[Haskell-beginners] monad question

[mike h]

That certainly helps me  David, thanks.
How then would you write
> tupled :: String -> (String, String)

 
with the parameter written explicitly? i.e.

tupled s = do …

or does the question not make sense in light of your earlier reply?

Thanks

Mike




> On 13 Oct 2017, at 19:35, David McBride <toad3k at gmail.com> wrote:
> 
> Functions are Monads.
> 
> :i Monad
> class Applicative m => Monad (m :: * -> *) where
>   (>>=) :: m a -> (a -> m b) -> m b
>   (>>) :: m a -> m b -> m b
>   return :: a -> m a
> ...
> instance Monad (Either e) -- Defined in ‘Data.Either’
> instance Monad [] -- Defined in ‘GHC.Base’
> ...
> instance Monad ((->) r) -- Defined in ‘GHC.Base’
> 
> That last instance means if I have a function whose first argument is type r, that is a monad.  And if you fill in the types of the various monad functions you would get something like this
> 
> (>>=) :: ((->) r) a -> (a -> ((-> r) b) -> ((-> r) b)
> (>>=) :: (r -> a) -> (a -> (r -> b)) -> (r -> b) -- simplified
> return :: a -> (r -> a)
> 
> So in the same way that (IO String) is a Monad and can use do notation, (a -> String) is also a Monad, and can also use do notation.  Hopefully that made sense.
> 
> On Fri, Oct 13, 2017 at 2:15 PM, mike h <mike_k_houghton at yahoo.co.uk <mailto:mike_k_houghton at yahoo.co.uk>> wrote:
> 
> I have
> 
> cap :: String -> String
> cap = toUpper
> 
> rev :: String -> String
> rev = reverse
> 
> then I make
> 
> tupled :: String -> (String, String)
> tupled = do
>     r <- rev
>     c <- cap
>     return (r, c)
> 
> and to be honest, yes it’s been a long day at work, and this is coding at home rather than coding (java) at work but
> I’m not sure how tupled  works!!!
> My first shot was supplying a param s like this
> 
> tupled :: String -> (String, String)
> tupled s = do
>     r <- rev s
>     c <- cap s
>     return (r, c)
> 
> which doesn’t compile. But how does the first version work? How does the string to be processed get into the rev and cap functions??
> 
> Thanks
> 
> Mike
> 
> 
> 
> 
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org <mailto:Beginners at haskell.org>
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners <http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners>
> 
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/beginners/attachments/20171013/8024e7fd/attachment.html>