CODEWITHSUNDEEP
haskellbytestring
sjakobi
sjakobi

Reputation: 3606

What is the type parameter in the bytestring builder internals for?

The core datatypes of Data.ByteString.Builder are

newtype Builder = Builder (forall r. BuildStep r -> BuildStep r)

type BuildStep a = BufferRange -> IO (BuildSignal a)

data BuildSignal a =
    Done {-# UNPACK #-} !(Ptr Word8) a
  | BufferFull
      {-# UNPACK #-} !Int 
      {-# UNPACK #-} !(Ptr Word8)
                     (BuildStep a)
  | InsertChunk
      {-# UNPACK #-} !(Ptr Word8)
                     S.ByteString
                     (BuildStep a)

What purpose does the type parameter (r or a) serve?

Upvotes: 6

Views: 145

Answers (2)

sjakobi
sjakobi

Reputation: 3606

Simon Meier, the author of this Builder design, has responded to my question on the bytestring issue tracker:

The type parameter was there to support the Put monad (a Writer spezialized to fill a buffer during the computation of its value).

Put is defined as

newtype Put a = Put { unPut :: forall r. (a -> BuildStep r) -> BuildStep r }

and exported from Data.ByteString.Builder.Internal which is hidden in the current bytestring release.

Upvotes: 0

Daniel Wagner
Daniel Wagner

Reputation: 152682

It is not needed. As proof, I have created a fork which does not change any of the public APIs -- only the API of modules named Internal -- but removes this type argument.

Upvotes: 5

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