Organize Compiler and Evaluator to be similar to each other

eval/compile.go

@@ -13,7 +13,8 @@ type Compiler struct {
 	compilerEphemeral
 }
 
-// compilerEphemeral wraps the ephemeral parts of a Compiler.
+// compilerEphemeral wraps the ephemeral parts of a Compiler, namely the parts
+// only valid through one startCompile-stopCompile cycle.
 type compilerEphemeral struct {
 	name, text string
 	scopes     []map[string]Type
@@ -50,10 +51,15 @@ func (cp *Compiler) startCompile(name, text string, scope map[string]Type) {
 	}
 }
 
+func (cp *Compiler) stopCompile() {
+	cp.compilerEphemeral = compilerEphemeral{}
+}
+
 // Compile compiles a ChunkNode into an Op, with the knowledge of current
 // scope. The supplied name and text are used in diagnostic messages.
 func (cp *Compiler) Compile(name, text string, n *parse.ChunkNode, scope map[string]Type) (op Op, err error) {
 	cp.startCompile(name, text, scope)
+	defer cp.stopCompile()
 	defer util.Recover(&err)
 	return cp.compileChunk(n), nil
 }

eval/eval.go

@@ -18,8 +18,7 @@ import (
 // has certain components replaced.
 type Evaluator struct {
 	Compiler *Compiler
-	name, text  string
-	context     string
+	evaluatorEphemeral
 	scope       map[string]*Value
 	env         *Env
 	searchPaths []string
@@ -27,6 +26,12 @@ type Evaluator struct {
 	statusCb    func([]Value)
 }
 
+// evaluatorEphemeral holds the ephemeral parts of an Evaluator, namely the
+// parts only valid through one startEval-stopEval cycle.
+type evaluatorEphemeral struct {
+	name, text, context string
+}
+
 func statusOk(vs []Value) bool {
 	for _, v := range vs {
 		v, ok := v.(*String)
@@ -150,19 +155,19 @@ func (ev *Evaluator) eval(name, text string, op Op) (err error) {
 	if op == nil {
 		return nil
 	}
-	defer util.Recover(&err)
+	ev.startEval(name, text)
 	defer ev.stopEval()
-	ev.name = name
-	ev.text = text
-	ev.context = "top"
+	defer util.Recover(&err)
 	op(ev)
 	return nil
 }
 
+func (ev *Evaluator) startEval(name, text string) {
+	ev.evaluatorEphemeral = evaluatorEphemeral{name, text, "top"}
+}
+
 func (ev *Evaluator) stopEval() {
-	ev.name = ""
-	ev.text = ""
-	ev.context = ""
+	ev.evaluatorEphemeral = evaluatorEphemeral{}
 }
 
 // errorf stops the ev.eval immediately by panicking with a diagnostic message.