Safe Haskell | Safe |
---|---|

Language | Haskell2010 |

- The ability to fold input
- Prompt and deterministic finalization

`Frame`

s differ from `Pipe`

s because they form restricted monads rather than
forming ordinary monads. This means you must rebind `do`

notation to use
restricted monads from the `index-core`

package. See the "Create Frames"
section for details. For even more details, consult the `index-core`

package.

- data C
- data O a
- type M a = O (Maybe a)
- data FrameF b x i where
- type Frame b m i j r = IFreeT (FrameF (m (), b)) (U m) (r := j) i
- type Stack m r = Frame C m (M ()) C r
- yieldF :: Monad m => m () -> b -> Frame b m i i ()
- awaitF :: Monad m => Frame b m (M a) (M a) (Maybe a)
- close :: Monad m => Frame b m (M a) C ()
- yield :: Monad m => b -> Frame b m i i ()
- await :: Monad m => Frame b m (M a) (M a) a
- catchD :: Monad m => m () -> Frame b m i j r -> Frame b m i j r
- catchF :: Monad m => m () -> Frame b m (M a) j r -> Frame b m (M a) j r
- finallyD :: Monad m => m () -> Frame b m i j r -> Frame b m i j r
- finallyF :: Monad m => m () -> Frame b m (M a) j r -> Frame b m (M a) j r
- (<-<) :: Monad m => Frame c m (M b) C r -> Frame b m (M a) C r -> Frame c m (M a) C r
- (>->) :: Monad m => Frame b m (M a) C r -> Frame c m (M b) C r -> Frame c m (M a) C r
- idF :: Monad m => Frame a m (M a) C r
- newtype FrameC m r a b = FrameC {}
- runFrame :: Monad m => Stack m r -> m r

# Types

The first step to convert `Pipe`

code to `Frame`

code is to translate the
types. All types of the form "`Pipe a b m r`

" become
"`Frame b m (M a) C r`

". For example, given the following type signatures
from the tutorial:

printer :: (Show a) => Pipe b C IO r take' :: Int -> Pipe b b IO () fromList :: (Monad m) => [b] -> Pipe () b m ()

... you would replace them with:

printer :: (Show a) => Frame C IO (M a) C r take' :: Int -> Frame a IO (M a) C () fromList :: (Monad m) => [a] -> Frame a m (M ()) C () -- To use the finalization example, change fromList's base monad to 'IO' fromList :: [a] -> Frame a IO (M ()) C ()

type M a = O (Maybe a) Source #

Index representing an open input end, receiving values of type `Maybe a`

data FrameF b x i where Source #

Base functor for a pipe that can close its input end

`b`

- Output type`x`

- Next step`i`

- Current step's index

# Create Frames

The second step to convert `Pipe`

code to `Frame`

code is to change your
module header to:

{-# LANGUAGE RebindableSyntax #-} import Control.IMonad.Do import Control.Frame import Prelude hiding (Monad(..))

Control.Frame replaces all `Pipe`

`await`

s and `yield`

s with their
corresponding `Frame`

counterparts. `Control.IMonad.Do`

rebinds `do`

notation to work with restricted monads, which also requires using the
`RebindableSyntax`

extension and hiding the `Monad`

class from the `Prelude`

.

You also must use the restricted monad utility functions, which have the
same name as their ordinary monad counterparts except with an '`R`

' suffix,
such as `foreverR`

instead of `forever`

. Finally, you must use `liftU`

instead of `lift`

to invoke operations in the base monad.

Finally, every terminating `Frame`

must be `close`

d exactly once before being
passed to composition.

printer = foreverR $ do a <- await liftU $ print a take' n = do replicateMR_ n $ do a <- await yield a close liftU $ putStrLn "You shall not pass!" fromList xs = do close mapMR_ yield xs

## Primitives

`yieldF`

guards against downstream termination by yielding the most
up-to-date finalization alongside each value, so that downstream can call
that finalizer if it terminates before requesting another value.

`awaitF`

intercepts upstream termination by returning a `Nothing`

if
upstream terminates before providing a value. Further attempts to request
input from upstream will terminate the current `Frame`

using the
return value provided from upstream.

While `awaitF`

is useful for folds, `yieldF`

is less useful for end-users of
this library and the higher-order `catchF`

/ `finallyF`

finalization
functions are much more user-friendly.

Composing two `Frame`

s requires that each `Frame`

invokes `close`

exactly
once. Anything else will not type-check. Leave out the `close`

statement
when writing library components and let the person assembling the components
for composition specify where the `close`

goes.

The earlier you `close`

the upstream `Frame`

, the earlier it is finalized.
However, once you `close`

it you may no longer `await`

.

