Add Filter() to List

And update collection templates to be backed by the untyped
Filter() implementations.

Fixes #578

nomdl/codegen/list.tmpl

@@ -134,11 +134,8 @@ func (l {{.Name}}) IterAllP(concurrency int, cb {{.Name}}IterAllCallback) {
 type {{.Name}}FilterCallback func(v {{userType .ElemType}}, i uint64) (keep bool)
 
 func (l {{.Name}}) Filter(cb {{.Name}}FilterCallback) {{.Name}} {
-	nl := New{{.Name}}()
-	l.IterAll(func(v {{userType .ElemType}}, i uint64) {
-		if cb(v, i) {
-			nl = nl.Append(v)
-		}
+	out := l.l.Filter(func(v {{$typesPackage}}Value, i uint64) bool {
+		return cb({{valueToUser "v" .ElemType}}, i)
 	})
-	return nl
+	return {{.Name}}{out, &ref.Ref{}}
 }

nomdl/codegen/map.tmpl

@@ -125,11 +125,8 @@ func (m {{.Name}}) IterAllP(concurrency int, cb {{.Name}}IterAllCallback) {
 type {{.Name}}FilterCallback func(k {{userType .KeyType}}, v {{userType .ValueType}}) (keep bool)
 
 func (m {{.Name}}) Filter(cb {{.Name}}FilterCallback) {{.Name}} {
-	nm := New{{.Name}}()
-	m.IterAll(func(k {{userType .KeyType}}, v {{userType .ValueType}}) {
-		if cb(k, v) {
-			nm = nm.Set(k, v)
-		}
+	out := m.m.Filter(func(k, v {{$typesPackage}}Value) bool {
+		return cb({{valueToUser "k" .KeyType}}, {{valueToUser "v" .ValueType}})
 	})
-	return nm
+	return {{.Name}}{out, &ref.Ref{}}
 }

nomdl/codegen/set.tmpl

@@ -105,13 +105,10 @@ func (s {{.Name}}) IterAllP(concurrency int, cb {{.Name}}IterAllCallback) {
 type {{.Name}}FilterCallback func(p {{userType .ElemType}}) (keep bool)
 
 func (s {{.Name}}) Filter(cb {{.Name}}FilterCallback) {{.Name}} {
-	ns := New{{.Name}}()
-	s.IterAll(func(v {{userType .ElemType}}) {
-		if cb(v) {
-			ns = ns.Insert(v)
-		}
+	out := s.s.Filter(func(v {{$typesPackage}}Value) bool {
+		return cb({{valueToUser "v" .ElemType}})
 	})
-	return ns
+	return {{.Name}}{out, &ref.Ref{}}
 }
 
 func (s {{.Name}}) Insert(p ...{{userType .ElemType}}) {{.Name}} {

types/list.go

@@ -86,6 +86,18 @@ func (l List) iterInternal(sem chan int, lf listIterAllFunc, offset uint64) {
 	wg.Wait()
 }
 
+type listFilterCallback func(v Value, index uint64) (keep bool)
+
+func (l List) Filter(cb listFilterCallback) List {
+	data := []Value{}
+	for i, v := range l.values {
+		if cb(v, uint64(i)) {
+			data = append(data, v)
+		}
+	}
+	return newListNoCopy(data, l.t)
+}
+
 type MapFunc func(v Value, index uint64) interface{}
 
 func (l List) Map(mf MapFunc) []interface{} {