Data.ByteString.Builder.Prim.Internal

• Warning:* this module is internal. If you find that you need it please contact the maintainers and explain what you are trying to do and discuss what you would need in the public API. It is important that you do this as the module may not be exposed at all in future releases.

The maintainers are glad to accept patches for further standard encodings of standard Haskell values.

If you need to write your own builder primitives, then be aware that you are writing code with all safety belts off; i.e., *this is the code that might make your application vulnerable to buffer-overflow attacks!* The Data.ByteString.Builder.Prim.Tests module provides you with utilities for testing your encodings thoroughly.

Fixed-size builder primitives

type Size = Int

The type used for sizes and sizeBounds of sizes.

data FixedPrim a

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

size :: FixedPrim a -> Int

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

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

emptyF :: FixedPrim a

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

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

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

Note that primitives are Contravariant 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)

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

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

Bounded-size builder primitives

data BoundedPrim a

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

boundedPrim :: Int -> (a -> Ptr Word8 -> IO (Ptr Word8)) -> BoundedPrim a

Since: 0.10.12.0

sizeBound :: BoundedPrim a -> Int

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

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

emptyB :: BoundedPrim a

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

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

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

Note that BoundedPrims are Contravariant 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)

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

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

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

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') (liftFixedToBounded char7) emptyB
 

toB :: FixedPrim a -> BoundedPrim a

Convert a FixedPrim to a BoundedPrim.

liftFixedToBounded :: FixedPrim a -> BoundedPrim a

Lift a FixedPrim to a BoundedPrim.

Shared operators

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

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.

 import Data.ByteString.Builder.Prim as P
showEncoding ((\x -> ('\'', (x, '\''))) >$< fixed3) 'x' = "'x'"
  where
    fixed3 = P.char7 >*< P.char7 >*< P.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

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"

Helpers

caseWordSize_32_64 :: a -> a -> a

Select an implementation depending on bitness. Throw a compile time error if bitness is neither 32 nor 64.

Deprecated

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

Deprecated: Use boundedPrim instead