Merge pull request #225 from ichiban/context-aware-exceptions

make errors' Imp_def a term representing a context

engine/clause.go

@@ -90,7 +90,7 @@ func compile(t Term) (clauses, error) {
 		for iter.Next() {
 			c, err := compileClause(head, iter.Current())
 			if err != nil {
-				return nil, TypeErrorCallable(body, nil)
+				return nil, TypeError(ValidTypeCallable, body, nil)
 			}
 			c.raw = t
 			cs = append(cs, c)
@@ -116,7 +116,7 @@ func compileClause(head Term, body Term) (clause, error) {
 	}
 	if body != nil {
 		if err := c.compileBody(body); err != nil {
-			return c, TypeErrorCallable(body, nil)
+			return c, TypeError(ValidTypeCallable, body, nil)
 		}
 	}
 	c.bytecode = append(c.bytecode, instruction{opcode: opExit})

engine/env.go

@@ -1,5 +1,10 @@
 package engine
 
+const (
+	varContext  = Variable("$context")
+	rootContext = Atom("root")
+)
+
 type color uint8
 
 const (
@@ -21,6 +26,13 @@ type binding struct {
 	// attributes?
 }
 
+var rootEnv = &Env{
+	binding: binding{
+		variable: varContext,
+		value:    rootContext,
+	},
+}
+
 // NewEnv creates an empty environment.
 func NewEnv() *Env {
 	return nil
@@ -29,6 +41,9 @@ func NewEnv() *Env {
 // Lookup returns a term that the given variable is bound to.
 func (e *Env) Lookup(k Variable) (Term, bool) {
 	node := e
+	if node == nil {
+		node = rootEnv
+	}
 	for {
 		if node == nil {
 			return nil, false
@@ -46,7 +61,11 @@ func (e *Env) Lookup(k Variable) (Term, bool) {
 
 // Bind adds a new entry to the environment.
 func (e *Env) Bind(k Variable, v Term) *Env {
-	ret := *e.insert(k, v)
+	node := e
+	if node == nil {
+		node = rootEnv
+	}
+	ret := *node.insert(k, v)
 	ret.color = black
 	return &ret
 }
@@ -67,7 +86,9 @@ func (e *Env) insert(k Variable, v Term) *Env {
 		ret.balance()
 		return &ret
 	default:
-		return e
+		ret := *e
+		ret.value = v
+		return &ret
 	}
 }
 

engine/env_test.go

@@ -12,9 +12,15 @@ func TestEnv_Bind(t *testing.T) {
 	var env *Env
 	assert.Equal(t, &Env{
 		color: black,
+		right: &Env{
+			binding: binding{
+				variable: "A",
+				value:    Atom("a"),
+			},
+		},
 		binding: binding{
-			variable: "A",
-			value:    Atom("a"),
+			variable: varContext,
+			value:    Atom("root"),
 		},
 	}, env.Bind("A", Atom("a")))
 }