Add Len()

is.go

@@ -4,6 +4,7 @@ import (
 	"io"
 	"log"
 	"os"
+	"reflect"
 	"testing"
 )
 
@@ -221,6 +222,23 @@ func (is *Is) NotZero(o interface{}) {
 	}
 }
 
+// Len checks the provided object to determine if it is the same length as the
+// provided length argument.
+//
+// If the object is not one of type array, slice or map, it will fail.
+func (is *Is) Len(o interface{}, l int) {
+	t := reflect.TypeOf(o)
+	if t.Kind() != reflect.Array &&
+		t.Kind() != reflect.Slice &&
+		t.Kind() != reflect.Map {
+		fail(is, "expected object '%s' to be of length '%d', but the object is not one of array, slice or map", objectTypeName(o), l)
+	}
+
+	if reflect.ValueOf(o).Len() != l {
+		fail(is, "expected object '%s' to be of length '%d'", objectTypeName(o), l)
+	}
+}
+
 // SetOutput changes the message output Writer from the default (os.Stdout).
 // This may be useful if the application under test takes over the console, or
 // if logging to a file is desired.

is_test.go

@@ -149,6 +149,15 @@ func TestIs(t *testing.T) {
 	is.OneOf(1, 2, 3, 1)
 	is.NotOneOf(1, 2, 3)
 
+	lens := []interface{}{
+		[]int{1, 2, 3},
+		[3]int{1, 2, 3},
+		map[int]int{1: 1, 2: 2, 3: 3},
+	}
+	for _, l := range lens {
+		is.Len(l, 3)
+	}
+
 	fail = func(is *Is, format string, args ...interface{}) {}
 	is.Equal((*testStruct)(nil), &testStruct{})
 	is.Equal(&testStruct{}, (*testStruct)(nil))