commit e3bd2658cd032c9d386fa580eebc5ca3e9ddc8a4
Author: silva guimaraes <silva__guimaraes@hotmail.com>
Date:   Sun May 25 02:35:11 2025 -0300

    Commit inicial

diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..d114169
--- /dev/null
+++ b/go.mod
@@ -0,0 +1,3 @@
+module sexp
+
+go 1.24.2
diff --git a/input.go b/input.go
new file mode 100644
index 0000000..0676d21
--- /dev/null
+++ b/input.go
@@ -0,0 +1,69 @@
+package main
+
+import (
+	"fmt"
+	"slices"
+)
+
+type Input struct {
+	index int
+	input []Lexeme
+}
+
+func (in Input) Index() int {
+	return in.index
+}
+
+func (in Input) String() string {
+	_ = in.input[in.index]
+	return fmt.Sprintf("%v", in.input[in.index:])
+}
+
+func (in *Input) Len() int {
+	// _ = in.input[in.index]
+	return len(in.input[in.index:])
+}
+
+func (in *Input) Peek(i int) Lexeme {
+	return in.input[in.index+i]
+}
+
+func (in *Input) Pop() Lexeme {
+	v := in.input[in.index]
+	in.index++
+	return v
+}
+
+func (in *Input) Seek(i int) {
+	_ = in.input[i]
+	in.index = i
+}
+
+func (in *Input) Take(amount int) *Input {
+	var out = &Input{
+		index: 0,
+		input: in.input[in.index : in.index+amount],
+	}
+	in.index += amount
+	return out
+}
+
+func (in *Input) Find(open, close *Token) (int, bool) {
+	level := 1
+	_ = in.input[in.index]
+	idx := slices.IndexFunc(in.input[in.index:], func(a Lexeme) bool {
+		switch {
+		case a.Equals(open):
+			level++
+		case a.Equals(close):
+			level--
+			return level == 0
+		}
+		return false
+	})
+	if idx > -1 {
+		return idx, true
+	} else {
+		return 0, false
+	}
+}
diff --git a/lex.go b/lex.go
new file mode 100644
index 0000000..b81c9b1
--- /dev/null
+++ b/lex.go
@@ -0,0 +1,159 @@
+package main
+
+import (
+	"fmt"
+	"regexp"
+	"strconv"
+	"strings"
+)
+
+type position struct {
+	pos          int
+	filepath     string
+	line, column int
+}
+
+func (p position) String() string {
+	return fmt.Sprintf("%s:%d:%d", p.filepath, p.line, p.column)
+}
+
+func (p *position) Position() position {
+	return *p
+}
+
+type Range [2]position
+
+type token[T comparable] struct {
+	position
+	value T
+}
+
+type Lexeme interface {
+	Position() position
+	Equals(Lexeme) bool
+}
+
+type Id string
+
+type Token token[string]
+
+type Int token[int]
+
+type Identifier token[Id]
+
+var (
+	Lparen      = &Token{value: "("}
+	Rparen      = &Token{value: ")"}
+	Space       = &Token{value: " "}
+	Tab         = &Token{value: "\t"}
+	Newline     = &Token{value: "\n"}
+	SingleQuote = &Token{value: "'"}
+	Let         = &Token{value: "let"}
+)
+
+func (t Token) String() string {
+	return fmt.Sprint(t.value)
+}
+
+func (t Int) String() string {
+	return fmt.Sprint(t.value)
+}
+
+func (t Identifier) String() string {
+	return fmt.Sprint(t.value)
+}
+
+var knownTokens = []*Token{
+	Lparen,
+	Rparen,
+	Space,
+	Tab,
+	Newline,
+	SingleQuote,
+	Let,
+}
+
+func (t *Token) Equals(a Lexeme) bool {
+	switch x := a.(type) {
+	case *Token:
+		return t.value == x.value
+	default:
+		return false
+	}
+}
+
+func (t *Int) Equals(a Lexeme) bool {
+	return false
+}
+
+func (t *Identifier) Equals(a Lexeme) bool {
+	switch x := a.(type) {
+	case *Identifier:
+		return t.value == x.value
+	default:
+		return false
+	}
+}
+
+var matchNumbers = regexp.MustCompile(`-?\d+`)
+var matchIdentifier = regexp.MustCompile(`[^\'() ]+`)
+
+func lex(source string) (*Input, error) {
+	var (
