commands: expand and complete template code

Expand and complete template code. The exline does understand template
code and can evaluate it. With this patch, the completion systems
supports writing the templates by showing the available template data
fields and template functions. Pressing <Tab> after }} will show the
evaluated template in the completion menu. Complex template code, such
as if-else statements work as well.

Examples:

:filter -f {{<Tab>
will show the possible template data fields and functions

:filter -f {{index (.To|email) 0}}<Tab>
will show the value of the template expression in the completion list

Pressing <Tab> twice after a completed template expression will
substitute the expression with its value.

:{{if match .Folder "INBOX"}}check-mail{{else}}cf INBOX{{end}}

Signed-off-by: Koni Marti <koni.marti@gmail.com>
Acked-by: Robin Jarry <robin@jarry.cc>

1 files changed, 52 insertions, 0 deletions

diff --git a/lib/templates/template.go b/lib/templates/template.go
index bf257669..6eae591a 100644
--- a/lib/templates/template.go
+++ b/lib/templates/template.go
@@ -6,6 +6,7 @@ import (
 	"io"
 	"os"
 	"path"
+	"reflect"
 	"text/template"
 
 	"git.sr.ht/~rjarry/aerc/models"
@@ -56,3 +57,54 @@ func ParseTemplate(name, content string) (*template.Template, error) {
 func Render(t *template.Template, w io.Writer, data models.TemplateData) error {
 	return t.Execute(w, data)
 }
+
+// builtins is a slice of keywords and functions built into the Go standard
+// library for templates. Since they are not exported, they are hardcoded here.
+var builtins = []string{
+	// from the Go standard library: src/text/template/parse/lex.go
+	"block",
+	"break",
+	"continue",
+	"define",
+	"else",
+	"end",
+	"if",
+	"range",
+	"nil",
+	"template",
+	"with",
+
+	// from the Go standard library: src/text/template/funcs.go
+	"and",
+	"call",
+	"html",
+	"index",
+	"slice",
+	"js",
+	"len",
+	"not",
+	"or",
+	"print",
+	"printf",
+	"println",
+	"urlquery",
+	"eq",
+	"ge",
+	"gt",
+	"le",
+	"lt",
+	"ne",
+}
+
+func Terms() []string {
+	var s []string
+	t := reflect.TypeOf((*models.TemplateData)(nil)).Elem()
+	for i := 0; i < t.NumMethod(); i++ {
+		s = append(s, "."+t.Method(i).Name)
+	}
+	for fnStr := range templateFuncs {
+		s = append(s, fnStr)
+	}
+	s = append(s, builtins...)
+	return s
+}