAlternativeStmt = try resolve(statement: alternativeStmt)
}
return .if(resolvedTestExpr, resolvedConsequentStmt, resolvedAlternativeStmt)
}
mutating private func handlePrintStatement(expr: Expression) throws -> ResolvedStatement {
let resolvedExpression = try resolve(expression: expr)
return .print(resolvedExpression)
}
mutating private func handleReturnStatement(returnToken: Token, expr: Expression?) throws -> ResolvedStatement {
if currentFunctionType == .none {
throw ResolverError.cannotReturnOutsideFunction
}
// NOTA BENE: We allow for an initializer to have a `return`
// statement if it does _not_ include an expression.
if let expr {
if currentFunctionType == .initializer {
throw ResolverError.cannotReturnValueFromInitializer
}
let resolvedExpr = try resolve(expression: expr)
return .return(returnToken, resolvedExpr)
}
return .return(returnToken, nil)
}
mutating private func handleBreak(breakToken: Token) throws -> ResolvedStatement {
if currentLoopType == .none {
throw ResolverError.cannotBreakOutsideLoop
}
return .break(breakToken)
}
mutating private func handleContinue(continueToken: Token) throws -> ResolvedStatement {
if currentLoopType == .none {
throw ResolverError.cannotContinueOutsideLoop
}
return .continue(continueToken)
}
mutating private func handleWhile(conditionExpr: Expression, bodyStmt: Statement) throws -> ResolvedStatement {
let previousLoopType = currentLoopType
currentLoopType = .loop
defer {
currentLoopType = previousLoopType
}
let resolvedConditionExpr = try resolve(expression: conditionExpr)
let resolvedBodyStmt = try resolve(statement: bodyStmt)
return .while(resolvedConditionExpr, resolvedBodyStmt)
}
mutating private func handleFor(initializerStmt: Statement?,
testExpr: Expression,
incrementExpr: Expression?,
bodyStmt: Statement) throws -> ResolvedStatement {
let previousLoopType = currentLoopType
currentLoopType = .loop
defer {
currentLoopType = previousLoopType
}
var resolvedInitializerStmt: ResolvedStatement? = nil
if let initializerStmt {
resolvedInitializerStmt = try resolve(statement: initializerStmt)
}
let resolvedTestExpr = try resolve(expression: testExpr)
var resolvedIncrementExpr: ResolvedExpression? = nil
if let incrementExpr {
resolvedIncrementExpr = try resolve(expression: incrementExpr)
}
let resolvedBodyStmt = try resolve(statement: bodyStmt)
return .for(resolvedInitializerStmt,
resolvedTestExpr,
resolvedIncrementExpr,
resolvedBodyStmt)
}
// Resolver for expressions
mutating private func resolve(expression: Expression) throws -> ResolvedExpression {
switch expression {
case .variable(let nameToken):
return try handleVariable(nameToken: nameToken)
case .assignment(let nameToken, let valueExpr):
return try handleAssignment(nameToken: nameToken, valueExpr: valueExpr)
case .binary(let leftExpr, let operToken, let rightExpr):
return try handleBinary(leftExpr: leftExpr, operToken: operToken, rightExpr: rightExpr)
case .unary(let operToken, let rightExpr):
return try handleUnary(operToken: operToken, rightExpr: rightExpr)
case .call(let calleeExpr, let rightParenToken, let args):
return try handleCall(calleeExpr: calleeExpr, rightParenToken: rightParenToken, args: args)
case .get(let instanceExpr, let propertyNameToken):
return try handleGet(instanceExpr: instanceExpr, propertyNameToken: propertyNameToken)
case .set(let instanceExpr, let propertyNameToken, let valueExpr):
return try handleSet(instanceExpr: instanceExpr,
propertyNameToken: propertyNameToken,
valueExpr: valueExpr)
case .this(let thisToken):
return try handleThis(thisToken: thisToken)
case .literal(let value):
return .literal(value)
case .grouping(let expr):
let resolvedExpr = try resolve(expression: expr)
return .grouping(resolvedExpr)
case .logical(let leftExpr, let operToken, let rightExpr):
return try handleLogical(leftExpr: leftExpr, operToken: operToken, rightExpr: rightExpr)
case .lambda(let params, let statements):
return try handleLambda(params: params, statements: statements, functionType: .lambda)
case .super(let superToken, let methodToken):
return try handleSuper(superToken: superToken, methodToken: methodToken)
case .list(let elements):
return try handleList(elements: elements)
case .subscriptGet(let listExpr, let indexExpr):
return try handleSubscriptGet(listExpr: listExpr, indexExpr: indexExpr)
case .subscriptSet(let listExpr, let indexExpr, let valueExpr):
return try handleSubscriptSet(listExpr: listExpr, indexExpr: indexExpr, valueExpr: valueExpr)
}
}
mutating private func handleVariable(nameToken: Token) throws -> ResolvedExpression {
if !scopeStack.isEmpty && scopeStack.lastMutable[nameToken.lexeme] == false {
throw ResolverError.variableAccessedBeforeInitialization
}
let depth = getDepth(name: nameToken.lexeme)
return .variable(nameToken, depth)
}
mutating private func handleAssignment(nameToken: Token, valueExpr: Expression) throws -> ResolvedExpression {
let resolveValueExpr = try resolve(expression: valueExpr)
let depth = getDepth(name: nameToken.lexeme)
return .assignment(nameToken, resolveValueExpr, depth)
}
mutating private func handleBinary(leftExpr: Expression,
operToken: Token,
rightExpr: Expression) throws -> ResolvedExpression {
let resolvedLeftExpr = try resolve(expression: leftExpr)
let resolvedRightExpr = try resolve(expression: rightExpr)
return .binary(resolvedLeftExpr, operToken, resolvedRightExpr)
}
mutating private func handleUnary(operToken: Token, rightExpr: Expression) throws -> ResolvedExpression {
let resolvedRightExpr = try resolve(expression: rightExpr)
return .unary(operToken, resolvedRightExpr)
}
mutating private func handleCall(calleeExpr: Expression,
rightParenToken: Token,
args: [Expression]) throws -> ResolvedExpression {
let resolvedCalleeExpr = try resolve(expression: calleeExpr)
let resolvedArgs = try args.map { arg in
try resolve(expression: arg)
}
return .call(resolvedCalleeExpr, rightParenToken, resolvedArgs)
}
mutating private func handleGet(instanceExpr: Expression,
propertyNameToken: Token) throws -> ResolvedExpression {
// Note that we don't attempt to resolve property names
// because they are defined and looked up at _runtime_.
