Rename Substr to Slice

That is whas was implemented, not Substr.

Also make the API more similar to Go's internal slice by making both the start and end indices optional.

See #990

tpl/template.go

@@ -188,12 +188,25 @@ func compareGetFloat(a interface{}, b interface{}) (float64, float64) {
 	return left, right
 }
 
-func Substr(a interface{}, pos, length int) (string, error) {
+// Slicing in Slice is done by specifying a half-open range with
+// two indices, lower and upper. 1 and 4 creates a slice including elements 1 through 3.
+// The start and/or end indices can be omitted by setting one or both of them to -1; they default to zero and the slice's length respectively
+func Slice(a interface{}, start, end int) (string, error) {
 	aStr, err := cast.ToStringE(a)
 	if err != nil {
 		return "", err
 	}
-	return aStr[pos:length], nil
+
+	if start != -1 && end != -1 {
+		return aStr[start:end], nil
+	} else if start == -1 && end == -1 {
+		return aStr[:], nil
+	} else if start == -1 {
+		return aStr[:end], nil
+	} else {
+		return aStr[start:], nil
+	}
+
 }
 
 func Split(a interface{}, delimiter string) ([]string, error) {
@@ -1324,7 +1337,7 @@ func init() {
 		"lt":          Lt,
 		"le":          Le,
 		"in":          In,
-		"substr":      Substr,
+		"slice":       Slice,
 		"split":       Split,
 		"intersect":   Intersect,
 		"isSet":       IsSet,

tpl/template_test.go

@@ -276,7 +276,7 @@ func TestIn(t *testing.T) {
 	}
 }
 
-func TestSubstr(t *testing.T) {
+func TestSlice(t *testing.T) {
 	for i, this := range []struct {
 		v1     interface{}
 		v2     int
@@ -286,14 +286,17 @@ func TestSubstr(t *testing.T) {
 		{"abc", 1, 2, "b"},
 		{"abc", 1, 3, "bc"},
 		{"abc", 0, 1, "a"},
+		{"abcdef", -1, -1, "abcdef"},
+		{"abcdef", -1, 2, "ab"},
+		{"abcdef", 2, -1, "cdef"},
 		{123, 1, 3, "23"},
 		{tstNoStringer{}, 0, 1, false},
 	} {
-		result, err := Substr(this.v1, this.v2, this.v3)
+		result, err := Slice(this.v1, this.v2, this.v3)
 
 		if b, ok := this.expect.(bool); ok && !b {
 			if err == nil {
-				t.Errorf("[%d] Substr didn't return an expected error", i)
+				t.Errorf("[%d] Slice didn't return an expected error", i)
 			}
 		} else {
 			if err != nil {