Data types like `IO (IO a)` are useful and their usage is described in the following blog post:

* http://www.haskellforall.com/2018/02/the-wizard-monoid.html

One function from the example uses `join` directly:

runWizard :: IO (IO a) -> IO a

runWizard = join

On Mon, Oct 14, 2019 at 7:55 AM Dannyu NDos <ndospark320@gmail.com> wrote:

join is surely useful for Maybe and [].

But IO (IO a)? ReadPrec (ReadPrec a)? ST s (ST s a)? When the heck could I encounter such types?

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