/
Parser.swift
691 lines (559 loc) · 22.5 KB
/
Parser.swift
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//
// Parser.swift
// slox
//
// Created by Danielle Kefford on 2/25/24.
//
struct Parser {
private var tokens: [Token]
private var cursor: Int = 0
private var currentToken: Token {
return tokens[cursor]
}
private var previousToken: Token {
return tokens[cursor - 1]
}
init(tokens: [Token]) {
self.tokens = tokens
}
mutating func parse() throws -> [Statement] {
var statements: [Statement] = []
while currentToken.type != .eof {
let statement = try parseDeclaration()
statements.append(statement)
}
return statements
}
// Statements are parsed in the following order:
//
// program → declaration* EOF ;
// declaration → classDecl
// | funDecl
// | varDecl
// | statement ;
// classDecl → "class" IDENTIFIER ( "<" IDENTIFIER )?
// "{" function* "}" ;
// funDecl → "fun" function ;
// function → IDENTIFIER "(" parameters? ")" block ;
// varDecl → "var" IDENTIFIER ( "=" expression )? ";" ;
// statement → exprStmt
// | forStmt
// | ifStmt
// | printStmt
// | jumpStmt
// | whileStmt
// | block ;
// exprStmt → expression ";" ;
// forStmt → "for" "(" ( varDecl | exprStmt | ";" )
// expression? ";"
// expression? ")" statement ;
// ifStmt → "if" "(" expression ")" statement
// ( "else" statement )? ;
// printStmt → "print" expression ";" ;
// jumpStmt → ( "return" expression? ";"
// | "break" ";"
// | "continue" ";" ) ;
// whileStmt → "while" "(" expression ")" statement ;
// block → "{" declaration* "}" ;
mutating private func parseDeclaration() throws -> Statement {
if currentTokenMatchesAny(types: [.class]) {
return try parseClassDeclaration()
}
// We look ahead to see if the next token is an identifer,
// and if so we assume this is a function declaration. Otherwise,
// if the current token is `fun`, then we have a lambda, and we
// will eventually parse it when we hit parsePrimary().
if currentTokenMatches(type: .fun) && nextTokenMatches(type: .identifier) {
advanceCursor()
return try parseFunctionDeclaration()
}
if currentTokenMatchesAny(types: [.var]) {
return try parseVariableDeclaration()
}
return try parseStatement()
}
mutating private func parseClassDeclaration() throws -> Statement {
guard case .identifier = currentToken.type else {
throw ParseError.missingClassName(currentToken)
}
let className = currentToken
advanceCursor()
var superclassExpr: Expression? = nil
if currentTokenMatchesAny(types: [.less]) {
guard case .identifier = currentToken.type else {
throw ParseError.missingSuperclassName(currentToken)
}
superclassExpr = .variable(currentToken)
advanceCursor()
}
if !currentTokenMatchesAny(types: [.leftBrace]) {
throw ParseError.missingOpenBraceBeforeClassBody(currentToken)
}
var methodStatements: [Statement] = []
var staticMethodStatements: [Statement] = []
while currentToken.type != .rightBrace && currentToken.type != .eof {
// Note that we don't look for/consume a `fun` token before
// calling `parseFunctionDeclaration()`. That's a deliberate
// design decision by the original author.
if currentTokenMatchesAny(types: [.class]) {
let staticMethodStatement = try parseFunctionDeclaration()
staticMethodStatements.append(staticMethodStatement)
} else {
let methodStatement = try parseFunctionDeclaration()
methodStatements.append(methodStatement)
}
}
if currentTokenMatchesAny(types: [.rightBrace]) {
return .class(className, superclassExpr, methodStatements, staticMethodStatements)
}
throw ParseError.missingClosingBrace(previousToken)
}
mutating private func parseFunctionDeclaration() throws -> Statement {
guard case .identifier = currentToken.type else {
throw ParseError.missingFunctionName(currentToken)
}
let functionName = currentToken
advanceCursor()
if !currentTokenMatchesAny(types: [.leftParen]) {
throw ParseError.missingOpenParenForFunctionDeclaration(currentToken)
}
let parameters = try parseParameters()
if !currentTokenMatchesAny(types: [.rightParen]) {
throw ParseError.missingCloseParenAfterArguments(currentToken)
}
if !currentTokenMatchesAny(types: [.leftBrace]) {
throw ParseError.missingOpenBraceBeforeFunctionBody(currentToken)
}
let functionBody = try parseBlock()
return .function(functionName, .lambda(parameters, functionBody))
}
mutating private func parseVariableDeclaration() throws -> Statement {
guard case .identifier = currentToken.type else {
throw ParseError.missingVariableName(currentToken)
}
let name = currentToken
advanceCursor()
var initializer: Expression? = nil
if currentTokenMatchesAny(types: [.equal]) {
initializer = try parseExpression()
}
if currentTokenMatchesAny(types: [.semicolon]) {
return .variableDeclaration(name, initializer);
}
throw ParseError.missingSemicolon(currentToken)
}
mutating private func parseStatement() throws -> Statement {
if currentTokenMatchesAny(types: [.for]) {
return try parseForStatement()
}
if currentTokenMatchesAny(types: [.if]) {
return try parseIfStatement()
}
if currentTokenMatchesAny(types: [.print]) {
return try parsePrintStatement()
}
if [.return, .break, .continue].contains(currentToken.type) {
return try parseJumpStatement()
}
if currentTokenMatchesAny(types: [.while]) {
return try parseWhileStatement()
}
if currentTokenMatchesAny(types: [.leftBrace]) {
let statements = try parseBlock()
return .block(statements)
}
return try parseExpressionStatement()
}
mutating private func parseForStatement() throws -> Statement {
if !currentTokenMatchesAny(types: [.leftParen]) {
throw ParseError.missingOpenParenForForStatement(currentToken)
}
var initializerStmt: Statement?
