purescript-node-stream-pipes

Interact with node streams in object mode using Pipes!

spago install node-stream-pipes

Raw objectMode Node streams are represented in Node.Stream.Object:

• Writable a accepts chunks of type a
• Readable a emits chunks of type a
• Transform a b transforms chunks from a to b

Non-Object streams can also be represented with these types; for example an fs.WriteStream can be coerced to Writable Buffer without issue.

Interop between these types and Node.Stream are provided in Node.Stream.Object:

• unsafeFrom{String,Buffer}{Writable,Readable,Transform}
• unsafeCoerce{Writable,Readable,Transform}

Streams in Node.Stream.Object can be converted to Producers, Consumers and Pipes with Pipes.Node.Stream:

• fromReadable :: forall a. <Readable a> -> Producer (Maybe a) <Aff> Unit
• fromWritable :: forall a. <Writable a> -> Consumer (Maybe a) <Aff> Unit
• fromTransform :: forall a b. <Transform a b> -> Pipe (Maybe a) (Maybe b) <Aff> Unit

Normally, pipe computations will not be executed once any computation in a pipeline exits.

To allow for resource cleanup and awareness that the stream is about to close, Maybe a is used occasionally in this package as an End-of-Stream marker:

-- foo.txt is "hello, world!\n"
chunks <- Pipes.Collect.toArray $ Pipes.FS.read "foo.txt" >-> Pipes.Node.Stream.inEOS (Pipes.Buffer.toString UTF8)
chunks `shouldEqual` [Just "hello, world!\n", Nothing]

Pipes from a -> b unaware of EOS can be lifted to Maybe a -> Maybe b with Pipes.Node.Stream.inEOS.

Producers of Maybe a can drop the EOS marker and emit a with Pipes.Node.Stream.unEOS.

Producers of a can have an EOS marker added with Pipes.Node.Stream.withEOS.

Pipes.PassThrough.js

import {PassThrough} from 'stream'

export const makePassThrough = () => new PassThrough()

Pipes.PassThrough.purs

module Pipes.PassThrough where

import Prelude

import Effect (Effect)
import Effect.Class (liftEffect)
import Effect.Aff (Aff)
import Pipes.Core (Pipe)
import Node.Stream.Object as ObjectStream
import Pipes.Node.Stream as Pipes.Node.Stream

type PassThroughStream a = ObjectStream.Transform a a

foreign import makeRaw :: Effect PassThroughStream

passThrough :: forall a. Pipe a a Aff Unit
passThrough = do
  raw <- liftEffect $ makeRaw
  Pipes.Node.Stream.fromTransform raw

This package provides utilities that explicitly use MonadRec to ensure stack-safety when dealing with producers of large amounts of data.

• Pipes.Collect provides stack-safe utilities for executing a pipeline and collecting results into a collection, Buffer, Monoid etc.
• Pipes.Construct provides stack-safe utilities for creating producers from in-memory collections.
• Pipes.Util provides some miscellaneous utilities missing from pipes.

Pipes for compression & decompression using zlib are provided in Pipes.Node.Zlib.

Read files with:

• Pipes.Node.FS.read <path>
• Pipes.Node.FS.read' <WriteStreamOptions> <path>

Pipes.Collect.toStringWith UTF8 $ Pipes.Node.FS.read "foo.txt" >-> Pipes.Stream.unEOS

Write files with:

• Pipes.Node.FS.write' <WriteStreamOptions> <path>
• Pipes.Node.FS.trunc <path>
• Pipes.Node.FS.create <path>
• Pipes.Node.FS.append <path>

  Pipes.Stream.withEOS (
    Pipes.Construct.eachArray ["id,name", "1,henry", "2,suzie"]
    >-> Pipes.Util.intersperse "\n"
    >-> Pipes.Buffer.fromString UTF8
  )
  >-> Pipes.Node.FS.create "foo.csv"