Type check

.gitignore

@@ -0,0 +1,2 @@
+testdata
+tmp

expression.go

@@ -0,0 +1,334 @@
+package main
+
+import (
+	"fmt"
+	"maps"
+)
+
+type Expression interface {
+	Step() (Expression, error)
+	Substitute(Id, Expression)
+	TypeCheck(TypeEnvironment) (Type, error)
+}
+
+type Declaration struct {
+	variables []*Symbol
+	body      []Expression
+}
+
+func (d *Declaration) Step() (Expression, error) {
+	for _, v := range d.variables {
+		if isValue(v.expr) {
+			continue
+		}
+		n, err := v.expr.Step()
+		if err != nil {
+			return nil, err
+		}
+		v.expr = n
+		return d, nil
+	}
+	for _, v := range d.variables {
+		for _, b := range d.body {
+			b.Substitute(v.id.value, v.expr)
+		}
+	}
+	if len(d.body) == 0 {
+		return &zero, nil
+	}
+	for i, b := range d.body {
+		if isValue(b) {
+			continue
+		}
+		n, err := b.Step()
+		if err != nil {
+			return nil, err
+		}
+		d.body[i] = n
+		return d, nil
+	}
+	return d.body[len(d.body)-1], nil
+}
+
+func (d *Declaration) Substitute(id Id, expr Expression) {
+	for _, v := range d.variables {
+		v.expr.Substitute(id, expr)
+	}
+	for _, b := range d.body {
+		b.Substitute(id, expr)
+	}
+}
+
+func (d *Declaration) TypeCheck(env TypeEnvironment) (Type, error) {
+	env = maps.Clone(env)
+	for _, v := range d.variables {
+		typ, err := v.expr.TypeCheck(env)
+		if err != nil {
+			return nil, err
+		}
+		env[v.id.value] = typ
+	}
+	var body []Type = make([]Type, len(d.body))
+	for i, e := range d.body {
+		typ, err := e.TypeCheck(env)
+		if err != nil {
+			return nil, err
+		}
+		body[i] = typ
+	}
+	if len(body) > 0 {
+		return body[len(body)-1], nil
+	} else {
+		return &PrimVoid, nil
+	}
+}
+
+type Conditional struct {
+	predicate Expression
+	ifTrue    Expression
+	ifFalse   Expression
+}
+
+func (c *Conditional) Step() (Expression, error) {
+	n, err := c.predicate.Step()
+	if err != nil {
+		return nil, err
+	}
+	c.predicate = n
+	if !isValue(n) {
+		return c, nil
+	}
+	b, ok := n.(*BoolValue)
+	if !ok {
+		return nil, fmt.Errorf("boolean value was expected")
+	}
+	if *b {
+		n, err = c.ifTrue.Step()
+		if err != nil {
+			return nil, err
+		}
+		c.ifTrue = n
+		if isValue(n) {
+			return n, nil
+		} else {
+			return c, nil
+		}
+	} else {
+		n, err = c.ifFalse.Step()
+		if err != nil {
+			return nil, err
+		}
+		c.ifTrue = n
+		if isValue(n) {
+			return n, nil
+		} else {
+			return c, nil
+		}
+	}
+}
+
+func (c *Conditional) Substitute(id Id, expr Expression) {
+	c.predicate.Substitute(id, expr)
+	c.ifTrue.Substitute(id, expr)
+	c.ifFalse.Substitute(id, expr)
+}
+
+func (c *Conditional) TypeCheck(env TypeEnvironment) (Type, error) {
+	typ, err := c.predicate.TypeCheck(env)
+	if err != nil {
+		return nil, err
+	}
+	if !typ.Equal(&PrimBoolean) { // TODO: truthiness
+		return nil, fmt.Errorf("expected boolean expression as predicate")
+	}
+	typ1, err := c.ifTrue.TypeCheck(env)
+	if err != nil {
+		return nil, err
+	}
+	typ2, err := c.ifFalse.TypeCheck(env)
+	if err != nil {
+		return nil, err
+	}
+	if !typ1.Equal(typ2) {
+		return nil, fmt.Errorf("type mismatch between conditionals. can't coerce %v and %v", typ1, typ2)
+	}
+	return typ1, nil
+}
+
+type BuiltInFunction struct {
+	t *FunctionType
+	f func([]Expression) (Expression, error)
+}
+
+func (b *BuiltInFunction) HasValue()                               {}
+func (b *BuiltInFunction) Substitute(Id, Expression)               {}
+func (b *BuiltInFunction) TypeCheck(TypeEnvironment) (Type, error) { return b.t, nil }