if currentTokenMatchesAny(types: [.semicolon]) {
initializerStmt = nil
} else if currentTokenMatchesAny(types: [.var]) {
initializerStmt = try parseVariableDeclaration()
} else {
initializerStmt = try parseExpressionStatement()
}
var testExpr: Expression = .literal(.boolean(true))
if !currentTokenMatches(type: .semicolon) {
testExpr = try parseExpression()
}
if !currentTokenMatchesAny(types: [.semicolon]) {
throw ParseError.missingSemicolonAfterForLoopCondition(currentToken)
}
var incrementExpr: Expression? = nil
if !currentTokenMatches(type: .rightParen) {
incrementExpr = try parseExpression()
}
if !currentTokenMatchesAny(types: [.rightParen]) {
throw ParseError.missingCloseParenForForStatement(currentToken)
}
let bodyStmt = try parseStatement()
return .for(initializerStmt, testExpr, incrementExpr, bodyStmt)
}
mutating private func parseIfStatement() throws -> Statement {
if !currentTokenMatchesAny(types: [.leftParen]) {
throw ParseError.missingOpenParenForIfStatement(currentToken)
}
let testExpr = try parseExpression()
if !currentTokenMatchesAny(types: [.rightParen]) {
throw ParseError.missingCloseParenForIfStatement(currentToken)
}
let consequentStmt = try parseStatement()
var alternativeStmt: Statement? = nil
if currentTokenMatchesAny(types: [.else]) {
alternativeStmt = try parseStatement()
}
return .if(testExpr, consequentStmt, alternativeStmt)
}
mutating private func parsePrintStatement() throws -> Statement {
let expr = try parseExpression()
if currentTokenMatchesAny(types: [.semicolon]) {
return .print(expr)
}
throw ParseError.missingSemicolon(currentToken)
}
mutating private func parseJumpStatement() throws -> Statement {
if currentTokenMatchesAny(types: [.return]) {
let returnToken = previousToken
var expr: Expression? = nil
if currentToken.type != .semicolon {
expr = try parseExpression()
}
if !currentTokenMatchesAny(types: [.semicolon]) {
throw ParseError.missingSemicolon(currentToken)
}
return .return(returnToken, expr)
}
if currentTokenMatchesAny(types: [.break]) {
let breakToken = previousToken
if !currentTokenMatchesAny(types: [.semicolon]) {
throw ParseError.missingSemicolon(currentToken)
}
return .break(breakToken)
}
if currentTokenMatchesAny(types: [.continue]) {
let continueToken = previousToken
if !currentTokenMatchesAny(types: [.semicolon]) {
throw ParseError.missingSemicolon(currentToken)
}
return .continue(continueToken)
}
throw ParseError.unsupportedJumpStatement(currentToken)
}
mutating private func parseWhileStatement() throws -> Statement {
if !currentTokenMatchesAny(types: [.leftParen]) {
throw ParseError.missingOpenParenForWhileStatement(currentToken)
}
let expr = try parseExpression()
if !currentTokenMatchesAny(types: [.rightParen]) {
throw ParseError.missingCloseParenForWhileStatement(currentToken)
}
let stmt = try parseStatement()
return .while(expr, stmt)
}
mutating private func parseBlock() throws -> [Statement] {
var statements: [Statement] = []
while currentToken.type != .rightBrace && currentToken.type != .eof {
let statement = try parseDeclaration()
statements.append(statement)
}
if currentTokenMatchesAny(types: [.rightBrace]) {
return statements
}
throw ParseError.missingClosingBrace(previousToken)
}
mutating private func parseExpressionStatement() throws -> Statement {
let expr = try parseExpression()
// NOTA BENE: If the expression is the last thing to be parsed,
// then we want to return that immediately so it can be evaluated
// and whose result can be printed in the REPL, and without burdening
// the user to add a semicolon at the end.
