没有因使用‘`return’而产生的(Monad m0)的实例

Question

我尝试用haskell和Qt创建一个程序，它以图形的形式显示从haskell函数获得的一些点。

这是一个正常工作的示例，但重点在于返回函数及其参数，而不是代码的其余部分！

main :: IO ()
main = do
    clazz <- newClass [
        defMethod' "factorial" (\_ txt ->
            let n = read $ T.unpack txt :: Integer
            in return . T.pack . show $ product [1..n] :: IO Text)]
    ctx <- newObject clazz ()
    runEngineLoop defaultEngineConfig {
        initialDocument = fileDocument "exemple4.qml",
        contextObject = Just $ anyObjRef ctx}

下面是我的代码：

main :: IO ()
main = do
    clazz <- newClass [
        defMethod' "init_tableau" (\_  -> 
            return $ map (^2) [0..20])]
    ctx <- newObject clazz ()
    runEngineLoop defaultEngineConfig {
        initialDocument = fileDocument "exemple4.qml",
        contextObject = Just $ anyObjRef ctx}

它应该只返回i^2的21个第一次出现，从0开始。

下面是所有的错误：

Build FAILED

/home/lowley/Documents/haskell/Qt/lancer-qml4.hs: line 10, column 9:
  No instance for (MethodSuffix (m0 [b0]))
    arising from a use of defMethod'
  The type variables `m0', `b0' are ambiguous
  Possible fix: add a type signature that fixes these type variable(s)
  Note: there are several potential instances:
    instance (Marshal a, CanGetFrom a ~ Yes, MethodSuffix b) =>
             MethodSuffix (a -> b)
      -- Defined in `Graphics.QML.Objects'
    instance (Marshal a, CanReturnTo a ~ Yes) => MethodSuffix (IO a)
      -- Defined in `Graphics.QML.Objects'
  Possible fix:
    add an instance declaration for (MethodSuffix (m0 [b0]))
  In the expression:
    defMethod' "init_tableau" (\ _ -> return $ map (^ 2) [0 .. 20])
  In the first argument of `newClass', namely
    `[defMethod' "init_tableau" (\ _ -> return $ map (^ 2) [0 .. 20])]'
  In a stmt of a 'do' block:
    clazz <- newClass
               [defMethod' "init_tableau" (\ _ -> return $ map (^ 2) [0 .. 20])]
/home/lowley/Documents/haskell/Qt/lancer-qml4.hs: line 11, column 13:
  No instance for (Monad m0) arising from a use of `return'
  The type variable `m0' is ambiguous
  Possible fix: add a type signature that fixes these type variable(s)
  Note: there are several potential instances:
    instance Monad ((->) r) -- Defined in `GHC.Base'
    instance Monad IO -- Defined in `GHC.Base'
    instance Monad [] -- Defined in `GHC.Base'
    ...plus two others
  In the expression: return
  In the expression: return $ map (^ 2) [0 .. 20]
  In the second argument of defMethod', namely
    `(\ _ -> return $ map (^ 2) [0 .. 20])'
/home/lowley/Documents/haskell/Qt/lancer-qml4.hs: line 11, column 27:
  No instance for (Num b0) arising from a use of `^'
  The type variable `b0' is ambiguous
  Possible fix: add a type signature that fixes these type variable(s)
  Note: there are several potential instances:
    instance Num Double -- Defined in `GHC.Float'
    instance Num Float -- Defined in `GHC.Float'
    instance Integral a => Num (GHC.Real.Ratio a)
      -- Defined in `GHC.Real'
    ...plus three others
  In the first argument of `map', namely `(^ 2)'
  In the second argument of `($)', namely `map (^ 2) [0 .. 20]'
  In the expression: return $ map (^ 2) [0 .. 20]
/home/lowley/Documents/haskell/Qt/lancer-qml4.hs: line 11, column 31:
  No instance for (Enum b0)
    arising from the arithmetic sequence `0 .. 20'
  The type variable `b0' is ambiguous
  Possible fix: add a type signature that fixes these type variable(s)
  Note: there are several potential instances:
    instance Enum Double -- Defined in `GHC.Float'
    instance Enum Float -- Defined in `GHC.Float'
    instance Integral a => Enum (GHC.Real.Ratio a)
      -- Defined in `GHC.Real'
    ...plus 7 others
  In the second argument of `map', namely `[0 .. 20]'
  In the second argument of `($)', namely `map (^ 2) [0 .. 20]'
  In the expression: return $ map (^ 2) [0 .. 20]

我认为这个错误很小，应该很快纠正，但我所有的尝试都失败了。

谢谢你

编辑:有关信息：

Prelude> let liste=[0..20] in map (^2) liste
[0,1,4,9,16,25,36,49,64,81,100,121,144,169,196,225,256,289,324,361,400]
Prelude>

我试过了：

defMethod' "init_tableau" (\_  ->
            let a=map (^2) [0..20] 
            in return a)]

使用大致相同的错误

编辑2：

Prelude> :t map (^ 2) [0..20]
map (^ 2) [0..20] :: (Enum b, Num b) => [b]

编辑3:使用此代码：

main :: IO ()
main = do
    clazz <- newClass [ 
        defMethod' "init_tableau" (\_  -> 
            return ( map (^2) [0..20] ) :: IO [Integer] )]
    ctx <- newObject clazz ()
    runEngineLoop defaultEngineConfig {
        initialDocument = fileDocument "exemple4.qml",
        contextObject = Just $ anyObjRef ctx}

我得到了这个错误：

/home/lowley/Documents/haskell/hsqml-demo-samples-0.3.4.0/qml/tableau1.hs:11:9:
    Couldn't match type `MarshalMode Integer ICanReturnTo ()'
                  with `Yes'
    In the expression:
      defMethod'
        "init_tableau"
        (\ _ -> return (map (^ 2) [0 .. 20]) :: IO [Integer])
    In the first argument of `newClass', namely
      `[defMethod'
          "init_tableau"
          (\ _ -> return (map (^ 2) [0 .. 20]) :: IO [Integer])]'
    In a stmt of a 'do' block:
      clazz <- newClass
                 [defMethod'
                    "init_tableau"
                    (\ _ -> return (map (^ 2) [0 .. 20]) :: IO [Integer])]

我还是无法解决这个问题。

Answer 1

我还没有测试过这一点，但我怀疑这里的问题是有许多合适的monad可以使用。如果您希望与工作示例完全并行，则可以通过向return表达式添加类型注释来显式地选择使用IO作为单体：

return $ map (^2) [0..20] :: IO [Integer]

Answer 2

Daniel Wagner的建议修复了您询问的错误。

您现在有了另一个不相关的错误：

Couldn't match type `MarshalMode Integer ICanReturnTo ()' with `Yes'

现在考虑一下类型

defMethod' :: (Typeable obj, MethodSuffix ms) => String -> (ObjRef obj -> ms) -> Member obj

您正试图将(\_ -> return ( map (^2) [0..20] ) :: IO [Integer] )作为第二个参数传递给它。

那么我们可以用forall a. a -> IO [Integer]统一ObjRef obj -> ms吗？

好吧，第一部分是统一的，因为a是免费的。但是IO [Integer]是MethodSuffix的一个实例吗？遵循类型类，你会发现这要求[Integer]是Marshal和follow that的一个实例，你会发现我们不能这样做。我在文档中没有看到一个很好的Marshal基本实例列表，但我在source中找到了它们。

因此，如果您尝试返回[Int]或Text而不是[Integer]，那么这很可能是类型检查。