+func (b *BuiltInFunction) Step() (Expression, error)               { panic("not implemented") }
+func (b BuiltInFunction) String() string                           { return "(lambda (..) (..))" }
+
+// func (f *FunctionValue) TypeCheck(TypeEnvironment) (Type, error) { return &PrimBoolean, nil }
+
+type Lambda struct {
+	t      *FunctionType
+	params []*Symbol
+	body   []Expression
+}
+
+func (l *Lambda) Step() (Expression, error) {
+	for _, arg := range l.params {
+		if arg.expr == nil {
+			panic("!")
+		}
+		for _, e := range l.body {
+			e.Substitute(arg.id.value, arg.expr)
+		}
+	}
+	if len(l.body) == 0 {
+		return &zero, nil
+	}
+	for i, expr := range l.body {
+		if isValue(expr) {
+			continue
+		}
+		r, err := expr.Step()
+		if err != nil {
+			return nil, err
+		}
+		l.body[i] = r
+		return l, nil
+	}
+	return l.body[len(l.body)-1], nil
+}
+
+func (l *Lambda) Substitute(id Id, expr Expression) {
+	for _, b := range l.body {
+		b.Substitute(id, expr)
+	}
+}
+
+func (l *Lambda) TypeCheck(env TypeEnvironment) (Type, error) {
+	env = maps.Clone(env)
+	for _, p := range l.params {
+		typ, err := p.expr.TypeCheck(env)
+		if err != nil {
+			return nil, err
+		}
+		env[p.id.value] = typ
+	}
+	var body []Type = make([]Type, len(l.body))
+	for i, e := range l.body {
+		typ, err := e.TypeCheck(env)
+		if err != nil {
+			return nil, err
+		}
+		body[i] = typ
+	}
+	if len(body) > 0 {
+		return body[len(body)-1], nil
+	} else {
+		return &PrimVoid, nil
+	}
+}
+
+type Symbol struct {
+	id   *Identifier
+	expr Expression
+}
+
+func (v *Symbol) Step() (Expression, error) {
+	if v.expr == nil {
+		return nil, fmt.Errorf("unbound symbol %s at %s", v.id, v.id.position)
+	}
+	return v.expr, nil
+}
+
+func (v *Symbol) Substitute(id Id, expr Expression) {
+	if v.id.value == id {
+		v.expr = expr
+	}
+}
+
+func (s *Symbol) TypeCheck(env TypeEnvironment) (Type, error) {
+	if typ, ok := env[s.id.value]; ok {
+		return typ, nil
+	}
+	if s.expr == nil {
+		return nil, fmt.Errorf("undeclared symbol %s at %s", s.id, s.id.position)
+	}
+	return s.expr.TypeCheck(env)
+}
+
+type List struct {
+	els []Expression
+}
+
+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 &zero, nil
+	}
+	args := v.els[1:]
+	switch x := v.els[0].(type) {
+	case *Lambda:
+		for i, p := range x.params {
+			p.expr = args[i]
+		}
+		return x, nil
+	case *BuiltInFunction:
+		return x.f(args)
+	default:
+		return nil, fmt.Errorf("not a function")
+	}
+}
+
+func (v *List) Substitute(id Id, expr Expression) {
+	for _, e := range v.els {
+		e.Substitute(id, expr)
+	}
+}
+
+func (v *List) TypeCheck(env TypeEnvironment) (Type, error) {
+	var et []Type
+	for _, e := range v.els {
+		typ, err := e.TypeCheck(env)
+		if err != nil {
+			return nil, err
+		}
+		et = append(et, typ)
+	}
+	if len(et) == 0 {
+		return &PrimAny, nil
+	}
+	tf, ok := et[0].(*FunctionType)
+	if !ok {
+		return nil, fmt.Errorf("expected function call. got %v", et[0])
+	}
+	args := et[1:]
+	// if !isVariadic(tf.params[len(tf.params)-1]) {
+	// }
+	if len(args) != len(tf.params) {
+		return nil, fmt.Errorf("parameter and arguments number mismatch.")
+	}
+	for i := range args {
+		if !tf.params[i].Equal(args[i]) {
+			return nil, fmt.Errorf("parameter type mismatch. expected %v got %v",
+				tf.params[i], args[i])
+		}
+	}
+	return tf.ret, nil
+	// for _, typ := range et {
+	// 	if !typ.Equal(et[0]) {
+	// 		return nil, fmt.Errorf("array type mismatch")
+	// 	}
+	// }
+	// return &ListType{et[0]}, nil
+}