## Pipe-like primitives

The following `Pipe`

-like primitives are built on top of the `Frame`

primitives. They behave identically to their `Pipe`

counterparts and can
be used as drop-in replacements for them.

yield :: Monad m => b -> Frame b m i i () Source #

`yield`

a value upstream alongside an empty finalizer

await :: Monad m => Frame b m (M a) (M a) a Source #

`await`

a value from upstream and terminate if upstream terminates

# Finalize Frames

The third (and optional) step to convert `Pipe`

code to `Frame`

code is to
register finalizers for your `Frame`

. These finalizers may be arbitrarily
nested:

printer = foreverR $ catchF (putStrLn "printer interrupted") $ do a <- await liftU $ print a take' n = finallyF (putStrLn "You shall not pass!") $ do replicateMR_ n $ do a <- catchF (putStrLn "take' interrupted") await yield a close fromList xs = catchF (putStrLn "fromList interrupted") $ do close mapMR_ yield xs

These convenience functions register block-level finalizers to be called if
another `Frame`

terminates first. The naming conventions are:

- "catch" functions (i.e.
`catchD`

/`catchF`

) call the finalizer only if another`Frame`

terminates before the block completes, but will not call the finalizer if the block terminates normally. - "finally" functions (i.e.
`finallyD`

/`finallyF`

) are like "catch" functions except that they also call the finalizer if the block terminates normally. - Functions that end in a '
`D`

' suffix (i.e.`catchD`

/`finallyD`

) only guard against downstream termination. - Functions that end in a '
`F`

' suffix (i.e.`catchF`

/`finallyF`

) guard against termination in both directions. You usually want these ones.

Note that finalization blocks that *begin* after the `close`

statement may
only use the '`D`

'-suffixed version as upstream has been closed off. This
is a consequence of a deficiency in Haskell's type system that will take
time to work around. However an '`F`

'-suffixed block that begins before a
`close`

statement may continue through it normally. So, for code blocks
after a `close`

statement, use `catchD`

/ `finallyD`

, otherwise use
`catchF`

/ `finallyF`

. In future releases, the '`D`

'-suffixed versions
will be removed and merged into the '`F`

'-suffixed versions.

catchD :: Monad m => m () -> Frame b m i j r -> Frame b m i j r Source #

`catchD m p`

calls the finalizer `m`

if a downstream `Frame`

terminates
before `p`

finishes.

catchF :: Monad m => m () -> Frame b m (M a) j r -> Frame b m (M a) j r Source #

`catchF m p`

calls the finalizer `m`

if any `Frame`

terminates before `p`

finishes.

finallyD :: Monad m => m () -> Frame b m i j r -> Frame b m i j r Source #

`finallyD m p`

calls the finalizer `m`

if a downstream `Frame`

terminates
before `p`

finishes or if `p`

completes normally.

finallyF :: Monad m => m () -> Frame b m (M a) j r -> Frame b m (M a) j r Source #

`finallyF m p`

calls the finalizer `m`

if any `Frame`

terminates before `p`

finishes or if `p`

completes normally.

# Compose Frames

The fourth step to convert `Pipe`

code to `Frame`

code is to replace (`<+<`

)
with (`<-<`

):

printer <-< take' 3 <-< fromList [1..]

Like `Pipe`

s, Frames form a `Category`

where composition pipes the output
from the upstream `Frame`

to the input of the downstream `Frame`

.
Additionally, composition guarantees the following behaviors:

`Frame`

s receive exactly one`Nothing`

if an upstream`Frame`

terminates.- Registered finalizers get called exactly once if a downstream
`Frame`

terminates. - Finalizers are always ordered from upstream to downstream.

The `Category`

laws cannot be broken, so you don't have to be careful when
using `Frame`

s.

Note that you may only compose `Frame`

s that begin open and end closed.

(<-<) :: Monad m => Frame c m (M b) C r -> Frame b m (M a) C r -> Frame c m (M a) C r infixr 9 Source #

(>->) :: Monad m => Frame b m (M a) C r -> Frame c m (M b) C r -> Frame c m (M a) C r infixr 9 Source #

Corresponds to (`>>>`

) from `Control.Category`

# Run Frames

The fifth step to convert `Pipe`

code to `Frame`

code is to use `runFrame`

instead of `runPipe`

:

`>>>`

1 2 3 fromList interrupted You shall not pass! printer interrupted`runFrame $ printer <-< take' 3 <-< fromList [1..]`

`>>>`

1 You shall not pass! take' interrupted printer interrupted`runFrame $ printer <-< take' 3 <-< fromList [1]`