feature: Add functions safeDefault, safeCoalesce, and nonNil

defaults.go

@@ -31,6 +31,14 @@ func dfault(d interface{}, given ...interface{}) interface{} {
 	return given[0]
 }
 
+// Similar to dfault but only considers nil as unset. 0, false, "" are considered set
+func safeDefault(d interface{}, given ...interface{}) interface{} {
+	if given == nil || len(given) == 0 || given[0] == nil {
+		return d
+	}
+	return given[0]
+}
+
 // empty returns true if the given value has the zero value for its type.
 func empty(given interface{}) bool {
 	g := reflect.ValueOf(given)
@@ -69,6 +77,16 @@ func coalesce(v ...interface{}) interface{} {
 	return nil
 }
 
+// safeCoalesce returns the first non-nil value
+func safeCoalesce(v ...interface{}) interface{} {
+	for _, val := range v {
+		if val != nil {
+			return val
+		}
+	}
+	return nil
+}
+
 // all returns true if empty(x) is false for all values x in the list.
 // If the list is empty, return true.
 func all(v ...interface{}) bool {

defaults_test.go

@@ -30,6 +30,55 @@ func TestDefault(t *testing.T) {
 	}
 }
 
+func TestSafeDefault(t *testing.T) {
+	tpl := `{{"" | safeDefault "foo"}}`
+	if err := runt(tpl, ""); err != nil {
+		t.Error(err)
+	}
+	tpl = `{{safeDefault "foo" 234}}`
+	if err := runt(tpl, "234"); err != nil {
+		t.Error(err)
+	}
+	tpl = `{{safeDefault "foo" 2.34}}`
+	if err := runt(tpl, "2.34"); err != nil {
+		t.Error(err)
+	}
+
+	tpl = `{{ .Nothing | safeDefault "123" }}`
+	if err := runt(tpl, "123"); err != nil {
+		t.Error(err)
+	}
+	tpl = `{{ safeDefault "123" }}`
+	if err := runt(tpl, "123"); err != nil {
+		t.Error(err)
+	}
+
+	tpl = `{{ .Nothing | safeDefault true }}`
+	if err := runt(tpl, "true"); err != nil {
+		t.Error(err)
+	}
+	tpl = `{{ false | safeDefault true }}`
+	if err := runt(tpl, "false"); err != nil {
+		t.Error(err)
+	}
+	tpl = `{{ true | safeDefault false }}`
+	if err := runt(tpl, "true"); err != nil {
+		t.Error(err)
+	}
+	tpl = `{{ .Nothing | safeDefault 100 }}`
+	if err := runt(tpl, "100"); err != nil {
+		t.Error(err)
+	}
+	tpl = `{{ 0 | safeDefault 100 }}`
+	if err := runt(tpl, "0"); err != nil {
+		t.Error(err)
+	}
+	tpl = `{{ 55 | safeDefault 0 }}`
+	if err := runt(tpl, "55"); err != nil {
+		t.Error(err)
+	}
+}
+
 func TestEmpty(t *testing.T) {
 	tpl := `{{if empty 1}}1{{else}}0{{end}}`
 	if err := runt(tpl, "0"); err != nil {
@@ -84,6 +133,27 @@ func TestCoalesce(t *testing.T) {
 	}
 }
 
+func TestSafeCoalesce(t *testing.T) {
+	tests := map[string]string{
+		`{{ safeCoalesce 1 }}`:                            "1",
+		`{{ safeCoalesce "" 0 nil 2 }}`:                   "",
+		`{{ safeCoalesce nil 0 "" 2 }}`:				   "0",
+		`{{ $two := 2 }}{{ safeCoalesce nil $two "" 0 }}`: "2",
+		`{{ $two := 2 }}{{ safeCoalesce "" $two 0 0 0 }}`: "",
+		`{{ $two := 2 }}{{ safeCoalesce "" $two 3 4 5 }}`: "",
+		`{{ safeCoalesce }}`:                              "<no value>",
+	}
+	for tpl, expect := range tests {
+		assert.NoError(t, runt(tpl, expect))
+	}
+
+	dict := map[string]interface{}{"top": map[string]interface{}{}}
+	tpl := `{{ coalesce .top.NoSuchThing .bottom .bottom.dollar "airplane"}}`
+	if err := runtv(tpl, "airplane", dict); err != nil {
+		t.Error(err)
+	}
+}
+
 func TestAll(t *testing.T) {
 	tests := map[string]string{
 		`{{ all 1 }}`:                            "true",

docs/defaults.md

@@ -25,6 +25,18 @@ The definition of "empty" depends on type:
 For structs, there is no definition of empty, so a struct will never return the
 default.
 
+## safeDefault
+
+Similar to `default` but only considers `nil` as unset value. `0`, `false`,
+`""` are not considered unset, unlike `default`.
+
+```
+safeDefault true .Enabled
+```
+
+In the above, if `.Enabled` evaluates to `false`, same will be returned.
+However, `default` would return `true` here.
+
 ## empty
 
 The `empty` function returns `true` if the given value is considered empty, and
@@ -58,6 +70,16 @@ The above will first check to see if `.name` is empty. If it is not, it will ret
 that value. If it _is_ empty, `coalesce` will evaluate `.parent.name` for emptiness.
 Finally, if both `.name` and `.parent.name` are empty, it will return `Matt`.
 
+## safeCoalesce
+
+Similar to `coalesce` but only considers `nil` as unset value.
+
+```
+safeCoalesce nil 0 1 2
+```
+
+The above evaluates to `0`.
+
 ## all
 
 The `all` function takes a list of values and returns true if all values are non-empty.

functions.go

@@ -237,8 +237,10 @@ var genericMap = map[string]interface{}{
 
 	// Defaults
 	"default":          dfault,
+	"safeDefault":	    safeDefault,
 	"empty":            empty,
 	"coalesce":         coalesce,
+	"safeCoalesce":	    safeCoalesce,
 	"all":              all,
 	"any":              any,
 	"compact":          compact,