Data.ByteString.Builder.Prim.Internal

Copyright	2010-2011 Simon Meier 2010 Jasper van der Jeugt
License	BSD3-style (see LICENSE)
Maintainer	Simon Meier <[email protected]>
Stability	unstable, private
Portability	GHC
Safe Haskell	Unsafe
Language	Haskell98

Fixed-size builder primitives

type Size = Int Source

The type used for sizes and sizeBounds of sizes.

data FixedPrim a Source

A builder primitive that always results in a sequence of bytes of a pre-determined, fixed size.

fixedPrim :: Int -> (a -> Ptr Word8 -> IO ()) -> FixedPrim a Source

size :: FixedPrim a -> Int Source

The size of the sequences of bytes generated by this FixedPrim.

runF :: FixedPrim a -> a -> Ptr Word8 -> IO () Source

emptyF :: FixedPrim a Source

The FixedPrim that always results in the zero-length sequence.

contramapF :: (b -> a) -> FixedPrim a -> FixedPrim b Source

Change a primitives such that it first applies a function to the value to be encoded.

Note that primitives are Contrafunctors http://hackage.haskell.org/package/contravariant. Hence, the following laws hold.

contramapF id = id
contramapF f . contramapF g = contramapF (g . f)

pairF :: FixedPrim a -> FixedPrim b -> FixedPrim (a, b) Source

Encode a pair by encoding its first component and then its second component.

storableToF :: forall a. Storable a => FixedPrim a Source

Bounded-size builder primitives

data BoundedPrim a Source

A builder primitive that always results in sequence of bytes that is no longer than a pre-determined bound.

boudedPrim :: Int -> (a -> Ptr Word8 -> IO (Ptr Word8)) -> BoundedPrim a Source

sizeBound :: BoundedPrim a -> Int Source

The bound on the size of sequences of bytes generated by this BoundedPrim.

runB :: BoundedPrim a -> a -> Ptr Word8 -> IO (Ptr Word8) Source

emptyB :: BoundedPrim a Source

The BoundedPrim that always results in the zero-length sequence.

contramapB :: (b -> a) -> BoundedPrim a -> BoundedPrim b Source

Change a BoundedPrim such that it first applies a function to the value to be encoded.

Note that BoundedPrims are Contrafunctors http://hackage.haskell.org/package/contravariant. Hence, the following laws hold.

contramapB id = id
contramapB f . contramapB g = contramapB (g . f)

pairB :: BoundedPrim a -> BoundedPrim b -> BoundedPrim (a, b) Source

Encode a pair by encoding its first component and then its second component.

eitherB :: BoundedPrim a -> BoundedPrim b -> BoundedPrim (Either a b) Source

Encode an Either value using the first BoundedPrim for Left values and the second BoundedPrim for Right values.

Note that the functions eitherB, pairB, and contramapB (written below using >$<) suffice to construct BoundedPrims for all non-recursive algebraic datatypes. For example,

maybeB :: BoundedPrim () -> BoundedPrim a -> BoundedPrim (Maybe a)
maybeB nothing just = maybe (Left ()) Right >$< eitherB nothing just
 

condB :: (a -> Bool) -> BoundedPrim a -> BoundedPrim a -> BoundedPrim a Source

Conditionally select a BoundedPrim. For example, we can implement the ASCII primitive that drops characters with Unicode codepoints above 127 as follows.

charASCIIDrop = condB (< '\128') (fromF char7) emptyB
 

toB :: FixedPrim a -> BoundedPrim a Source

Convert a FixedPrim to a BoundedPrim.

liftFixedToBounded :: FixedPrim a -> BoundedPrim a Source

Lift a FixedPrim to a BoundedPrim.

Shared operators

(>$<) :: Contravariant f => (b -> a) -> f a -> f b infixl 4 Source

A fmap-like operator for builder primitives, both bounded and fixed size.

Builder primitives are contravariant so it's like the normal fmap, but backwards (look at the type). (If it helps to remember, the operator symbol is like ($) but backwards.)

We can use it for example to prepend and/or append fixed values to an primitive.

showEncoding ((\x -> ('\'', (x, '\''))) >$< fixed3) 'x' = "'x'"
  where
    fixed3 = char7 >*< char7 >*< char7

Note that the rather verbose syntax for composition stems from the requirement to be able to compute the size / size bound at compile time.

(>*<) :: Monoidal f => f a -> f b -> f (a, b) infixr 5 Source

A pairing/concatenation operator for builder primitives, both bounded and fixed size.

For example,

toLazyByteString (primFixed (char7 >*< char7) ('x','y')) = "xy"

We can combine multiple primitives using >*< multiple times.

toLazyByteString (primFixed (char7 >*< char7 >*< char7) ('x',('y','z'))) = "xyz"