if currentToken.type == .eof || currentTokenMatchesAny(types: [.semicolon]) {
return .expression(expr)
}
throw ParseError.missingSemicolon(currentToken)
}
// The parsing strategy below follows these rules of precedence
// in _ascending_ order:
//
// expression → assignment ;
// assignment → ( call "." )? IDENTIFIER "=" assignment
// | logicOr ;
// logicOr → logicAnd ( "or" logicAnd )* ;
// logicAnd → equality ( "and" equality )* ;
// equality → comparison ( ( "!=" | "==" ) comparison )* ;
// comparison → term ( ( ">" | ">=" | "<" | "<=" ) term )* ;
// term → factor ( ( "-" | "+" ) factor )* ;
// factor → unary ( ( "/" | "*" ) unary )* ;
// unary → ( "!" | "-" ) unary
// | postfix ;
// postfix → primary ( "(" arguments? ")" | "." IDENTIFIER | "[" logicOr "]" )* ;
// primary → NUMBER | STRING | "true" | "false" | "nil"
// | "(" expression ")"
// | "[" arguments? "]"
// | "this"
// | IDENTIFIER
// | lambda
// | "super" "." IDENTIFIER ;
// lambda → "fun" "(" parameters? ")" block ;
//
mutating private func parseExpression() throws -> Expression {
return try parseAssignment()
}
mutating private func parseAssignment() throws -> Expression {
let expr = try parseLogicOr()
if currentTokenMatchesAny(types: [.equal]) {
let equalToken = previousToken
let valueExpr = try parseAssignment()
if case .variable(let name) = expr {
return .assignment(name, valueExpr)
} else if case .get(let instanceExpr, let propertyNameToken) = expr {
return .set(instanceExpr, propertyNameToken, valueExpr)
}
throw ParseError.invalidAssignmentTarget(equalToken)
}
return expr
}
mutating private func parseLogicOr() throws -> Expression {
var expr = try parseLogicAnd()
while currentTokenMatchesAny(types: [.or]) {
let oper = previousToken
let right = try parseLogicAnd()
expr = .logical(expr, oper, right)
}
return expr
}
mutating private func parseLogicAnd() throws -> Expression {
var expr = try parseEquality()
while currentTokenMatchesAny(types: [.and]) {
let oper = previousToken
let right = try parseEquality()
expr = .logical(expr, oper, right)
}
return expr
}
mutating private func parseEquality() throws -> Expression {
var expr = try parseComparison()
while currentTokenMatchesAny(types: [.bangEqual, .equalEqual]) {
let oper = previousToken
let rightExpr = try parseComparison()
expr = .binary(expr, oper, rightExpr)
}
return expr
}
mutating private func parseComparison() throws -> Expression {
var expr = try parseTerm()
while currentTokenMatchesAny(types: [.less, .lessEqual, .greater, .greaterEqual]) {
let oper = previousToken
let rightExpr = try parseTerm()
expr = .binary(expr, oper, rightExpr)
}
return expr
}
mutating private func parseTerm() throws -> Expression {
var expr = try parseFactor()
while currentTokenMatchesAny(types: [.plus, .minus]) {
let oper = previousToken
let rightExpr = try parseFactor()
expr = .binary(expr, oper, rightExpr)
}
return expr
}
mutating private func parseFactor() throws -> Expression {
var expr = try parseUnary()
while currentTokenMatchesAny(types: [.slash, .star]) {
let oper = previousToken
let rightExpr = try parseUnary()
expr = .binary(expr, oper, rightExpr)
}
return expr
}
mutating private func parseUnary() throws -> Expression {
if currentTokenMatchesAny(types: [.bang, .minus]) {
let oper = previousToken
let expr = try parseUnary()
return .unary(oper, expr)
}
return try parsePostfix()
}
mutating private func parsePostfix() throws -> Expression {
var expr = try parsePrimary()
while true {
if currentTokenMatchesAny(types: [.leftParen]) {
let args = try parseArguments()
if !currentTokenMatchesAny(types: [.rightParen]) {
throw ParseError.missingCloseParenAfterArguments(currentToken)
}
expr = .call(expr, previousToken, args)
} else if currentTokenMatchesAny(types: [.dot]) {
if !currentTokenMatchesAny(types: [.identifier]) {
throw ParseError.missingIdentifierAfterDot(currentToken)
}
expr = .get(expr, previousToken)
} else if currentTokenMatchesAny(types: [.leftBracket]) {
let indexExpr = try parseLogicOr()
if !currentTokenMatchesAny(types: [.rightBracket]) {
throw ParseError.missingCloseBracketForSubscriptAccess(currentToken)
}
if currentTokenMatchesAny(types: [.equal]) {
let valueExpr = try parseExpression()
expr = .subscriptSet(expr, indexExpr, valueExpr)
} else {
expr = .subscriptGet(expr, indexExpr)
}
} else {
break
}
}
return expr
}
mutating private func parsePrimary() throws -> Expression {
if currentTokenMatchesAny(types: [.fun]) {
return try parseLambda()
}
if currentTokenMatchesAny(types: [.false]) {
return .literal(.boolean(false))
}
if currentTokenMatchesAny(types: [.true]) {
return .literal(.boolean(true))
}
if currentTokenMatchesAny(types: [.nil]) {
return .literal(.nil)
}
if currentTokenMatchesAny(types: [.number]) {
let number = Double(previousToken.lexeme)!
