Ditch IndexVarer interface in favor of IndexSetter.

This complicates indexing a little bit, but makes it easier to implement new
containers.

eval/compile.go

@@ -323,19 +323,38 @@ func (cp *compiler) indexingVar(n *parse.Indexing, msg string) VariableOp {
 		if variable == nil {
 			ec.errorf(p, "variable $%s does not exisit, compiler bug", varname)
 		}
-		for i, op := range indexOps {
-			indexer, ok := variable.Get().(IndexVarer)
+		if len(indexOps) == 0 {
+			// Just a variable, return directly.
+			return variable
+		}
+
+		// Indexing. Do Index up to the last but one index.
+		value := variable.Get()
+		n := len(indexOps)
+		for i, op := range indexOps[:n-1] {
+			indexer, ok := value.(Indexer)
 			if !ok {
-				ec.errorf( /* from p to */ indexBegins[i], "cannot be indexing for setting (type %T)", variable.Get())
-			}
-			values := op(ec)
-			if len(values) != 1 {
-				ec.errorf(indexBegins[i], "index must eval to a single Value (got %v)", values)
+				ec.errorf( /* from p to */ indexBegins[i], "cannot be indexed (value is %s, type %s)", value.Repr(), value.Type())
 			}
 
-			variable = indexer.IndexVar(values[0])
+			indicies := op(ec)
+			if len(indicies) != 1 {
+				ec.errorf(indexBegins[i], "index must eval to a single Value (got %v)", indicies)
+			}
+
+			value = indexer.Index(indicies[0])
 		}
-		return variable
+		// Now this must be an IndexSetter.
+		indexSetter, ok := value.(IndexSetter)
+		if !ok {
+			ec.errorf( /* from p to */ indexBegins[n-1], "cannot be indexed for setting (value is %s, type %s)", value.Repr(), value.Type())
+		}
+		// XXX Duplicate code.
+		indicies := indexOps[n-1](ec)
+		if len(indicies) != 1 {
+			ec.errorf(indexBegins[n-1], "index must eval to a single Value (got %v)", indicies)
+		}
+		return elemVariable{indexSetter, indicies[0]}
 	}
 }
 

eval/value.go

@@ -42,9 +42,10 @@ type Indexer interface {
 	Index(idx Value) Value
 }
 
-// IndexVarer is anything that can be indexed by a Value and yields a Variable.
-type IndexVarer interface {
-	IndexVar(idx Value) Variable
+// IndexSetter is a Value whose elements can be get as well as set.
+type IndexSetter interface {
+	Indexer
+	IndexSet(idx Value, v Value)
 }
 
 // Caller is anything may be called on an evalCtx with a list of Value's.
@@ -283,8 +284,15 @@ func (l List) Index(idx Value) Value {
 	return (*l.inner)[i]
 }
 
-func (l List) IndexVar(idx Value) Variable {
-	return listElem{l, intIndex(idx)}
+func (l List) IndexSet(idxv Value, v Value) {
+	idx := intIndex(idxv)
+	if idx < 0 {
+		idx += len(*l.inner)
+	}
+	if idx < 0 || idx >= len(*l.inner) {
+		throw(ErrIndexOutOfRange)
+	}
+	(*l.inner)[idx] = v
 }
 
 // Map is a map from string to Value.
@@ -328,8 +336,8 @@ func (m Map) Index(idx Value) Value {
 	return v
 }
 
-func (m Map) IndexVar(idx Value) Variable {
-	return mapElem{m, idx}
+func (m Map) IndexSet(idx Value, v Value) {
+	(*m.inner)[idx] = v
 }
 
 // Closure is a closure.

eval/variable.go

@@ -24,43 +24,18 @@ func (iv ptrVariable) Get() Value {
 	return *iv.valuePtr
 }
 
-// listElem is the Variable generated by indexing a List.
-type listElem struct {
-	parent List
-	idx    int
+// elemVariable is an element of a IndexSetter.
+type elemVariable struct {
+	container IndexSetter
+	index     Value
 }
 
-func (le listElem) Set(val Value) {
-	(*le.parent.inner)[le.index()] = val
+func (ev elemVariable) Set(val Value) {
+	ev.container.IndexSet(ev.index, val)
 }
 
-func (le listElem) Get() Value {
-	return (*le.parent.inner)[le.index()]
-}
-
-func (le listElem) index() int {
-	idx := le.idx
-	if idx < 0 {
-		idx += len(*le.parent.inner)
-	}
-	if idx < 0 || idx >= len(*le.parent.inner) {
-		throw(ErrIndexOutOfRange)
-	}
-	return idx
-}
-
-// mapElem is the Variable generated by indexing a Map.
-type mapElem struct {
-	parent Map
-	idx    Value
-}
-
-func (me mapElem) Set(val Value) {
-	(*me.parent.inner)[me.idx] = val
-}
-
-func (me mapElem) Get() Value {
-	return (*me.parent.inner)[me.idx]
+func (ev elemVariable) Get() Value {
+	return ev.container.Index(ev.index)
 }
 
 type envVariable struct {