diff --git a/cmdlang/builtins.go b/cmdlang/builtins.go
index 46fd5ba..f98dc4c 100644
--- a/cmdlang/builtins.go
+++ b/cmdlang/builtins.go
@@ -25,10 +25,10 @@ func echoBuiltin(ctx context.Context, args invocationArgs) (object, error) {
 	return asStream(line.String()), nil
 }
 
-func toUpperBuiltin(ctx context.Context, args invocationArgs) (object, error) {
+func toUpperBuiltin(ctx context.Context, inStream stream, args invocationArgs) (object, error) {
 	// Handle args
 	return mapFilterStream{
-		in: args.inStream,
+		in: inStream,
 		mapFn: func(x object) (object, bool) {
 			s, ok := x.(string)
 			if !ok {
@@ -86,8 +86,8 @@ func (f *fileLinesStream) close() error {
 	return nil
 }
 
-func errorTestBuiltin(ctx context.Context, args invocationArgs) (object, error) {
-	return &timeBombStream{args.inStream, 2}, nil
+func errorTestBuiltin(ctx context.Context, inStream stream, args invocationArgs) (object, error) {
+	return &timeBombStream{inStream, 2}, nil
 }
 
 type timeBombStream struct {
@@ -104,9 +104,5 @@ func (ms *timeBombStream) next() (object, error) {
 }
 
 func (ms *timeBombStream) close() error {
-	closable, ok := ms.in.(closableStream)
-	if ok {
-		return closable.close()
-	}
-	return nil
+	return ms.in.close()
 }
diff --git a/cmdlang/eval.go b/cmdlang/eval.go
index 2785872..b00026e 100644
--- a/cmdlang/eval.go
+++ b/cmdlang/eval.go
@@ -45,10 +45,17 @@ func (e evaluator) evalCmd(ctx context.Context, ec *evalCtx, ast *astCmd) (objec
 		return nil, err
 	}
 
-	return cmd.invoke(ctx, invocationArgs{
-		args:     args,
-		inStream: ec.currentStream,
-	})
+	if ec.currentStream != nil {
+		if si, ok := cmd.(streamInvokable); ok {
+			return si.invokeWithStream(ctx, ec.currentStream, invocationArgs{args: args})
+		} else {
+			if err := ec.currentStream.close(); err != nil {
+				return nil, err
+			}
+		}
+	}
+
+	return cmd.invoke(ctx, invocationArgs{args: args})
 }
 
 func (e evaluator) evalArg(ctx context.Context, ec *evalCtx, n astCmdArg) (object, error) {
diff --git a/cmdlang/inst.go b/cmdlang/inst.go
index d94e3d9..88c9d1f 100644
--- a/cmdlang/inst.go
+++ b/cmdlang/inst.go
@@ -13,10 +13,10 @@ type Inst struct {
 func New() *Inst {
 	rootEC := evalCtx{}
 	rootEC.addCmd("echo", invokableFunc(echoBuiltin))
-	rootEC.addCmd("toUpper", invokableFunc(toUpperBuiltin))
+	rootEC.addCmd("toUpper", invokableStreamFunc(toUpperBuiltin))
 	rootEC.addCmd("cat", invokableFunc(catBuiltin))
 
-	rootEC.addCmd("testTimebomb", invokableFunc(errorTestBuiltin))
+	rootEC.addCmd("testTimebomb", invokableStreamFunc(errorTestBuiltin))
 
 	return &Inst{
 		rootEC: &rootEC,
diff --git a/cmdlang/objs.go b/cmdlang/objs.go
index 934da5a..8f372e7 100644
--- a/cmdlang/objs.go
+++ b/cmdlang/objs.go
@@ -17,8 +17,7 @@ func (s strObject) String() string {
 }
 
 type invocationArgs struct {
-	args     []object
-	inStream stream
+	args []object
 }
 
 func (ia invocationArgs) expectArgn(x int) error {
@@ -44,8 +43,23 @@ type invokable interface {
 	invoke(ctx context.Context, args invocationArgs) (object, error)
 }
 
