package lexer import "monkey/pkg/token" // Lexer represents the lexer type Lexer struct { input string // current position in input position int // current reading position in input (after a char) readPosition int // current character under examination ch byte } // New returns a new lexer func New(input string) *Lexer { l := &Lexer{input: input} l.readChar() return l } // Read the current character and advances our position in the input string. func (l *Lexer) readChar() { // if we've reached the end of the input, we set the current character to 0, // which is the ASCII code for NUL. if l.readPosition >= len(l.input) { l.ch = 0 } else { l.ch = l.input[l.readPosition] } l.position = l.readPosition l.readPosition++ } func (l *Lexer) readIdentifier() string { position := l.position for isLetter(l.ch) { l.readChar() } return l.input[position:l.position] } func (l *Lexer) readNumber() string { position := l.position for isDigit(l.ch) { l.readChar() } return l.input[position:l.position] } // we don't care about white space characters, we skip them when we find them. func (l *Lexer) skipWhitespace() { for l.ch == ' ' || l.ch == '\t' || l.ch == '\n' || l.ch == '\r' { l.readChar() } } // NextToken reads the next token from the lexer and returns the current token. func (l *Lexer) NextToken() token.Token { var tok token.Token l.skipWhitespace() switch l.ch { case '=': tok = newToken(token.ASSIGN, l.ch) case '+': tok = newToken(token.PLUS, l.ch) case '-': tok = newToken(token.MINUS, l.ch) case '!': tok = newToken(token.BANG, l.ch) case '*': tok = newToken(token.ASTERISK, l.ch) case '/': tok = newToken(token.SLASH, l.ch) case '<': tok = newToken(token.LT, l.ch) case '>': tok = newToken(token.GT, l.ch) case ';': tok = newToken(token.SEMICOLON, l.ch) case ',': tok = newToken(token.COMMA, l.ch) case '(': tok = newToken(token.LPAREN, l.ch) case ')': tok = newToken(token.RPAREN, l.ch) case '{': tok = newToken(token.LBRACE, l.ch) case '}': tok = newToken(token.RBRACE, l.ch) case 0: tok.Literal = "" tok.Type = token.EOF default: if isLetter(l.ch) { tok.Literal = l.readIdentifier() tok.Type = token.LookupIdent(tok.Literal) return tok } else if isDigit(l.ch) { tok.Type = token.INT tok.Literal = l.readNumber() return tok } else { tok = newToken(token.ILLEGAL, l.ch) } } l.readChar() return tok } func newToken(tokenType token.TokenType, ch byte) token.Token { return token.Token{Type: tokenType, Literal: string(ch)} } func isLetter(ch byte) bool { return 'a' <= ch && ch <= 'z' || 'A' <= ch && ch <= 'Z' || ch == '_' } func isDigit(ch byte) bool { return '0' <= ch && ch <= '9' }