Re: Is join useful for every monad?

You can use the (-> c) instance in fun ways both :: (a -> b) -> (a, a) -> (b, b) both = join (***) dup :: a -> (a, a) dup = join (,) between `join` (char '"') On 10/13/19 11:55 PM, Dannyu NDos 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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