+type streamInvokable interface {
+	invokable
+	invokeWithStream(context.Context, stream, invocationArgs) (object, error)
+}
+
 type invokableFunc func(ctx context.Context, args invocationArgs) (object, error)
 
 func (i invokableFunc) invoke(ctx context.Context, args invocationArgs) (object, error) {
 	return i(ctx, args)
 }
+
+type invokableStreamFunc func(ctx context.Context, inStream stream, args invocationArgs) (object, error)
+
+func (i invokableStreamFunc) invoke(ctx context.Context, args invocationArgs) (object, error) {
+	return i(ctx, nil, args)
+}
+
+func (i invokableStreamFunc) invokeWithStream(ctx context.Context, inStream stream, args invocationArgs) (object, error) {
+	return i(ctx, inStream, args)
+}
diff --git a/cmdlang/streams.go b/cmdlang/streams.go
index e8d7637..b7f4e5c 100644
--- a/cmdlang/streams.go
+++ b/cmdlang/streams.go
@@ -7,21 +7,25 @@ import (
 
 // stream is an object which returns a collection of objects from a source.
 // These are used to create pipelines
+//
+// The stream implementation can expect close to be called if at least one next() call is made.  Otherwise
+// closableStream cannot assume that close will be called (the pipe may be left unconsumed, for example).
+//
+// It is the job of the final iterator to call close. Any steam that consumes from another stream must
+// implement this, and call close on the parent stream.
 type stream interface {
 	// next pulls the next object from the stream.  If an object is available, the result is the
 	// object and a nil error.  If no more objects are available, error returns io.EOF.
 	// Otherwise, an error is returned.
 	next() (object, error)
+
+	close() error
 }
 
 // forEach will iterate over all the items of a stream. The iterating function can return an error, which will
 // be returned as is. A stream that has consumed every item will return nil. The stream will automatically be closed.
 func forEach(s stream, f func(object) error) (err error) {
-	defer func() {
-		if c, ok := s.(closableStream); ok {
-			c.close()
-		}
-	}()
+	defer s.close()
 
 	var sv object
 	for sv, err = s.next(); err == nil; sv, err = s.next() {
@@ -35,19 +39,6 @@ func forEach(s stream, f func(object) error) (err error) {
 	return nil
 }
 
-// closableStream is a stream that has opened resources that must be closed when the stream is
-// consumed.  The stream implementation can expect close to be called if at least one next() call is made.  Otherwise
-// closableStream cannot assume that close will be called (the pipe may be left unconsumed, for example).
-//
-// It is the job of the final iterator to call close. Any steam that consumes from another stream must
-// implement this, and call close on the parent stream.
-type closableStream interface {
-	stream
-
-	// close closes the stream
-	close() error
-}
-
 // asStream converts an object to a stream.  If t is already a stream, it's returned as is.
 // Otherwise, a singleton stream is returned.
 func asStream(v object) stream {
@@ -63,6 +54,8 @@ func (s emptyStream) next() (object, error) {
 	return nil, io.EOF
 }
 
+func (s emptyStream) close() error { return nil }
+
 type singletonStream struct {
 	t        any
 	consumed bool
@@ -76,6 +69,8 @@ func (s *singletonStream) next() (object, error) {
 	return s.t, nil
 }
 
+func (s *singletonStream) close() error { return nil }
+
 type mapFilterStream struct {
 	in    stream
 	mapFn func(x object) (object, bool)
@@ -96,9 +91,5 @@ func (ms mapFilterStream) next() (object, error) {
 }
 
 func (ms mapFilterStream) close() error {
-	closable, ok := ms.in.(closableStream)
-	if ok {
-		return closable.close()
-	}
-	return nil
+	return ms.in.close()
 }