lex.go

@@ -3,6 +3,7 @@ package main
 import (
 	"fmt"
 	"regexp"
+	"slices"
 	"strconv"
 	"strings"
 )
@@ -37,9 +38,9 @@ type Id string
 
 type Token token[string]
 
-type Int token[int]
+type IntToken token[int]
 
-type Float token[float64]
+type FloatToken token[float64]
 
 type Identifier token[Id]
 
@@ -52,13 +53,16 @@ var (
 	SingleQuote = &Token{value: "'"}
 	Let         = &Token{value: "let"}
 	LambdaTok   = &Token{value: "lambda"}
+	If          = &Token{value: "if"}
+	TrueTok     = &Token{value: "t"}
+	NilTok      = &Token{value: "nil"}
 )
 
 func (t Token) String() string {
 	return fmt.Sprint(t.value)
 }
 
-func (t Int) String() string {
+func (t IntToken) String() string {
 	return fmt.Sprint(t.value)
 }
 
@@ -75,6 +79,9 @@ var knownTokens = []*Token{
 	SingleQuote,
 	Let,
 	LambdaTok,
+	If,
+	TrueTok,
+	NilTok,
 }
 
 func (t *Token) Equals(a Lexeme) bool {
@@ -86,8 +93,8 @@ func (t *Token) Equals(a Lexeme) bool {
 	}
 }
 
-func (t *Int) Equals(a Lexeme) bool   { return false }
+func (t *IntToken) Equals(a Lexeme) bool   { return false }
-func (t *Float) Equals(a Lexeme) bool { return false }
+func (t *FloatToken) Equals(a Lexeme) bool { return false }
 
 func (t *Identifier) Equals(a Lexeme) bool {
 	switch x := a.(type) {
@@ -110,6 +117,15 @@ func lex(source string) (*Input, error) {
 outer:
 	for len(source) > 0 {
 		p := position{pos: pos, line: line, column: column, filepath: "stdin"}
+		if source[0] == ';' {
+			idx := slices.Index([]byte(source), '\n')
+			if idx < 0 {
+				break
+			} else {
+				source = source[idx:]
+				continue
+			}
+		}
 		for _, try := range knownTokens {
 			if !strings.HasPrefix(source, try.value) {
 				continue
@@ -137,7 +153,7 @@ outer:
 			a := source[index[0]:index[1]]
 			conv, err := strconv.ParseFloat(a, 64)
 			if err == nil {
-				var i = &Float{position: p, value: conv}
+				var i = &FloatToken{position: p, value: conv}
 				tokens = append(tokens, i)
 				l := len(a)
 				source = source[l:]
@@ -151,7 +167,7 @@ outer:
 			a := source[index[0]:index[1]]
 			conv, err := strconv.Atoi(a)
 			if err == nil {
-				var i = &Int{position: p, value: conv}
+				var i = &IntToken{position: p, value: conv}
 				tokens = append(tokens, i)
 				l := len(a)
 				source = source[l:]

main.go

@@ -5,144 +5,14 @@ import (
 	"fmt"
 	"log"
 	"os"
+	"reflect"
 )
 
 var source = ""
 
