text/template: export isTrue

The definition of 'truth' used by if etc. is not trivial to compute, so publish
the implementation to allow custom template functions to have the
same definition as the template language itself.

Fixes #12033.

Change-Id: Icdfd6039722d7d3f984ba0905105eb3253e14831
Reviewed-on: https://go-review.googlesource.com/14593Reviewed-by: Andrew Gerrand <adg@golang.org>

src/html/template/template.go

@@ -9,6 +9,7 @@ import (
 	"io"
 	"io/ioutil"
 	"path/filepath"
+	"reflect"
 	"sync"
 	"text/template"
 	"text/template/parse"
@@ -413,3 +414,10 @@ func parseGlob(t *Template, pattern string) (*Template, error) {
 	}
 	return parseFiles(t, filenames...)
 }
+
+// IsTrue reports whether the value is 'true', in the sense of not the zero of its type,
+// and whether the value has a meaningful truth value. This is the definition of
+// truth used by if and other such actions.
+func IsTrue(val reflect.Value) (truth, ok bool) {
+	return template.IsTrue(val)
+}

src/text/template/exec.go

@@ -242,7 +242,7 @@ func (s *state) walk(dot reflect.Value, node parse.Node) {
 func (s *state) walkIfOrWith(typ parse.NodeType, dot reflect.Value, pipe *parse.PipeNode, list, elseList *parse.ListNode) {
 	defer s.pop(s.mark())
 	val := s.evalPipeline(dot, pipe)
-	truth, ok := isTrue(val)
+	truth, ok := IsTrue(val)
 	if !ok {
 		s.errorf("if/with can't use %v", val)
 	}
@@ -257,9 +257,10 @@ func (s *state) walkIfOrWith(typ parse.NodeType, dot reflect.Value, pipe *parse.
 	}
 }
 
-// isTrue reports whether the value is 'true', in the sense of not the zero of its type,
-// and whether the value has a meaningful truth value.
-func isTrue(val reflect.Value) (truth, ok bool) {
+// IsTrue reports whether the value is 'true', in the sense of not the zero of its type,
+// and whether the value has a meaningful truth value. This is the definition of
+// truth used by if and other such actions.
+func IsTrue(val reflect.Value) (truth, ok bool) {
 	if !val.IsValid() {
 		// Something like var x interface{}, never set. It's a form of nil.
 		return false, true

src/text/template/funcs.go

@@ -265,7 +265,7 @@ func call(fn interface{}, args ...interface{}) (interface{}, error) {
 // Boolean logic.
 
 func truth(a interface{}) bool {
-	t, _ := isTrue(reflect.ValueOf(a))
+	t, _ := IsTrue(reflect.ValueOf(a))
 	return t
 }
 
@@ -300,9 +300,8 @@ func or(arg0 interface{}, args ...interface{}) interface{} {
 }
 
 // not returns the Boolean negation of its argument.
-func not(arg interface{}) (truth bool) {
-	truth, _ = isTrue(reflect.ValueOf(arg))
-	return !truth
+func not(arg interface{}) bool {
+	return !truth(arg)
 }
 
 // Comparison.