Add a utility for checking if vars are set:

...since strict-mode is now enabled

main.go

@@ -27,7 +27,8 @@ func ReadEnvVars(rawEnv []string) (environ map[string]string) {
 func WriteTemplateToStream(tplSource string, environ map[string]string, outStream io.Writer) {
     tpl := template.New("_root_")
     tpl.Funcs(template.FuncMap{
-        "split": TplSplitStr,
+        "split":  TplSplitStr,
+        "exists": TplCheckExists,
     })
     tpl.Option("missingkey=error")
     _, err := tpl.Parse(tplSource)
@@ -47,14 +48,27 @@ func TplSplitStr(args ...interface{}) ([]string, error) {
     if len(args) > 2 {
         parsedLimit, ok := args[2].(int)
         if !ok {
-            err := errors.New("Limit parameter (3rd)is not integer")
+            err := errors.New("Limit arg (3rd) to `split` is not integer")
             return nil, err
         }
         limit = parsedLimit
     }
     return strings.SplitN(rawValue, sep, limit), nil
 }
 
+func TplCheckExists(args ...interface{}) (bool, error) {
+    datamap, ok := args[0].(map[string]string)
+    if !ok {
+        return false, errors.New("data-map arg (1st) to `exists` should be a map[string]string, did you mean '.' or '$'?")
+    }
+    key, ok := args[1].(string)
+    if !ok {
+        return false, errors.New("lookup-key arg (2nd) to `exists` should be a string")
+    }
+    _, exists := datamap[key]
+    return exists, nil
+}
+
 func main() {
     WriteTemplateToStream(ReadTemplate(os.Stdin), ReadEnvVars(os.Environ()), os.Stdout)
 }

main_test.go

@@ -25,7 +25,7 @@ func ExampleWriteTemplateToStream_basic() {
     "TARGET_PLANET": "earth",
   }
   WriteTemplateToStream("Hello, {{ .TARGET_PLANET }}!\n", env, buf)
-  WriteTemplateToStream("{{ if not .BEGIN_INVASION}}We mean you no{{ else }}Prepare yourselves for{{ end }} harm.", env, buf)
+  WriteTemplateToStream("{{ if not (exists . \"BEGIN_INVASION\")}}We mean you no{{ else }}Prepare yourselves for{{ end }} harm.", env, buf)
   fmt.Println(buf.String())
   // Output:
   // Hello, earth!