[Haskell-cafe] Re: Grokking monads by not overlooking concatMap

[Achim Schneider]

Peter Hercek <phercek at gmail.com> wrote:

> Achim Schneider wrote:
> > Peter Hercek <phercek at gmail.com> wrote:
> >>> Haskell wins the wickedness of design contest by using [()] and []
> >>> as truth values.
> >> Maybe you wanted to say: "... by using [()] as True value and [] as
> >>   False value" ... which does not seem that wicked (at least to
> >> me).
> >>
> > Strangely enough, it reads exactly the same to me. IMHO, not working
> > like filter is the thing that makes this wicked: guard doesn't care
> > about what is returned, you could as well say (\_ ->
> > [("foo","bar")])
> 
> Right, should be a list with one element in it to represent True.
> So maybe it is a bit worse than C since list with two elements
>   gives you two True values :-D
> On the other side it is nice that guard works the expected way
>   (using the same implementation) with other MonadPlus-es too.
>
Yes ;)

You can actually do stuff like

f'''' = concatMap 
        (\x -> concatMap 
            (\y -> concatMap 
                (\_ -> [(x,y)])
                (if x*y == 8 then replicate (x+y) () else []))
            [2..8])
        [1..4]

, resulting in
[(1,8),(1,8),(1,8),(1,8),(1,8),(1,8),(1,8),(1,8),(1,8),(2,4),(2,4),
(2,4),(2,4),(2,4),(2,4),(4,2),(4,2),(4,2),(4,2),(4,2),(4,2)]

or

Prelude> "Hello World" >>= (\c -> if (c == 'o') then replicate 10 c else [c])
"Helloooooooooo Woooooooooorld"
-- 
(c) this sig last receiving data processing entity. Inspect headers for
past copyright information. All rights reserved. Unauthorised copying,
hiring, renting, public performance and/or broadcasting of this
signature prohibited.