Safe Haskell  SafeInferred 

 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 indexcore
package. See the "Create Frames"
section for details. For even more details, consult the indexcore
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 ()
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
uptodate 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 endusers of
this library and the higherorder catchF
/ finallyF
finalization
functions are much more userfriendly.
Composing two Frame
s requires that each Frame
invokes close
exactly
once. Anything else will not typecheck. 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
.
Pipelike 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 dropin replacements for them.
await :: Monad m => Frame b m (M a) (M a) aSource
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 blocklevel finalizers to be called if
another Frame
terminates first. The naming conventions are:
 "catch" functions (i.e.
catchD
/catchF
) call the finalizer only if anotherFrame
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 rSource
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 rSource
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 rSource
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 rSource
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 oneNothing
if an upstreamFrame
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 b m (M a) C r > Frame c m (M b) C r > Frame c m (M a) C rSource
Corresponds to (>>>
) from Control.Category
Run Frames
The fifth step to convert Pipe
code to Frame
code is to use runFrame
instead of runPipe
:
>>>
runFrame $ printer << take' 3 << fromList [1..]
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