haskell - 如何在 Haskell 中编写自应用函数？

Question

我尝试了以下代码，但它会生成类型错误。

sa f = f f

• Occurs check: cannot construct the infinite type: t ~ t -> t1
• In the first argument of ‘f’, namely ‘f’
  In the expression: f f
  In an equation for ‘sa’: sa f = f f
• Relevant bindings include
    f :: t -> t1
      (bound at fp-through-lambda-calculus-michaelson.hs:9:4)
    sa :: (t -> t1) -> t1
      (bound at fp-through-lambda-calculus-michaelson.hs:9:1)

Answer 1

使用新类型来构造无限类型。

newtype Eventually a = NotYet (Eventually a -> a)

sa :: Eventually a -> a
sa eventually@(NotYet f) = f eventually

在 GHC 中，最终 和 f 将是内存中的同一个对象。