-func (v *Value[T]) Step() (Expression, error) { return v, nil }
-func (v *Value[T]) Replace(Id, Expression)    {}
-
-func (v *Quoted) Step() (Expression, error) { return v, nil }
-func (v *Quoted) Replace(Id, Expression)    {}
-
-func (d *Declaration) Step() (Expression, error) {
-	for _, v := range d.variables {
-		if isValue(v.expr) {
-			continue
-		}
-		n, err := v.expr.Step()
-		if err != nil {
-			return nil, err
-		}
-		v.expr = n
-		return d, nil
-	}
-	for _, v := range d.variables {
-		for _, b := range d.body {
-			b.Replace(v.id.value, v.expr)
-		}
-	}
-	if len(d.body) == 0 {
-		return zero, nil
-	}
-	for i, b := range d.body {
-		if isValue(b) {
-			continue
-		}
-		n, err := b.Step()
-		if err != nil {
-			return nil, err
-		}
-		d.body[i] = n
-		return d, nil
-	}
-	return d.body[len(d.body)-1], nil
-}
-
-func (d *Declaration) Replace(id Id, expr Expression) {
-	for _, v := range d.variables {
-		v.expr.Replace(id, expr)
-	}
-	for _, b := range d.body {
-		b.Replace(id, expr)
-	}
-}
-
-func (v *Symbol) Step() (Expression, error) {
-	if v.expr == nil {
-		return nil, fmt.Errorf("unbound symbol %s at %s", v.id, v.id.position)
-	}
-	return v.expr, nil
-}
-
-func (v *Symbol) Replace(id Id, expr Expression) {
-	if v.id.value == id {
-		v.expr = expr
-	}
-}
-
-func (l *Lambda) Step() (Expression, error) {
-	f := func(args []Expression) (Expression, error) {
-		if len(args) != len(l.args) {
-			return nil, fmt.Errorf("unexpected number of arguments")
-		}
-		for i, arg := range l.args {
-			for _, e := range l.body {
-				e.Replace(arg.id.value, args[i])
-			}
-		}
-		if len(l.body) == 0 {
-			return zero, nil
-		}
-		for i, expr := range l.body {
-			if isValue(expr) {
-				continue
-			}
-			r, err := expr.Step()
-			if err != nil {
-				return nil, err
-			}
-			l.body[i] = r
-		}
-		return l.body[len(l.body)-1], nil
-	}
-	return &Value[Function]{f}, nil
-}
-
-func (l *Lambda) Replace(id Id, expr Expression) {
-	for _, b := range l.body {
-		b.Replace(id, expr)
-	}
-}
-
-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 zero, nil
-	}
-	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 (v *List) Replace(id Id, expr Expression) {
-	for _, e := range v.els {
-		e.Replace(id, expr)
-	}
-}
-
 func isValue(e Expression) bool {
-	switch e.(type) {
+	_, ok := e.(Value)
-	case *Value[int], *Value[float64], *Value[Function], *Quoted:
+	return ok
-		return true
-	default:
-		return false
-	}
 }
 
 func fullStep(e Expression) (Expression, error) {
@@ -156,70 +26,65 @@ func fullStep(e Expression) (Expression, error) {
 	return e, nil
 }
 
