[Haskell-cafe] what monad is this?

[Brent Yorgey]

ghci> :set -XBlockArguments
ghci> let be = id in do be do be do ()
()

On Wed, Feb 21, 2024 at 3:18 AM Johannes Waldmann <
johannes.waldmann at htwk-leipzig.de> wrote:

> Dear Cafe,
>
> I was surprised to see  ghc  accept this code
>
> ghci> do let {x = False}; not x
> True
>
> because I initially thought: there's a `do`, so there must be some monad,
> but which is it? some implicit Identity monad perhaps?
>
> But the type is indeed plain `Bool`, and the explanation is (I think):
> there is a `do` but there is no monad, since the translation according to
> (
> https://www.haskell.org/onlinereport/haskell2010/haskellch3.html#x8-470003.14
> )
> does never produce `(>>=)`.
>
> So, all is fine, nothing to see here.
> Perhaps keep in mind as an edge case, useful to confuse students.
> Actually, to motivate them to read the language standard ...
>
> - J.W.
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