+		pos, line, column int
+		tokens            []Lexeme
+	)
+outer:
+	for len(source) > 0 {
+		p := position{pos: pos, line: line, column: column, filepath: "sourceinput"}
+		for _, try := range knownTokens {
+			if !strings.HasPrefix(source, try.value) {
+				continue
+			}
+			var (
+				l = len(try.value)
+				t = &Token{position: p, value: try.value}
+			)
+			source = source[l:]
+			pos += l
+			switch try {
+			case Newline:
+				line += 1
+				column = 0
+			case Tab, Space:
+				column += l
+			default:
+				column += l
+				tokens = append(tokens, t)
+			}
+			continue outer
+		}
+		index := matchNumbers.FindStringSubmatchIndex(source)
+		if index != nil && index[0] == 0 {
+			a := source[index[0]:index[1]]
+			conv, err := strconv.Atoi(a)
+			if err == nil {
+				var i = &Int{position: p, value: conv}
+				tokens = append(tokens, i)
+				l := len(a)
+				source = source[l:]
+				column += l
+				pos += l
+				continue outer
+			}
+		}
+		index = matchIdentifier.FindStringIndex(source)
+		if index != nil && index[0] == 0 {
+			a := source[index[0]:index[1]]
+			var id = &Identifier{position: p, value: Id(a)}
+			tokens = append(tokens, id)
+			l := len(a)
+			source = source[l:]
+			column += l
+			pos += l
+			continue outer
+		}
+		return nil, fmt.Errorf("unrecognized token '%c' at %s", source[0], p)
+	}
+	return &Input{input: tokens}, nil
+}
diff --git a/main.go b/main.go
new file mode 100644
index 0000000..712d034
--- /dev/null
+++ b/main.go
@@ -0,0 +1,117 @@
+package main
+
+import (
+	"fmt"
+	"os"
+)
+
+var source = `
+	(+ 1 2 3 4)
+	`
+
+func (v *Value[T]) Step() (Expression, error) {
+	return v, nil
+}
+
+func (v *Quoted) Step() (Expression, error) {
+	return v, nil
+}
+
+func (d *Declaration) Step() (Expression, error) {
+	return &Value[int]{0}, nil
+}
+
+func (v *Identifier) Step() (Expression, error) {
+	switch v.value {
+	case "+":
+		f := func(args []Expression) (Expression, error) {
+			sum := 0
+			for _, arg := range args {
+				if !isValue(arg) {
+					panic("!")
+				}
+				switch x := arg.(type) {
+				case *Value[int]:
+					sum += x.value
+				case *Value[*Int]:
+					sum += x.value.value
+				default:
+					return nil, fmt.Errorf("invalid type")
+
+				}
+			}
+			return &Value[int]{sum}, nil
+		}
+		return &Value[Function]{f}, nil
+	default:
+		panic("!")
+	}
+}
+
+func (v *List) Step() (Expression, error) {
+	for i, expr := range v.els {
+		if !isValue(expr) {
+			e, err := expr.Step()
+			if err != nil {
+				return nil, err
+			}
+			v.els[i] = e
+			return v, nil
+		}
+	}
+	if len(v.els) == 0 {
+		return nil, fmt.Errorf("empty list")
+	}
+	f, ok := v.els[0].(*Value[Function])
+	if !ok {
+		return nil, fmt.Errorf("not a function")
+	}
+	r, err := f.value(v.els[1:])
+	if err != nil {
+		return nil, err
+	}
+	return r, nil
+}
+
+func isValue(e Expression) bool {
+	switch e.(type) {
+	case *Value[int], *Value[*Int], *Value[Function], *Quoted:
+		return true
+	default:
+		return false
+	}
+}
+
+func fullStep(e Expression) (Expression, error) {
+	for !isValue(e) {
+		n, err := e.Step()
+		if err != nil {
+			return n, nil
+		}
+		e = n
+	}
+	return e, nil
+}
+
+func main() {
+	if len(os.Args) > 1 {
+		source = os.Args[1]
+	}
+	tk, err := lex(source)
+	if err != nil {
+		panic(err)
+	}
+	p, err := consume(tk)
+	if err != nil {
+		panic(err)
+	}
+	s, err := p.Step()
+	if err != nil {
