[Haskell-beginners] How to unnest "do"

[Brent Yorgey]

On Sun, Jan 27, 2013 at 09:29:27PM +0100, Emmanuel Touzery wrote:
> >     stats :: String -> String
> >     stats =
> >         unwords .
> >         sequence [show . length . words,
> >                   show . length . lines,
> >                   show . length]
> >
> >
> [..]
> 
> 
> > This improves the statistics code slightly, but uses some monadic
> > machinery you may not be familiar with.  Another way to read the 'stats'
> > function is this:
> >
> >     stats :: String -> String
> >     stats str =
> >         unwords [show . length . words $ str,
> >                  show . length . lines $ str,
> >                  show . length $ str]
> >
> 
> 
> I'm sorry, may I ask on which monad here is "sequence" operating?
> 
> I can see that sequence here turns [a->a] into a->[a], I'm just not sure
> which is the monad at play here... I just need a little bit more
> explanation about this code before I get it.

It is the ((->) a)  monad, also known as the reader monad.

-Brent