On 13 Oct 2017, at 20:13, David McBride <toad3k@gmail.com> wrote:

If you are using do notation, you can't.  If you aren't you can write

tupled s = (rev s, cap s)

Your old tupled is equivalent to this

tupled = rev >>= \s -> cap >>= \c -> return (s, c)

which is quite different.

On Fri, Oct 13, 2017 at 3:05 PM, mike h <mike_k_houghton@yahoo.co.uk> wrote:

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@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@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




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