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

Language | Haskell2010 |

`Pipe`

is a monad transformer that enriches the base monad with the ability
to `await`

or `yield`

data to and from other `Pipe`

s.

For an extended tutorial, consult Control.Pipe.Tutorial.

- data PipeF a b x
- type Pipe a b = FreeT (PipeF a b)
- type Producer b = Pipe () b
- type Consumer b = Pipe b Void
- type Pipeline = Pipe () Void
- await :: Monad m => Pipe a b m a
- yield :: Monad m => b -> Pipe a b m ()
- pipe :: Monad m => (a -> b) -> Pipe a b m r
- (<+<) :: Monad m => Pipe b c m r -> Pipe a b m r -> Pipe a c m r
- (>+>) :: Monad m => Pipe a b m r -> Pipe b c m r -> Pipe a c m r
- idP :: Monad m => Pipe a a m r
- newtype PipeC m r a b = PipeC {}
- runPipe :: Monad m => Pipeline m r -> m r

# Introduction

I completely expose the `Pipe`

data type and internals in order to encourage
people to write their own `Pipe`

functions. This does not compromise the
correctness or safety of the library at all and you can feel free to use the
constructors directly without violating any laws or invariants.

I promote using the `Monad`

and `Category`

instances to build and compose
pipes, but this does not mean that they are the only option. In fact, any
combinator provided by other iteratee libraries can be recreated for pipes,
too. However, this core library does not provide many of the functions
found in other libraries in order to encourage people to find principled and
theoretically grounded solutions rather than devise ad-hoc solutions
characteristic of other iteratee implementations.

# Types

The `Pipe`

type is strongly inspired by Mario Blazevic's `Coroutine`

type in
his concurrency article from Issue 19 of The Monad Reader and is formulated
in the exact same way.

His `Coroutine`

type is actually a free monad transformer (i.e. `FreeT`

)
and his `InOrOut`

functor corresponds to `PipeF`

.

The base functor for the `Pipe`

type

type Pipe a b = FreeT (PipeF a b) Source #

The base type for pipes

`a`

- The type of input received from upstream pipes`b`

- The type of output delivered to downstream pipes`m`

- The base monad`r`

- The type of the return value

# Create Pipes

`yield`

and `await`

are the only two primitives you need to create pipes.
Since `Pipe a b m`

is a monad, you can assemble `yield`

and `await`

statements using ordinary `do`

notation. Since `Pipe a b`

is also a monad
transformer, you can use `lift`

to invoke the base monad. For example, you
could write a pipe stage that requests permission before forwarding any
output:

check :: (Show a) => Pipe a a IO r check = forever $ do x <- await lift $ putStrLn $ "Can '" ++ (show x) ++ "' pass?" ok <- read <$> lift getLine when ok (yield x)

await :: Monad m => Pipe a b m a Source #

Wait for input from upstream.

`await`

blocks until input is available from upstream.

pipe :: Monad m => (a -> b) -> Pipe a b m r Source #

Convert a pure function into a pipe

pipe f = forever $ do x <- await yield (f x)

# Compose Pipes

`Pipe`

s form a `Category`

, meaning that you can compose `Pipe`

s and also
define an identity `Pipe`

.

`Pipe`

composition binds the output of the upstream `Pipe`

to the input of
the downstream `Pipe`

. Like Haskell functions, `Pipe`

s are lazy, meaning
that upstream `Pipe`

s are only evaluated as far as necessary to generate
enough input for downstream `Pipe`

s. If any `Pipe`

terminates, it also
terminates every `Pipe`

composed with it.

If you want to define a proper `Category`

instance you have to wrap the
`Pipe`

type using the newtype `PipeC`

in order to rearrange the type
variables.

This means that if you want to compose pipes using (`.`

) from the `Category`

type class, you end up with a newtype mess:

unPipeC (PipeC p1 . PipeC p2)

You can avoid this by using convenient operators that do this newtype wrapping and unwrapping for you:

p1 <+< p2 = unPipeC $ PipeC p1 . PipeC p2 idP = unPipeC id

The `Category`

instance obeys the `Category`

laws. In other words:

- Composition is truly associative. The result of composition produces the
exact same composite
`Pipe`

regardless of how you group composition, so it is perfectly safe to omit the parentheses altogether:

(p1 <+< p2) <+< p3 = p1 <+< (p2 <+< p3) = p1 <+< p2 <+< p3

p <+< idP = p idP <+< p = p

The `Category`

laws are "correct by construction", meaning that you cannot
break them despite the library's internals being fully exposed. The above
equalities are true using the strongest denotational semantics possible in
Haskell, namely that both sides of the equals sign correspond to the exact
same value in Haskell, constructor-for-constructor, value-for-value. You
cannot create a function that can distinguish the results.

Actually, all other class instances in this library provide the same strong
guarantees for their corresponding laws. I only emphasize the guarantee for
the `Category`

instance because it is one of the most distinguishing
features of this library, making it far more extensible than other
implementations.

(>+>) :: Monad m => Pipe a b m r -> Pipe b c m r -> Pipe a c m r infixl 9 Source #

Corresponds to (`>>>`

) from `Control.Category`

# Run Pipes

Note that you can also unwrap a `Pipe`

a single step at a time using
`runFreeT`

(since `Pipe`

is just a type synonym for a free monad
transformer). This will take you to the next *external* `await`

or `yield`

statement.

This means that a closed `Pipeline`

will unwrap to a single step, in which
case you would have been better served by `runPipe`

. This directly follows
from the `Category`

laws, which guarantee that you cannot resolve a
composite pipe into its component pipes. When you compose two pipes, the
internal await and yield statements fuse and completely disappear.

`runFreeT`

is ideal for more advanced users who wish to write their own
`Pipe`

functions while waiting for me to find more elegant solutions.

runPipe :: Monad m => Pipeline m r -> m r Source #

Run the `Pipe`

monad transformer, converting it back into the base monad.

`runPipe`

imposes two conditions:

- The pipe's input, if any, is trivially satisfiable (i.e.
`()`

) - The pipe does not
`yield`

any output

The latter restriction makes `runPipe`

less polymorphic than it could be,
and I settled on the restriction for three reasons:

- It prevents against accidental data loss.
- It prevents wastefully draining a scarce resource by gratuitously demanding values from it.
- It encourages an idiomatic pipe programming style where input is consumed
in a structured way using a
`Consumer`

.

If you believe that discarding output is the appropriate behavior, you can specify this by explicitly feeding your output to a pipe that gratuitously discards it:

runPipe $ forever await <+< p