词法分析

TDD：测试驱动开发

先写测试用例，再进行词法分析逻辑的完善。

语法分析

递归下降语法分析伪代码

function parseProgram() {
    program = newProgramASTNode()
    advanceTokens()
    for (currentToken() != EOF_TOKEN) {
        statement = null
        if (currentToken() == LET_TOKEN) {
            statement = parseLetStatement()
        } else if (currentToken() == RETURN_TOKEN) {
            statement = parseReturnStatement()
        } else if (currentToken() == IF_TOKEN) {
            statement = parseIfStatement()
        }
        if (statement != null) {
            program.Statements.push(statement)
        }
        advanceTokens()
    }
    return program
}
function parseLetStatement() {
    advanceTokens()
    identifier = parseIdentifier()
    advanceTokens()
    if currentToken() != EQUAL_TOKEN {
        parseError("no equal sign!")
        return null
    }
    advanceTokens()
    value = parseExpression()
    variableStatement = newVariableStatementASTNode()
    variableStatement.identifier = identifier
    variableStatement.value = value
    return variableStatement
}
function parseIdentifier() {
    identifier = newIdentifierASTNode()
    identifier.token = currentToken()
    return identifier
}
function parseExpression() {
    if (currentToken() == INTEGER_TOKEN) {
        if (nextToken() == PLUS_TOKEN) {
            return parseOperatorExpression()
        } else if (nextToken() == SEMICOLON_TOKEN) {
            return parseIntegerLiteral()
        }
    } else if (currentToken() == LEFT_PAREN) {
        return parseGroupedExpression()
    }
// [...]
}
function parseOperatorExpression() {
    operatorExpression = newOperatorExpression()
    operatorExpression.left = parseIntegerLiteral()
    operatorExpression.operator = currentToken()
    operatorExpression.right = parseExpression()
    return operatorExpression()
}