diff --git a/filter.go b/filter.go
index 300dc5e..49e1439 100644
--- a/filter.go
+++ b/filter.go
@@ -2,6 +2,8 @@ package prompt
 
 import "strings"
 
+type Filter func([]string, string, bool) []string
+
 func FilterHasPrefix(completions []string, sub string, ignoreCase bool) []string {
 	if sub == "" {
 		return completions
diff --git a/filter_test.go b/filter_test.go
new file mode 100644
index 0000000..5cb9ad3
--- /dev/null
+++ b/filter_test.go
@@ -0,0 +1,72 @@
+package prompt
+
+import (
+	"testing"
+	"reflect"
+)
+
+func TestFilter(t *testing.T) {
+	var scenarioTable = [] struct {
+		scenario   string
+		filter     Filter
+		list       []string
+		substr     string
+		ignoreCase bool
+		expected   []string
+	} {
+		{
+			scenario:   "Contains don't ignore case",
+			filter:     FilterContains,
+			list:       []string{"abcde", "fghij", "ABCDE"},
+			substr:     "cd",
+			ignoreCase: false,
+			expected:  []string{"abcde"},
+		},
+		{
+			scenario:   "Contains ignore case",
+			filter:     FilterContains,
+			list:       []string{"abcde", "fghij", "ABCDE"},
+			substr:     "cd",
+			ignoreCase: true,
+			expected:   []string{"abcde", "ABCDE"},
+		},
+		{
+			scenario:   "HasPrefix don't ignore case",
+			filter:     FilterHasPrefix,
+			list:       []string{"abcde", "fghij", "ABCDE"},
+			substr:     "abc",
+			ignoreCase: false,
+			expected:  []string{"abcde"},
+		},
+		{
+			scenario:   "HasPrefix ignore case",
+			filter:     FilterHasPrefix,
+			list:       []string{"abcde", "fabcj", "ABCDE"},
+			substr:     "abc",
+			ignoreCase: true,
+			expected:   []string{"abcde", "ABCDE"},
+		},
+		{
+			scenario:   "HasSuffix don't ignore case",
+			filter:     FilterHasSuffix,
+			list:       []string{"abcde", "fcdej", "ABCDE"},
+			substr:     "cde",
+			ignoreCase: false,
+			expected:  []string{"abcde"},
+		},
+		{
+			scenario:   "HasSuffix ignore case",
+			filter:     FilterHasSuffix,
+			list:       []string{"abcde", "fcdej", "ABCDE"},
+			substr:     "cde",
+			ignoreCase: true,
+			expected:   []string{"abcde", "ABCDE"},
+		},
+	}
+
+	for _, s := range scenarioTable {
+		if actual := s.filter(s.list, s.substr, s.ignoreCase); !reflect.DeepEqual(actual, s.expected) {
+			t.Errorf("%s: Should be %#v, but got %#v", s.scenario, s.expected, actual)
+		}
+	}
+}