Turtle.Shell

You can think of Shell as [] + IO + Managed. In fact, you can embed all three of them within a Shell:

select ::        [a] -> Shell a
liftIO ::      IO a  -> Shell a
using  :: Managed a  -> Shell a

Those three embeddings obey these laws:

do { x <- select m; select (f x) } = select (do { x <- m; f x })
do { x <- liftIO m; liftIO (f x) } = liftIO (do { x <- m; f x })
do { x <- with   m; using  (f x) } = using  (do { x <- m; f x })

select (return x) = return x
liftIO (return x) = return x
using  (return x) = return x

... and select obeys these additional laws:

select xs <|> select ys = select (xs <|> ys)
select empty = empty

You typically won't build Shells using the Shell constructor. Instead, use these functions to generate primitive Shells:

• empty, to create a Shell that outputs nothing
• return, to create a Shell that outputs a single value
• select, to range over a list of values within a Shell
• liftIO, to embed an IO action within a Shell
• using, to acquire a Managed resource within a Shell

Then use these classes to combine those primitive Shells into larger Shells:

• Alternative, to concatenate Shell outputs using (<|>)
• Monad, to build Shell comprehensions using do notation

If you still insist on building your own Shell from scratch, then the Shell you build must satisfy this law:

-- For every shell `s`:
_foldIO s (FoldM step begin done) = do
    x  <- begin
    x' <- _foldIO s (FoldM step (return x) return)
    done x'

... which is a fancy way of saying that your Shell must call 'begin' exactly once when it begins and call 'done' exactly once when it ends.

Shell

newtype Shell a

A (Shell a) is a protected stream of a's with side effects

Constructors

Shell

Fields

• _foldIO :: forall r. FoldM IO a r -> IO r

Instances

Monad Shell

Methods

(>>=) :: Shell a -> (a -> Shell b) -> Shell b

(>>) :: Shell a -> Shell b -> Shell b

return :: a -> Shell a

fail :: String -> Shell a

Functor Shell

Methods

fmap :: (a -> b) -> Shell a -> Shell b

(<$) :: a -> Shell b -> Shell a

Applicative Shell

Methods

pure :: a -> Shell a

(<*>) :: Shell (a -> b) -> Shell a -> Shell b

(*>) :: Shell a -> Shell b -> Shell b

(<*) :: Shell a -> Shell b -> Shell a

MonadIO Shell

Methods

liftIO :: IO a -> Shell a

Alternative Shell

Methods

empty :: Shell a

(<|>) :: Shell a -> Shell a -> Shell a

some :: Shell a -> Shell [a]

many :: Shell a -> Shell [a]

MonadPlus Shell

Methods

mzero :: Shell a

mplus :: Shell a -> Shell a -> Shell a

MonadManaged Shell

Methods

using :: Managed a -> Shell a

Monoid a => Num (Shell a)

Shell forms a semiring, this is the closest approximation

Methods

(+) :: Shell a -> Shell a -> Shell a

(-) :: Shell a -> Shell a -> Shell a

(*) :: Shell a -> Shell a -> Shell a

negate :: Shell a -> Shell a

abs :: Shell a -> Shell a

signum :: Shell a -> Shell a

fromInteger :: Integer -> Shell a

IsString a => IsString (Shell a)

Methods

fromString :: String -> Shell a

Monoid a => Monoid (Shell a)

Methods

mempty :: Shell a

mappend :: Shell a -> Shell a -> Shell a

mconcat :: [Shell a] -> Shell a

foldIO :: MonadIO io => Shell a -> FoldM IO a r -> io r

Use a FoldM IO to reduce the stream of a's produced by a Shell

fold :: MonadIO io => Shell a -> Fold a b -> io b

Use a Fold to reduce the stream of a's produced by a Shell

sh :: MonadIO io => Shell a -> io ()

Run a Shell to completion, discarding any unused values

view :: (MonadIO io, Show a) => Shell a -> io ()

Run a Shell to completion, printing any unused values

Embeddings

select :: Foldable f => f a -> Shell a

Convert a list to a Shell that emits each element of the list

liftIO :: MonadIO m => forall a. IO a -> m a

Lift a computation from the IO monad.

using :: MonadManaged m => forall a. Managed a -> m a