统一记录类型

Question

作为学习练习，我正在尝试定义一个新类型作为函数的持有者，这些函数可以将 Show-able 值转换为 Effects，即：

newtype ShEff a = ShEff (forall eff. Show a => a -> Eff eff Unit)

然而，这：

f :: forall a. ShEff a
f = ShEff logShow

编译失败并出现此错误：

  Could not match type

    ( console :: CONSOLE
    | t2
    )

  with type

    eff1


while trying to match type Eff
                             ( console :: CONSOLE
                             | t2
                             )
  with type Eff eff1
while checking that expression logShow
  has type t0 -> Eff eff1 Unit
in value declaration f

你能给我指出正确的方向吗？

Answer 1

logShow的类型是

forall eff. Show a => a -> Eff (console :: CONSOLE | eff) Unit

所以你不能将它存储在 ShEff 中，因为它必须适用于 every eff，而 logShow 仅适用于包含 CONSOLE 效果的行。

您有两个选择：

将 eff 类型参数移到 ShEff 之外：

newtype ShEff eff a = ShEff (a -> Eff eff Unit)

f :: forall a eff. Show a => ShEff (console :: CONSOLE | eff) a
f = ShEff logShow

在ShEff内添加约束：

newtype ShEff a = ShEff (forall eff. a -> Eff (console :: CONSOLE | eff) Unit)

f :: forall a eff. Show a => ShEff a
f = ShEff logShow

另请注意，在这两种情况下，我都将 Show a 约束移到了 ShEff 之外。

统一记录类型

Unifying record type

purescript