return .literal(.number(number))
}
if currentTokenMatchesAny(types: [.string]) {
assert(previousToken.lexeme.hasPrefix("\"") && previousToken.lexeme.hasSuffix("\""))
let string = String(previousToken.lexeme.dropFirst().dropLast())
return .literal(.string(string))
}
if currentTokenMatchesAny(types: [.leftParen]) {
let expr = try parseExpression()
if currentTokenMatchesAny(types: [.rightParen]) {
return .grouping(expr)
}
throw ParseError.missingClosingParenthesis(currentToken)
}
if currentTokenMatchesAny(types: [.leftBracket]) {
let elements = try parseArguments()
if currentTokenMatchesAny(types: [.rightBracket]) {
return .list(elements)
}
throw ParseError.missingClosingBracket(previousToken)
}
if currentTokenMatchesAny(types: [.super]) {
let superToken = previousToken
if !currentTokenMatchesAny(types: [.dot]) {
throw ParseError.missingDotAfterSuper(currentToken)
}
guard case .identifier = currentToken.type else {
throw ParseError.expectedSuperclassMethodName(currentToken)
}
let methodToken = currentToken
advanceCursor()
return .super(superToken, methodToken)
}
if currentTokenMatchesAny(types: [.this]) {
return .this(previousToken)
}
if currentTokenMatchesAny(types: [.identifier]) {
return .variable(previousToken)
}
throw ParseError.expectedPrimaryExpression(currentToken)
}
mutating private func parseLambda() throws -> Expression {
if !currentTokenMatchesAny(types: [.leftParen]) {
throw ParseError.missingOpenParenForFunctionDeclaration(currentToken)
}
let parameters = try parseParameters()
if !currentTokenMatchesAny(types: [.rightParen]) {
throw ParseError.missingCloseParenAfterArguments(currentToken)
}
if !currentTokenMatchesAny(types: [.leftBrace]) {
throw ParseError.missingOpenBraceBeforeFunctionBody(currentToken)
}
let functionBody = try parseBlock()
return .lambda(parameters, functionBody)
}
// Utility grammar rules:
//
// parameters → IDENTIFIER ( "," IDENTIFIER )* ;
// arguments → expression ( "," expression )* ;
//
mutating private func parseParameters() throws -> [Token] {
var parameters: [Token] = []
if currentToken.type != .rightParen {
repeat {
guard case .identifier = currentToken.type else {
throw ParseError.missingParameterName(currentToken)
}
let newParameter = currentToken
advanceCursor()
parameters.append(newParameter)
} while currentTokenMatchesAny(types: [.comma])
}
return parameters
}
mutating private func parseArguments() throws -> [Expression] {
var args: [Expression] = []
if currentToken.type != .rightParen {
repeat {
let newArg = try parseExpression()
args.append(newArg)
} while currentTokenMatchesAny(types: [.comma])
}
return args
}
// Other utility methods
mutating private func currentTokenMatchesAny(types: [TokenType]) -> Bool {
for type in types {
if currentTokenMatches(type: type) {
advanceCursor()
return true
}
}
return false
}
private func currentTokenMatches(type: TokenType) -> Bool {
if currentToken.type == .eof {
return false
}
return currentToken.type == type
}
private func nextTokenMatches(type: TokenType) -> Bool {
if currentToken.type == .eof {
return false
}
let nextToken = tokens[cursor + 1]
if nextToken.type == .eof {
return false
}
return nextToken.type == type
}
// TODO: Figure out if we actually need this, and if so
// how to wire it up.
mutating private func synchronize() {
advanceCursor()
while currentToken.type != .eof {
if previousToken.type == .semicolon {
return
}
switch currentToken.type {
case .class, .fun, .var, .for, .if, .while, .print, .return:
return
default:
break
}
advanceCursor()
}
}
// TODO: We need to make sure we don't run out of tokens here
mutating private func advanceCursor() {
cursor += 1
}
}