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/os/readfile.go

@@ -62,12 +62,13 @@ type ReadFileFunc struct {
 	init *interfaces.Init
 	last types.Value // last value received to use for diff
 
-	filename   *string // the active filename
 	recWatcher *recwatch.RecWatcher
 	events     chan error // internal events
 	wg         *sync.WaitGroup
 
-	result *string // last calculated output
+	args     []types.Value
+	filename *string     // the active filename
+	result   types.Value // last calculated output
 }
 
 // String returns a simple name for this function. This is needed so this struct
@@ -214,28 +215,49 @@ func (obj *ReadFileFunc) Stream(ctx context.Context) error {
 				continue // still waiting for input values
 			}
 
-			// read file...
-			content, err := os.ReadFile(*obj.filename)
+			args, err := interfaces.StructToCallableArgs(obj.last) // []types.Value, error)
 			if err != nil {
-				return errwrap.Wrapf(err, "error reading file")
+				return err
 			}
-			result := string(content) // convert to string
+			obj.args = args
 
-			if obj.result != nil && *obj.result == result {
+			result, err := obj.Call(ctx, obj.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 *ReadFileFunc) Call(ctx context.Context, args []types.Value) (types.Value, error) {
+	filename := args[0].Str()
+
+	// read file...
+	content, err := os.ReadFile(filename)
+	if err != nil {
+		return nil, errwrap.Wrapf(err, "error reading file")
+	}
+
+	return &types.StrValue{
+		V: string(content), // convert to string
+	}, nil
+}