-func stdlib(expr Expression) {
+func builtinFunctions(expr Expression) {
-	functions := map[Id]Function{
+	functions := map[Id]BuiltInFunction{
-		"+": func(args []Expression) (Expression, error) {
+		// "quote": {
-			sum := 0
+		// 	t: &FunctionType{
-			for _, arg := range args {
+		// 		ret: &PrimInteger,
-				if !isValue(arg) {
+		// 		params: ,
-					panic("!")
+		// 	},
+		// 	f: func(e []Expression) (Expression, error) {
+		// 		if len(e) == 1 {
+		// 			return &Quoted{e[0]}, nil
+		// 		}
+		// 		return &Quoted{&List{e}}, nil
+		// 	},
+		// },
+		"+": {
+			t: &FunctionType{
+				ret:    &PrimInteger,
+				params: []Type{&PrimInteger, &PrimInteger},
+			},
+			f: func(args []Expression) (Expression, error) {
+				var sum IntValue = 0
+				for _, arg := range args {
+					if !isValue(arg) {
+						panic("not a value")
+					}
+					switch x := arg.(type) {
+					case *IntValue:
+						sum += *x
+					default:
+						panic("type error")
+					}
 				}
-				switch x := arg.(type) {
+				return &sum, nil
-				case *Value[int]:
+			},
-					sum += x.value
-				default:
-					return nil, fmt.Errorf("invalid type")
-				}
-			}
-			return &Value[int]{sum}, nil
-		},
-		"-": func(args []Expression) (Expression, error) {
-			v0, ok := args[0].(*Value[int])
-			if !ok {
-				return nil, fmt.Errorf("integer value was expected")
-			}
-			v1, ok := args[1].(*Value[int])
-			if !ok {
-				return nil, fmt.Errorf("integer value was expected")
-			}
-			return &Value[int]{v0.value - v1.value}, nil
-		},
-		"*": func(args []Expression) (Expression, error) {
-			v0, ok := args[0].(*Value[int])
-			if !ok {
-				return nil, fmt.Errorf("integer value was expected")
-			}
-			v1, ok := args[1].(*Value[int])
-			if !ok {
-				return nil, fmt.Errorf("integer value was expected")
-			}
-			return &Value[int]{v0.value * v1.value}, nil
-		},
-		"/": func(args []Expression) (Expression, error) {
-			v0, ok := args[0].(*Value[int])
-			if !ok {
-				return nil, fmt.Errorf("integer value was expected")
-			}
-			v1, ok := args[1].(*Value[int])
-			if !ok {
-				return nil, fmt.Errorf("integer value was expected")
-			}
-			return &Value[int]{v0.value / v1.value}, nil
-		},
-		"/.": func(args []Expression) (Expression, error) {
-			v0, ok := args[0].(*Value[float64])
-			if !ok {
-				return nil, fmt.Errorf("float value was expected")
-			}
-			v1, ok := args[1].(*Value[float64])
-			if !ok {
-				return nil, fmt.Errorf("float value was expected")
-			}
-			return &Value[float64]{float64(v0.value) / float64(v1.value)}, nil
 		},
+		// "-": func(args []Expression) (Expression, error) {
+		// 	v0, ok := args[0].(*IntValue)
+		// 	if !ok {
+		// 		return nil, fmt.Errorf("integer value was expected")
+		// 	}
+		// 	v1, ok := args[1].(*IntValue)
+		// 	if !ok {
+		// 		return nil, fmt.Errorf("integer value was expected")
+		// 	}
+		// 	var result IntValue = (*v0 - *v1)
+		// 	return &result, nil
+		// },
+		// "=": func(args []Expression) (Expression, error) {
+		// 	if reflect.TypeOf(args[0]) != reflect.TypeOf(args[1]) {
+		// 		return nil, fmt.Errorf("unmatched types")
+		// 	}
+		// 	v0 := reflect.ValueOf(args[0])
+		// 	v1 := reflect.ValueOf(args[1])
+		// 	var result BoolValue = BoolValue(v0.Equal(v1))
+		// 	return &result, nil
+		// },
 	}
 	for id, f := range functions {
-		expr.Replace(id, &Value[Function]{f})
+		expr.Substitute(id, &f)
 	}
 }
 
@@ -247,10 +112,14 @@ func main() {
 	if err != nil {
 		logger.Fatal(err)
 	}
-	stdlib(p)
+	builtinFunctions(p)
+	_, err = p.TypeCheck(make(TypeEnvironment))
+	if err != nil {
+		logger.Fatal(err)
+	}
 	result, err := fullStep(p)
 	if err != nil {
 		logger.Fatal(err)
 	}
-	fmt.Println(result)
+	fmt.Println(reflect.TypeOf(result), result)
 }

parse.go

@@ -8,56 +8,12 @@ import (
 
 var (
 	noMatch = errors.New("no match")
-	zero    = &Value[int]{0}
 )
 
-type Function func([]Expression) (Expression, error)
-
-type Expression interface {
-	Step() (Expression, error)
-	Replace(Id, Expression)
-}
-
-type List struct {
-	els []Expression
-}
-
-type Value[T interface{ int | float64 | Function }] struct {
-	value T
-	// rng   Range
-}
-
-type Quoted struct {
-	expr Expression
-}
-
-type Declaration struct {
-	variables []*Symbol
-	body      []Expression
-}
-
-type Lambda struct {
-	args []*Symbol
-	body []Expression
-}
-
-type Symbol struct {
-	id   *Identifier
-	expr Expression
-}
-
 // type Progn struct {
 // 	*List
 // }
 
