[Haskell-beginners] monad question
mike h
mike_k_houghton at yahoo.co.uk
Fri Oct 13 19:05:28 UTC 2017
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
>
>
>
>
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