lang: core, funcs: Port some functions to CallableFunc API

Some modern features of our function engine and language might require
this new API, so port what we can and figure out the rest later.

lang/core/golang/template.go

@@ -86,7 +86,7 @@ type TemplateFunc struct {
 	init *interfaces.Init
 	last types.Value // last value received to use for diff
 
-	result *string // last calculated output
+	result types.Value // last calculated output
 }
 
 // String returns a simple name for this function. This is needed so this struct
@@ -364,38 +364,54 @@ func (obj *TemplateFunc) Stream(ctx context.Context) error {
 			}
 			obj.last = input // store for next
 
-			st := input.Struct()
-
-			tmpl := st[templateArgNameTemplate].Str()
-			vars, exists := st[templateArgNameVars]
-			if !exists {
-				vars = nil
-			}
-
-			result, err := obj.run(ctx, tmpl, vars)
+			args, err := interfaces.StructToCallableArgs(input) // []types.Value, error)
 			if err != nil {
-				return err // no errwrap needed b/c helper func
+				return err
 			}
 
-			if obj.result != nil && *obj.result == result {
+			result, err := obj.Call(ctx, args)
+			if err != nil {
+				return err
+			}
+
+			// if the result is still the same, skip sending an update...
+			if obj.result != nil && result.Cmp(obj.result) == nil {
 				continue // result didn't change
 			}
-			obj.result = &result // store new result
+			obj.result = result // store new result
 
 		case <-ctx.Done():
 			return nil
 		}
 
 		select {
-		case obj.init.Output <- &types.StrValue{
-			V: *obj.result,
-		}:
+		case obj.init.Output <- obj.result: // send
+			// pass
 		case <-ctx.Done():
 			return nil
 		}
 	}
 }
 
+// Call this function with the input args and return the value if it is possible
+// to do so at this time.
+func (obj *TemplateFunc) Call(ctx context.Context, args []types.Value) (types.Value, error) {
+	tmpl := args[0].Str()
+
+	var vars types.Value // nil
+	if len(args) == 2 {
+		vars = args[1]
+	}
+
+	result, err := obj.run(ctx, tmpl, vars)
+	if err != nil {
+		return nil, err // no errwrap needed b/c helper func
+	}
+	return &types.StrValue{
+		V: result,
+	}, nil
+}
+
 // safename renames the functions so they're valid inside the template. This is
 // a limitation of the template library, and it might be worth moving to a new
 // one.