-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 {
@@ -68,7 +24,7 @@ func (v List) String() string {
 
 func (v Lambda) String() string {
 	var args List
-	for _, arg := range v.args {
+	for _, arg := range v.params {
 		args.els = append(args.els, arg)
 	}
 	var s []string
@@ -78,10 +34,6 @@ func (v Lambda) String() string {
 	return fmt.Sprintf("(lambda %v %v)", &args, strings.Join(s, " "))
 }
 
-func (v Function) String() string {
-	return "(lambda (..) (..))"
-}
-
 func (t Symbol) String() string {
 	return fmt.Sprintf("%v", t.id.value)
 }
@@ -109,6 +61,17 @@ func parseSymbol(in *Input) (Expression, error) {
 		return nil, noMatch
 	}
 	switch x := l.(type) {
+	case *Token:
+		switch {
+		case x.Equals(TrueTok):
+			v := BoolValue(true)
+			return &v, nil
+		case x.Equals(NilTok):
+			v := BoolValue(false)
+			return &v, nil
+		default:
+			return nil, noMatch
+		}
 	case *Identifier:
 		return &Symbol{id: x}, nil
 	default:
@@ -122,10 +85,12 @@ func parseValue(in *Input) (Expression, error) {
 		return nil, noMatch
 	}
 	switch x := l.(type) {
-	case *Int:
+	case *IntToken:
-		return &Value[int]{x.value}, nil
+		r := IntValue(x.value)
-	case *Float:
+		return &r, nil
-		return &Value[float64]{x.value}, nil
+	case *FloatToken:
+		r := FloatValue(x.value)
+		return &r, nil
 	default:
 		return nil, noMatch
 	}
@@ -172,6 +137,10 @@ func parseLet(in *Input) (Expression, error) {
 		return nil, fmt.Errorf("unmatched parenthesis starting at %s", openingParens.Position())
 	}
 	decl := in.Take(idx)
+	_, ok = in.Pop()
+	if !ok {
+		return nil, noMatch
+	}
 	tok, ok := decl.Pop()
 	if !ok {
 		return nil, noMatch
@@ -254,7 +223,43 @@ func parseLambda(in *Input) (Expression, error) {
 	if !ok {
 		return nil, noMatch
 	}
-	return &Lambda{args: args, body: body}, nil
+	return &Lambda{params: args, body: body}, nil
+}
+
+func parseConditional(in *Input) (Expression, error) {
+	openingParens, ok := in.Pop()
+	if !ok {
+		return nil, noMatch
+	}
+	if !openingParens.Equals(Lparen) {
+		return nil, noMatch
+	}
+	idx, ok := in.Find(Lparen, Rparen)
+	if !ok {
+		return nil, fmt.Errorf("unmatched parenthesis starting at %s", openingParens.Position())
+	}
+	decl := in.Take(idx)
+	_, ok = in.Pop()
+	tok, ok := decl.Pop()
+	if !ok {
+		return nil, noMatch
+	}
+	if !tok.Equals(If) {
+		return nil, noMatch
+	}
+	predicate, err := consume(decl)
+	if err != nil {
+		return nil, err
+	}
+	ifTrue, err := consume(decl)
+	if err != nil {
+		return nil, err
+	}
+	ifFalse, err := consume(decl)
+	if err != nil {
+		return nil, err
+	}
+	return &Conditional{predicate: predicate, ifTrue: ifTrue, ifFalse: ifFalse}, nil
 }
 
 type ParserFunc func(*Input) (Expression, error)
@@ -263,6 +268,7 @@ func consume(in *Input) (n Expression, err error) {
 	var parseFunctions = []ParserFunc{
 		parseQuotedObject,
 		parseLambda,
+		parseConditional,
 		parseLet,
 		parseList,
 		parseSymbol,