let resolvedInstanceExpr = try resolve(expression: instanceExpr)
return .get(resolvedInstanceExpr, propertyNameToken)
}
mutating private func handleSet(instanceExpr: Expression,
propertyNameToken: Token,
valueExpr: Expression) throws -> ResolvedExpression {
// As with `get` expressions, we do _not_ try to
// resolve property names.
let resolvedInstanceExpr = try resolve(expression: instanceExpr)
let resolvedValueExpr = try resolve(expression: valueExpr)
return .set(resolvedInstanceExpr, propertyNameToken, resolvedValueExpr)
}
mutating private func handleThis(thisToken: Token) throws -> ResolvedExpression {
guard case .class = currentClassType else {
throw ResolverError.cannotReferenceThisOutsideClass
}
let depth = getDepth(name: thisToken.lexeme)
return .this(thisToken, depth)
}
mutating private func handleLogical(leftExpr: Expression,
operToken: Token,
rightExpr: Expression) throws -> ResolvedExpression {
let resolvedLeftExpr = try resolve(expression: leftExpr)
let resolvedRightExpr = try resolve(expression: rightExpr)
return .logical(resolvedLeftExpr, operToken, resolvedRightExpr)
}
mutating private func handleLambda(params: [Token],
statements: [Statement],
functionType: FunctionType) throws -> ResolvedExpression {
beginScope()
let previousFunctionType = currentFunctionType
let previousLoopType = currentLoopType
currentFunctionType = functionType
currentLoopType = .none
defer {
endScope()
currentFunctionType = previousFunctionType
currentLoopType = previousLoopType
}
for param in params {
try declareVariable(name: param.lexeme)
defineVariable(name: param.lexeme)
}
let resolvedStatements = try statements.map { statement in
try resolve(statement: statement)
}
return .lambda(params, resolvedStatements)
}
mutating private func handleSuper(superToken: Token, methodToken: Token) throws -> ResolvedExpression {
switch currentClassType {
case .none:
throw ResolverError.cannotReferenceSuperOutsideClass
case .class:
throw ResolverError.cannotReferenceSuperWithoutSubclassing
case .subclass:
break
}
let depth = getDepth(name: superToken.lexeme)
return .super(superToken, methodToken, depth)
}
mutating private func handleList(elements: [Expression]) throws -> ResolvedExpression {
let resolvedElements = try elements.map { element in
return try resolve(expression: element)
}
return .list(resolvedElements)
}
mutating private func handleSubscriptGet(listExpr: Expression, indexExpr: Expression) throws -> ResolvedExpression {
let resolvedListExpr = try resolve(expression: listExpr)
let resolvedIndexExpr = try resolve(expression: indexExpr)
return .subscriptGet(resolvedListExpr, resolvedIndexExpr)
}
mutating private func handleSubscriptSet(listExpr: Expression,
indexExpr: Expression,
valueExpr: Expression) throws -> ResolvedExpression {
let resolvedListExpr = try resolve(expression: listExpr)
let resolvedIndexExpr = try resolve(expression: indexExpr)
let resolvedValueExpr = try resolve(expression: valueExpr)
return .subscriptSet(resolvedListExpr, resolvedIndexExpr, resolvedValueExpr)
}
// Internal helpers
mutating private func beginScope() {
scopeStack.append([:])
}
mutating private func endScope() {
scopeStack.removeLast()
}
mutating private func declareVariable(name: String) throws {
// ACHTUNG!!! Only variables declared/defined in local
// blocks are tracked by the resolver, which is why
// we bail here since the stack is empty in the
// global environment.
if scopeStack.isEmpty {
return
}
if scopeStack.lastMutable.keys.contains(name) {
throw ResolverError.variableAlreadyDefined(name)
}
scopeStack.lastMutable[name] = false
}
mutating private func defineVariable(name: String) {
// ACHTUNG!!! Only variables declared/defined in local
// blocks are tracked by the resolver, which is why
// we bail here since the stack is empty in the
// global environment.
if scopeStack.isEmpty {
return
}
scopeStack.lastMutable[name] = true
}
private func getDepth(name: String) -> Int {
var i = scopeStack.count - 1
while i >= 0 {
if let _ = scopeStack[i][name] {
return scopeStack.count - 1 - i
}
i = i - 1
}
// If we get here, the variable must be defined
// in the global environment, and not tracked by the
// resolver, and so we return the depth required to
// fetch the value of that variable.
return scopeStack.count
}
}