syntax - `~` 在 Haskell 中意味着什么？

Question

我正在研究mtl库并尝试做一些我自己的MonadTransformers。我正在检查 Control.Monad.State.StateT 声明，在所有代码中，我看到了以下语法:

execStateT :: (Monad m) => StateT s m a -> s -> m s
execStateT m s = do
  ~(_, s') <- runStateT m s
  return s'

这个~操作数是什么意思？

Answer 1

这是 Haskell 中惰性模式的表示法。我不能说我很熟悉但是from here :

It is called a lazy pattern, and has the form ~pat. Lazy patterns are irrefutable: matching a value v against ~pat always succeeds, regardless of pat. Operationally speaking, if an identifier in pat is later "used" on the right-hand-side, it will be bound to that portion of the value that would result if v were to successfully match pat, and ⊥ otherwise.

此外，this section可能有用。