haskell - 如何提升返回带有单例的字符串的函数？

Question

将堆栈与 lts-9.2(和 singletons-2.1)一起使用，我有这个:

   $(singletons [d|
       data EventScans
        = PassThrough
        | SuiteProgress

       -- cn :: EventScans -> [Char]
       cn PassThrough = "all-events"
       cn SuiteProgress = "suite-progress"
    |])

我不能给这个函数一个类型签名，推断的类型签名是 cn :: IsString t => EventScans -> t . AFAIK IsString不单调。

没有 cn 上的类型签名我收到了 Cn按预期键入家庭。如果我尝试添加类型签名，我会得到:

* Expected kind `[Char]', but `"all-events"' has kind `Symbol'
* In the type `"all-events"'
  In the type family declaration for `Cn' (haskell-stack-ghc)

这样做的正确方法是什么？

编辑 如果我尝试包含类型签名 cn :: IsString t => EventScans -> t我收到以下错误:

Variable `a_agPb' used as both a kind and a type
 Did you intend to use TypeInType? (haskell-stack-ghc)
Not in scope: type constructor or class `SIsString'
  Perhaps you meant `IsString' (imported from Data.String) (haskell-stack-ghc)

Answer 1

从我在 github 上打开的相关问题来看，这是因为 IsString还没有单调。见 https://github.com/goldfirere/singletons/issues/267详情。