add TemplateFile to the templates so user can give a path to a templa…

…te (#1)

* add TemplateFile to the templates so user can give a path to a template

README.md

@@ -122,6 +122,7 @@ Here's a compact overview:
 | [TCPAddrsCSV](https://godoc.org/github.com/sgreben/flagvar#TCPAddrsCSV)        | 127.0.0.1:10,:123               | []net.TCPAddr                              |
 | [Template](https://godoc.org/github.com/sgreben/flagvar#Template)    | "{{.Size}}"        | *template.Template                   |
 | [Templates](https://godoc.org/github.com/sgreben/flagvar#Templates)   | "{{.Size}}"        | []*template.Template                 |
+| [TemplateFile](https://godoc.org/github.com/sgreben/flagvar#TemplateFile)   | "/path/to/template.file"        | string                 |
 | [Time](https://godoc.org/github.com/sgreben/flagvar#Time)        | "10:30 AM"         | time.Time                            |
 | [Times](https://godoc.org/github.com/sgreben/flagvar#Times)       | "10:30 AM"         | []time.Time                          |
 | [UDPAddr](https://godoc.org/github.com/sgreben/flagvar#UDPAddr)        | 127.0.0.1:10               | net.UDPAddr                              |

template.go

@@ -2,6 +2,7 @@ package flagvar
 
 import (
 	"fmt"
+	"io/ioutil"
 	"text/template"
 )
 
@@ -67,3 +68,44 @@ func (fv *Templates) Set(v string) error {
 func (fv *Templates) String() string {
 	return fmt.Sprint(fv.Texts)
 }
+
+// Template is a `flag.Value` for `text.Template` arguments.
+// The value of the `Root` field is used as a root template when specified.
+// The value specified on the command line is the path to the template
+type TemplateFile struct {
+	Root *template.Template
+
+	Value *template.Template
+	Text  string
+}
+
+// Help returns a string suitable for inclusion in a flag help message.
+func (fv *TemplateFile) Help() string {
+	return "file system path to a go template"
+}
+
+// Set is flag.Value.Set
+func (fv *TemplateFile) Set(v string) error {
+
+	_template, err := ioutil.ReadFile(v)
+
+	if err != nil {
+		return err
+	}
+
+	root := fv.Root
+	if root == nil {
+		root = template.New("")
+	}
+	t, err := root.New(fmt.Sprintf("%T(%p)", fv, fv)).Parse(string(_template))
+	if err == nil {
+		fv.Text = v
+		fv.Value = t
+	}
+	return err
+}
+
+func (fv *TemplateFile) String() string {
+	return fv.Text
+}
+

template_test.go

@@ -114,3 +114,14 @@ func TestTemplatesFail(t *testing.T) {
 		t.Fail()
 	}
 }
+
+func TestTemplateFile(t *testing.T) {
+	fv := flagvar.TemplateFile{}
+	var fs flag.FlagSet
+	fs.Var(&fv, "template", "")
+
+	err := fs.Parse([]string{"-template", "./noSuchFile.tpl"})
+	if err == nil {
+		t.Fail()
+	}
+}