Clojure's Transducers in Haskell
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Haskell Transducers

Transducers for Haskell. See explanation in my blogpost Clojure's Transducers in Haskell.

Transducers are used for stream manipulation, where the stream type is unknown a priori: It can be list mapping, reductions, used for Conduits, or for your own types or needs. The input and output stream types are decoupled from eachother.


The transducers library expose three types: Reduced, Reducer and Transducer.

The Reducer type is a generalisation over the function one would usually pass to foldl, and has the following definition:

data Reducer s a b = Reducer { initState :: s,
                               complete :: s -> b -> b,
                               step :: s -> b -> a -> (s, Reduced b)

The s in a Reducer is its internal state. initState specifies the initial state of the Reducer, when it has not yet been used.

At the end of the "stream", complete is called with the state and the current accumulated value, and must return a new accumulated value. Usually you would pass back the accumulated value (const id), but complete can be used for cleanup or final changes to the value.

The step function is called for every element of type a in the "stream" or "process" the Reducer is ran over. It returns the new state, and a Reduced accumulated result.

The Reduced type describes a return value from step, and its definition is as follows:

data Reduced a = Continue a
               | Reduced a
               deriving (Eq, Ord, Show, Read, Functor)

A value of Continue signals that we would like to receive more input, if there are any. Reduced signals that we will not use further input for anything, and that the function calling can short-circuit if it would like to.

Reduced is also a comonad, in case you would like to perform extract, duplicate or extend on it.

A transducer is a function from Reducer to Reducer, and does not care about the reducing value:

{-# LANGUAGE RankNTypes #-}

type Transducer s t a b = forall r. Reducer s a r -> Reducer t b r

You should not let transducers specify constraints on s, a proper transducer will work for any s.

The transducers library provides a lot of predefined transducers, for example map, filter, take, drop, partitionBy, dedupe.

To run a reducing function, you would like to use reduce: It works like foldl' but for Reducers. You can use stateless to make a Reducer that works like functions passed to foldl':

import qualified Data.Transducer as T

T.reduce (T.stateless (+)) == foldl' (+)

T.reduce (T.take 10 $ T.stateless (+)) 0 [1..] == foldl' (+) 0 (take 10 [1..])

To run a transducer over a list, use sequence:

import qualified Data.Transducer as T

*> T.sequence ( (+10)) [1..10]
*> T.sequence T.double [1..10]
*> T.sequence T.dedupe [1, 1, 2, 3, 3, 4, 3]

You can compose transducers via function composition to create a new transducer. Note that transducer composition is contravariant, so read evaluation order from left to right, not right to left:

import qualified Data.Transducer as T

*> map (* 4) . map (+ 10) $ [10]
-- map f . map g $ lst == map (f . g) lst

*> T.sequence ( (* 4) . (+ 10)) [10]
-- f . g == (g . f)

*> T.sequence (T.double . T.take 10) [1..]

Transducers can be converted into conduits by using the toConduit function provided in Data.Transducer.Conduit:

import Data.Conduit
import qualified Data.Conduit.List as CL
import qualified Data.Transducer as T
import Data.Transducer.Conduit

*> CL.sourceList [1..] $$ toConduit (T.take 2 . T.double) =$ CL.mapM_ print

They can also in theory be created from a conduit. Help implementing the fromConduit function is appreciated.

You can also create your own transducers and your own functions that manipulate them. See Clojure's Transducers in Haskell for more information.

Real World Production Usage

Use conduits instead. This library will (most likely) not be published nor maintained.


Copyright © 2016 Jean Niklas L'orange

Distributed under the BSD 3-clause license, which is available in the file LICENSE.