[Haskell-cafe] what monad is this?

[Mark McConnell]

 To my surprise, hlint (version 3.3.6) did not suggest taking out the do.  I ran the following through hlint.
jk :: Booljk = do  let x = False  not x


    On Wednesday, February 21, 2024 at 04:23:49 AM EST, Noon van der Silk <noonsilk at gmail.com> wrote:  
 
 Yeah, it's a bit confusing; I think some tutorials/books do go over the `do` simplification steps. I guess conceptually `do` doesn't mean `Definitely a Monad`, it means `Maybe a Monad` or more `Probably should be a Monad, and if not, just remove the do!`.
On Wed, 21 Feb 2024 at 09:18, 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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-- 
Noon van der Silk

http://silky.github.io/

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