Begin refacting "Item" into a Data struct

Data struct contains an embeded Type tag. Eventually all types will be
contained within a Data. The first type to be contained with Data is the
Symbol type, which was previously (and incorrectly) called Atom.

Author: rread

env.go

@@ -0,0 +1,61 @@
+package main
+
+import "fmt"
+
+type Binding map[*Data]Item
+type Env struct {
+	vars  Binding
+	outer *Env
+}
+
+func NewEnv(outer *Env) *Env {
+	return &Env{
+		vars:  make(Binding),
+		outer: outer,
+	}
+
+}
+
+func (e *Env) BindName(name string, i Item) {
+	sym := internSymbol(name)
+	e.vars[sym] = i
+}
+
+func Symbolp(sym *Data) bool {
+	return sym.Type == SymbolType
+}
+
+func (e *Env) Bind(sym *Data, i Item) {
+	if !Symbolp(sym) {
+		panic(fmt.Errorf("sym is not a Symbol: %v", sym))
+	}
+	e.vars[sym] = i
+}
+
+func (e *Env) Var(sym *Data) (Item, error) {
+	if !Symbolp(sym) {
+		panic(fmt.Errorf("sym is not a Symbol: %v", sym))
+	}
+	v, ok := e.vars[sym]
+	if !ok {
+		return nil, fmt.Errorf("Undefined symbol: %v", sym)
+	}
+	return v, nil
+}
+
+func (env *Env) Find(sym *Data) *Env {
+	if !Symbolp(sym) {
+		panic(fmt.Errorf("sym is not a Symbol: %v", sym))
+	}
+	if _, ok := env.vars[sym]; ok {
+		return env
+	} else if env.outer != nil {
+		return env.outer.Find(sym)
+	} else {
+		return env
+	}
+}
+
+func (env *Env) FindVar(sym *Data) (Item, error) {
+	return env.Find(sym).Var(sym)
+}