type.go

@@ -0,0 +1,94 @@
+package main
+
+import "fmt"
+
+type Type interface {
+	Equal(Type) bool
+}
+
+type PrimitiveType string
+
+type ListType struct{ t Type }
+
+type QuotedType struct{ t Type }
+
+type FunctionType struct {
+	ret    Type
+	params []Type
+}
+
+// type VariadicListType ListType
+
+type TypeEnvironment map[Id]Type
+
+var (
+	PrimVoid    PrimitiveType = "void"
+	PrimAny     PrimitiveType = "any"
+	PrimBoolean PrimitiveType = "boolean"
+	PrimInteger PrimitiveType = "integer"
+	PrimFloat   PrimitiveType = "float"
+)
+
+func (a *PrimitiveType) Equal(b Type) bool {
+	switch b := b.(type) {
+	case *PrimitiveType:
+		return a == b
+	default:
+		return false
+	}
+}
+
+func (p PrimitiveType) String() string {
+	return string(p)
+}
+
+func (a *ListType) Equal(b Type) bool {
+	switch b := b.(type) {
+	case *ListType:
+		return a.t.Equal(b.t)
+	default:
+		return false
+	}
+}
+
+func (l ListType) String() string {
+	return fmt.Sprintf("[list %v]", l.t)
+}
+
+func (a *QuotedType) Equal(b Type) bool {
+	switch b := b.(type) {
+	case *QuotedType:
+		return a.t.Equal(b.t)
+	default:
+		return false
+	}
+}
+
+func (q QuotedType) String() string {
+	return fmt.Sprintf("[quote %v]", q.t)
+}
+
+func (fa *FunctionType) Equal(b Type) bool {
+	fb, ok := b.(*FunctionType)
+	if !ok {
+		return false
+	}
+	if len(fa.params) != len(fb.params) {
+		return false
+	}
+	for i := range fa.params {
+		if !fa.params[i].Equal(fb.params[i]) {
+			return false
+		}
+	}
+	return fa.ret.Equal(fb.ret)
+}
+
+// func (fa *VariadicListType) Equal(b Type) bool {
+// 	return false
+// }
+//
+// func isVariadic(t Type) bool {
+// 	_, ok := t.(*VariadicListType)
+// 	return ok
+// }

value.go

@@ -0,0 +1,48 @@
+package main
+
+import "fmt"
+
+var (
+	zero IntValue  = 0
+	True BoolValue = true
+	Frue BoolValue = false
+)
+
+type Value interface {
+	Expression
+	HasValue()
+}
+
+type IntValue int
+
+func (i *IntValue) HasValue()                               {}
+func (i *IntValue) Step() (Expression, error)               { return i, nil }
+func (i *IntValue) Substitute(Id, Expression)               {}
+func (i *IntValue) TypeCheck(TypeEnvironment) (Type, error) { return &PrimInteger, nil }
+func (i *IntValue) String() string                          { return fmt.Sprint(*i) }
+
+type FloatValue float32
+
+func (f *FloatValue) HasValue()                               {}
+func (f *FloatValue) Step() (Expression, error)               { return f, nil }
+func (f *FloatValue) Substitute(Id, Expression)               {}
+func (f *FloatValue) TypeCheck(TypeEnvironment) (Type, error) { return &PrimFloat, nil }
+func (f *FloatValue) String() string                          { return fmt.Sprint(*f) }
+
+type BoolValue bool
+
+func (b *BoolValue) HasValue()                               {}
+func (b *BoolValue) Step() (Expression, error)               { return b, nil }
+func (b *BoolValue) Substitute(Id, Expression)               {}
+func (b *BoolValue) TypeCheck(TypeEnvironment) (Type, error) { return &PrimBoolean, nil }
+func (b BoolValue) String() string                           { return fmt.Sprint(bool(b)) }
+
+type Quoted struct{ expr Expression }
+
+func (q *Quoted) HasValue()                 {}
+func (q *Quoted) Step() (Expression, error) { return q, nil }
+func (q *Quoted) Substitute(Id, Expression) {}
+func (q Quoted) String() string             { return fmt.Sprintf("'%v", q.expr) }
+func (q *Quoted) TypeCheck(env TypeEnvironment) (Type, error) {
+	return &QuotedType{&PrimVoid}, nil
+}