feat: add lexer

Dockerfile

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+ARG GO_VERSION=1.21.1
+
+FROM --platform=${BUILDPLATFORM} golang:${GO_VERSION}-alpine AS base
+WORKDIR /src
+# RUN --mount=type=bind,target=go.mod \
+#   --mount=type=bind,target=go.sum \
+#   go mod download
+
+FROM base as tests
+WORKDIR /src
+RUN --mount=type=bind,target=. \
+  go test ./...

compose.yaml

@@ -0,0 +1,15 @@
+---
+version: "3"
+
+services:
+  base:
+    build:
+      target: base
+    volumes:
+      - .:/src
+    working_dir: /src
+
+  format:
+    extends:
+      service: base
+    entrypoint: "gofmt -w *.go"

go.mod

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+module git.cybercyst.me/monkeylang
+
+go 1.21.1

lexer/lexer.go

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+package lexer
+
+import "git.cybercyst.me/monkeylang/token"
+
+type Lexer struct {
+	input        string
+	position     int
+	readPosition int
+	ch           byte
+}
+
+func New(input string) *Lexer {
+	l := &Lexer{input: input}
+	l.readChar()
+	return l
+}
+
+func (l *Lexer) readChar() {
+	if l.readPosition >= len(l.input) {
+		l.ch = 0
+	} else {
+		l.ch = l.input[l.readPosition]
+	}
+
+	l.position = l.readPosition
+	l.readPosition += 1
+}
+
+func (l *Lexer) NextToken() token.Token {
+	var tok token.Token
+
+	switch l.ch {
+	case '=':
+		tok = newToken(token.ASSIGN, l.ch)
+	case ';':
+		tok = newToken(token.SEMICOLON, l.ch)
+	case '(':
+		tok = newToken(token.LPAREN, l.ch)
+	case ')':
+		tok = newToken(token.RPAREN, l.ch)
+	case ',':
+		tok = newToken(token.COMMA, l.ch)
+	case '+':
+		tok = newToken(token.PLUS, l.ch)
+	case '{':
+		tok = newToken(token.LBRACE, l.ch)
+	case '}':
+		tok = newToken(token.RBRACE, l.ch)
+	case 0:
+		tok.Literal = ""
+		tok.Type = token.EOF
+	}
+
+	l.readChar()
+	return tok
+}
+
+func newToken(tokenType token.TokenType, ch byte) token.Token {
+	return token.Token{Type: tokenType, Literal: string(ch)}
+}

lexer/lexer_test.go

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+package lexer
+
+import (
+	"testing"
+
+	"git.cybercyst.me/monkeylang/token"
+)
+
+func TextNextToken(t *testing.T) {
+	input := `=+(){},;`
+
+	tests := []struct {
+		expectedType    token.TokenType
+		expectedLiteral string
+	}{
+		{token.ASSIGN, "="},
+		{token.PLUS, "+"},
+		{token.LPAREN, "("},
+		{token.RPAREN, ")"},
+		{token.LBRACE, "{"},
+		{token.RBRACE, "}"},
+		{token.COMMA, ","},
+		{token.SEMICOLON, ";"},
+	}
+
+	l := New(input)
+
+	for i, tt := range tests {
+		tok := l.NextToken()
+		if tok.Type != tt.expectedType {
+			t.Fatalf("tests[%d] - tokentype wrong. expected=%q, got=%q",
+				i, tt.expectedType, tok.Type)
+		}
+
+		if tok.Literal != tt.expectedLiteral {
+			t.Fatalf("tests[%d] - literal wrong. expected=%q, got=%q",
+				i, tt.expectedLiteral, tok.Literal)
+		}
+	}
+}

main.go

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+package main
+
+import "fmt"
+
+func main() {
+	fmt.Println("Hello, World!")
+}

token/token.go

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+package token
+
+type TokenType string
+
+type Token struct {
+	Type    TokenType
+	Literal string
+}
+
+const (
+	ILLEGAL = "ILLEGAL"
+	EOF     = "EOF"   // Identifiers + literals
+	IDENT   = "IDENT" // add, foobar, x, y, ...
+	INT     = "INT"   // 1343456
+
+	// Operators
+	ASSIGN = "="
+	PLUS   = "+"
+
+	// Delimiters
+	COMMA     = ","
+	SEMICOLON = ";"
+	LPAREN    = "("
+	RPAREN    = ")"
+	LBRACE    = "{"
+	RBRACE    = "}"
+
+	// Keywords
+	FUNCTION = "FUNCTION"
+	LET      = "LET"
+)