如何在弗雷格的非纯本机函数中声明可变类型的 Maybe？

Question

native-gen 工具为 showOpenDialog javafx.stage.FileChooser 中的方法像这样

data FileChooser = mutable native javafx.stage.FileChooser where
 native showOpenDialog :: FileChooser -> Window -> IO File

编译导致消息

Non pure native type File must be MutableIO
    File in IO actions.

现在设置

native showOpenDialog :: FileChooser -> Window -> MutableIO File

导致

FileChooser.showOpenDialog has an illegal
    return type for a method that is not pure, perhaps ST s (MutableIO
    File) would work

但遵循建议会再次出现第一条错误消息。

编译器接受IOMutable File作为return类型，这是有道理的因为它是一个 IO 操作，return 是一个可变类型。

如果可能，应修改编译器错误消息以避免用户方面感到沮丧。

但是，在这种特殊情况下，文件可以为空等核心类型不是File而是Maybe File。但是随后仅使用 IOMutable (Maybe File) 会导致相当令人惊讶的消息

The type MutableIO (Maybe File) is illegal,
    Maybe File must be a native type.

关于如何正确声明此类型有什么建议吗？

Answer 1

native-gen生成的代码是错误的，因为File已经declared as IO in native-gen but File is actually defined as a stateful (not IO) native type as can be seen from here。

IOMutable 定义为 type IOMutable d = IO (MutableIO d)。对于您的情况，可变本机类型 (MutableIO d) 可以为空，因此以下内容应该有效：

data FileChooser = mutable native javafx.stage.FileChooser where
  native showOpenDialog :: FileChooser -> Window -> IO (Maybe (MutableIO File))

如何在弗雷格的非纯本机函数中声明可变类型的 Maybe？

How do I declare Maybe of a mutable type in an non-pure native function in Frege?

frege