+		panic(err)
+	}
+	result, err := fullStep(s)
+	if err != nil {
+		panic(err)
+	}
+	fmt.Println(result)
+}
diff --git a/parse.go b/parse.go
new file mode 100644
index 0000000..c099a59
--- /dev/null
+++ b/parse.go
@@ -0,0 +1,164 @@
+package main
+
+import (
+	"errors"
+	"fmt"
+	"strings"
+)
+
+var noMatch = errors.New("no match")
+
+type Function func([]Expression) (Expression, error)
+
+type Expression interface {
+	Step() (Expression, error)
+	// Visit(func(Expression) error)
+}
+
+type List struct {
+	els []Expression
+}
+
+type Value[T interface{ *Int | int | Function }] struct {
+	value T
+}
+
+type Quoted struct {
+	expr Expression
+}
+
+type Declaration struct {
+	id   *Identifier
+	expr Expression
+}
+
+func (v Value[T]) String() string {
+	return fmt.Sprint(v.value)
+}
+
+func (v Quoted) String() string {
+	return fmt.Sprintf("'%s", v.expr)
+}
+
+func (v List) String() string {
+	var s []string
+	for _, e := range v.els {
+		s = append(s, fmt.Sprint(e))
+	}
+	return fmt.Sprintf("(%s)", strings.Join(s, " "))
+}
+
+func parseQuotedObject(in *Input) (Expression, error) {
+	start := in.Index()
+	l := in.Pop()
+	switch {
+	case l.Equals(SingleQuote):
+		n, err := consume(in)
+		if err != nil {
+			in.Seek(start)
+			return nil, err
+		}
+		return &Quoted{n}, nil
+	default:
+		in.Seek(start)
+		return nil, noMatch
+	}
+}
+
+func parseSymbol(in *Input) (Expression, error) {
+	start := in.Index()
+	l := in.Pop()
+	switch x := l.(type) {
+	case *Identifier:
+		return x, nil
+	default:
+		in.Seek(start)
+		return nil, noMatch
+	}
+}
+
+func parseValue(in *Input) (Expression, error) {
+	start := in.Index()
+	l := in.Pop()
+	switch x := l.(type) {
+	case *Int:
+		return &Value[*Int]{x}, nil
+	default:
+		in.Seek(start)
+		return nil, noMatch
+	}
+}
+
+func parseList(in *Input) (Expression, error) {
+	start := in.Index()
+	openingParens := in.Pop()
+	if !openingParens.Equals(Lparen) {
+		in.Seek(start)
+		return nil, noMatch
+	}
+	idx, ok := in.Find(Lparen, Rparen)
+	if !ok {
+		in.Seek(start)
+		return nil, fmt.Errorf("unmatched parenthesis starting at %s", openingParens.Position())
+	}
+	if idx == 0 {
+		in.Seek(start)
+		return nil, fmt.Errorf("no function declared at %s", openingParens.Position())
+	}
+	list := in.Take(idx)
+	var args = new(List)
+	for list.Len() > 0 {
+		arg, err := consume(list)
+		if err != nil {
+			in.Seek(start)
+			return nil, err
+		}
+		args.els = append(args.els, arg)
+	}
+	_ = in.Pop()
+	return args, nil
+}
+
+func parseLet(in *Input) (Expression, error) {
+	start := in.Index()
+	token := in.Pop()
+	if !token.Equals(Let) {
+		in.Seek(start)
+		return nil, noMatch
+	}
+	id, ok := in.Pop().(*Identifier)
+	if !ok {
+		in.Seek(start)
+		return nil, noMatch
+	}
+	expr, err := consume(in)
+	if err != nil {
+		in.Seek(start)
+		return nil, noMatch
+	}
+	return &Declaration{id: id, expr: expr}, nil
+}
+
+type ParserFunc func(*Input) (Expression, error)
+
+func consume(in *Input) (n Expression, err error) {
+	var parseFunctions = []ParserFunc{
+		parseQuotedObject,
+		parseList,
+		parseSymbol,
+		parseValue,
+		parseLet,
+	}
+	for _, f := range parseFunctions {
+		n, err := f(in)
+		if err != nil {
+			if errors.Is(err, noMatch) {
+				continue
+			} else {
+				return nil, err
+			}
+		}
+		return n, nil
+	}
+	return nil, fmt.Errorf("unrecognized construction: %s", in)
+}