如何对 Exists 使用类型约束

Question

data Foo a = Foo a

我可以创建一个 Exists https://github.com/purescript/purescript-exists

的数组

[(mkExists (Foo 0)), (mkExists (Foo "x"))]

如何使用类类型？我想得到 ["0", "x"]

getStrings :: Array (Exists Foo) -> Array String
getStrings list = map (runExists get) list
  where
  get :: forall a. Show a => Foo a -> String
  get (Foo a) = show a

No type class instance was found for
Prelude.Show _0
The instance head contains unknown type variables. Consider adding a type annotation.

Answer 1

一种选择是在 Foo 的定义中捆绑 show 函数，如下所示：

import Prelude
import Data.Exists

data Foo a = Foo a (a -> String)

type FooE = Exists Foo

mkFooE :: forall a. (Show a) => a -> FooE
mkFooE a = mkExists (Foo a show)

getStrings :: Array FooE -> Array String
getStrings = map (runExists get)
  where
  get :: forall a. Foo a -> String
  get (Foo a toString) = toString a

--

items :: Array FooE
items = [mkFooE 0, mkFooE 0.5, mkFooE "test"]

items' :: Array String
items' = getStrings items

如何对 Exists 使用类型约束

How to use type constrains with Exists

purescript