environments now have outer environment pointers

env.go

@@ -10,22 +10,30 @@ func (u UnknownSymbolError) Error() string {
 	return fmt.Sprintf(`unknown symbol "%v"`, u.symbol)
 }
 
-type environment map[symbol]interface{}
+type environment struct {
+	items map[symbol]interface{}
+	outer *environment
+}
 
 func (e environment) get(key symbol) (interface{}, error) {
-	v, ok := e[key]
+	v, ok := e.items[key]
 	if ok {
 		debugPrint(fmt.Sprintf(`found key "%v": %v`, key, v))
 		return v, nil
 	}
+
+	if e.outer != nil {
+		return e.outer.get(key)
+	}
+
 	return nil, UnknownSymbolError{key}
 }
 
 func (e environment) set(key symbol, val interface{}) {
-	e[key] = val
+	e.items[key] = val
 }
 
 func (e environment) defined(key symbol) bool {
-	_, ok := e[key]
-	return ok
+	_, err := e.get(key)
+	return err == nil
 }

skeam.go

@@ -20,7 +20,7 @@ func (s sexp) String() string {
 
 type symbol string
 
-var universe = &environment{
+var universe = &environment{map[symbol]interface{}{
 	"#t":     true,
 	"#f":     false,
 	"+":      proc(addition),
@@ -31,7 +31,7 @@ var universe = &environment{
 	"quote":  special(quote),
 	"if":     special(_if),
 	"set!":   special(set),
-}
+}, nil}
 
 // parses the string lexeme into a value that can be eval'd
 func atom(t token) (interface{}, error) {

special.go

@@ -14,61 +14,100 @@ type nargsInvalidError struct {
 }
 
 func (n nargsInvalidError) Error() string {
-	return fmt.Sprintf(`received %d arguments in *%v*, expected %d`, n.received, n.name, n.expected)
+	return fmt.Sprintf(`received %d arguments in *%v*, expected %d`,
+		n.received, n.name, n.expected)
 }
 
+// defines the built-in "define" construct.  e.g.:
+//
+//  (define x 5)
+//
+// would create the symbol "x" and set its value to 5.
 func define(env *environment, args ...interface{}) (interface{}, error) {
 	if len(args) != 2 {
 		return nil, nargsInvalidError{2, len(args), "define"}
 	}
+
 	s, ok := args[0].(symbol)
 	if !ok {
 		return nil, fmt.Errorf(`first argument to *define* must be symbol, received %v`, reflect.TypeOf(args[0]))
 	}
+
 	v, err := eval(args[1], env)
 	if err != nil {
 		return nil, err
 	}
 	env.set(s, v)
+
 	return nil, nil
 }
 
+// defines the built-in "quote" construct.  e.g.:
+//
+//  (quote (1 2 3))
+//
+// would evaluate to the list (1 2 3).  That is, quote is a function of arity 1
+// that is effectively a no-op; the input value is not evaluated, which
+// prevents evaluation of the first element of the list, in this case 1.
 func quote(_ *environment, args ...interface{}) (interface{}, error) {
 	if len(args) != 1 {
 		return nil, nargsInvalidError{1, len(args), "quote"}
 	}
+
 	return args[0], nil
 }
 
+// defines the built-in "if" contruct.  e.g.:
+//
+//  (if #t "foo" "bar")
+//
+// would evaluate to "foo", while the following:
+//
+//  (if #f "foo" "bar")
+//
+// would evaluate to "bar"
 func _if(env *environment, args ...interface{}) (interface{}, error) {
 	if len(args) != 3 {
 		return nil, nargsInvalidError{3, len(args), "if"}
 	}
+
 	v, err := eval(args[0], env)
 	if err != nil {
 		return nil, err
 	}
+
 	if b, ok := v.(bool); ok && !b {
 		return eval(args[2], env)
 	}
 	return eval(args[1], env)
 }
 
+// defines the built-in "set" construct, which is used to set the value of an
+// existing symbol in the provided environment.  e.g.:
+//
+//  (set! x 5)
+//
+// would set the symbol x to the value 5, if and only if the symbol x was
+// previously defined.
 func set(env *environment, args ...interface{}) (interface{}, error) {
 	if len(args) != 2 {
 		return nil, nargsInvalidError{2, len(args), "set!"}
 	}
+
 	s, ok := args[0].(symbol)
 	if !ok {
 		return nil, fmt.Errorf(`first argument to *set!* must be symbol, received %v`, reflect.TypeOf(args[0]))
 	}
+
 	if !env.defined(s) {
 		return nil, fmt.Errorf(`cannot *set!* undefined symbol %v`, s)
 	}
+
 	v, err := eval(args[1], env)
 	if err != nil {
 		return nil, err
 	}
 	env.set(s, v)
+
 	return nil, nil
 }