Control.Monad.Codec

The Codec monad provides functions for encoding and decoding complex data structures with unique integer numbers. In the simplest case the entire sturecture can be transformed to unique atom (see example1 below). When it is not sufficient to encode the input object with one codec, more complex codec structure can be used (see example2 below). In order to simplify codec manipulations the library relies on a lens package which provides lenses for typical data types.

Example:

 example1 = evalCodec empty $ do
     let xs = "abcabd"
     ys <- mapM (encode id) xs
     zs <- mapM (decode id) ys
     return $ zip zs ys

>>> example1
>>> [('a',0),('b',1),('c',2),('a',0),('b',1),('d',3)]

 example2 = evalCodec (empty, empty) $ do
     let xs = zip "abcabd" [1, 34342, 5435, 34342, 124, 1]
     ys <- forM xs $ \(x, y) ->
         (,) <$> encode _1 x <*> encode _2 y
     zs <- forM ys $ \(i, j) -> 
         (,) <$> decode _1 i <*> decode _2 j
     return (zs, ys)

>>> fst example2
>>> [('a',1),('b',34342),('c',5435),('a',34342),('b',124),('d',1)]
>>> snd example2
>>> [(0,0),(1,1),(2,2),(0,1),(1,3),(3,0)]

Documentation

data Codec c a

A Codec monad preserves mappings between objects and respective codec components.

Instances

Monad (Codec c)

Functor (Codec c)

Functor (Codec c) => Applicative (Codec c)

data AtomCodec a

Atomic Codec component, which represents to and fro mapping between a objects and unique intergers.

Constructors

AtomCodec

Fields

to :: !(Map a Int)

from :: !(IntMap a)

empty :: AtomCodec a

Empty codec component.

type AtomLens c a = Simple Lens c (AtomCodec a)

Just a type synonym for a lens between codec and codec component.

maybeEncode :: Ord a => AtomLens c a -> a -> Codec c (Maybe Int)

Encode the object with codec component identified by the lens. Return Nothing if the object is not present in the atomic codec component.

encode :: Ord a => AtomLens c a -> a -> Codec c Int

Encode the object with codec component identified by the lens.

encode' :: Ord a => AtomLens c a -> a -> Codec c Int

Version of encode which doesn't update the return componenent of the atom codec. It is useful when we know that particular value (e.g. value of a condition observation) won't be decoded afterwards so there is no need to store it and waste memory.

maybeDecode :: Ord a => AtomLens c a -> Int -> Codec c (Maybe a)

Decode the number with codec component identified by the lens. Return Nothing if the object is not present in the atomic codec component.

decode :: Ord a => AtomLens c a -> Int -> Codec c a

Decode the number with codec component identified by the lens. Report error when the number is not present in the codec component.

runCodec :: c -> Codec c a -> (a, c)

Run the Codec monad with the initial codec value. Return both the result and the final codec state. The obtained codec can be used next to perform subsequent decoding or encoding.

evalCodec :: c -> Codec c a -> a

Evaluate the Codec monad with the initial codec value. Only the monad result will be returned.

execCodec :: c -> Codec c a -> c

Execute the Codec monad with the initial codec value. Only the final codec state will be returned.