diff --git a/concat.go b/concat.go
index d2a9af5..0abb899 100644
--- a/concat.go
+++ b/concat.go
@@ -12,11 +12,13 @@ type concatIterator[T any] struct {
 }
 
 func Concat[T any](itb1 Iterable[T], itb2 Iterable[T]) Iterable[T] {
-	if itb1 == nil {
-		itb1 = empty[T]()
-	}
-	if itb2 == nil {
-		itb2 = empty[T]()
+	switch {
+	case itb1 == nil && itb2 == nil:
+		return empty[T]()
+	case itb2 == nil:
+		return itb1
+	case itb1 == nil:
+		return itb2
 	}
 	return &concatIterable[T]{itb1, itb2}
 }
diff --git a/filter.go b/filter.go
index c7a5562..8bbdc48 100644
--- a/filter.go
+++ b/filter.go
@@ -13,12 +13,10 @@ type filterIterator[T any] struct {
 
 func Filter[T any](itb Iterable[T], predicate func(v T) bool) Iterable[T] {
 	if itb == nil {
-		itb = empty[T]()
+		return empty[T]()
 	}
 	if predicate == nil {
-		predicate = func(v T) bool {
-			return true
-		}
+		return itb
 	}
 	return &filterIterable[T]{itb, predicate}
 }
diff --git a/map.go b/map.go
index dca687a..dd9b11b 100644
--- a/map.go
+++ b/map.go
@@ -1,29 +1,29 @@
 package gcf
 
 type mapIterable[T any, R any] struct {
-	itb      Iterable[T]
-	selector func(T) R
+	itb     Iterable[T]
+	mapFunc func(T) R
 }
 
 type mapIterator[T any, R any] struct {
-	it       Iterator[T]
-	selector func(T) R
-	current  R
+	it      Iterator[T]
+	mapFunc func(T) R
+	current R
 }
 
-func Map[T any, R any](itb Iterable[T], f func(T) R) Iterable[R] {
+func Map[T any, R any](itb Iterable[T], mapFunc func(T) R) Iterable[R] {
 	if itb == nil {
-		itb = empty[T]()
+		return empty[R]()
 	}
-	if f == nil {
+	if mapFunc == nil {
 		r := zero[R]()
-		f = func(v T) R { return r }
+		mapFunc = func(v T) R { return r }
 	}
-	return &mapIterable[T, R]{itb, f}
+	return &mapIterable[T, R]{itb, mapFunc}
 }
 
 func (itb *mapIterable[T, R]) Iterator() Iterator[R] {
-	return &mapIterator[T, R]{itb.itb.Iterator(), itb.selector, zero[R]()}
+	return &mapIterator[T, R]{itb.itb.Iterator(), itb.mapFunc, zero[R]()}
 }
 
 func (it *mapIterator[T, R]) MoveNext() bool {
@@ -31,7 +31,7 @@ func (it *mapIterator[T, R]) MoveNext() bool {
 		it.current = zero[R]()
 		return false
 	}
-	it.current = it.selector(it.it.Current())
+	it.current = it.mapFunc(it.it.Current())
 	return true
 }