Node.Thunk

Node.Thunk is a library for PureScript which helps to interface with asynchronous Node.js code:

Wrapping Node.js async code with FFI and thunks

This simple example shows how to use Node.Thunk to wrap a part of Node.js's fs module:

import Node.Thunk

foreign import fs "var fs = require('fs');" :: {
  readFile :: ThunkFn1 String String
}

readFile = runThunkFn1 fs.readFile

The type ThunkFn1 a b means that a function takes a single argument of type a and produces a result of type b. To convert ThunkFn1 a b into a PureScript function a -> Thunk b we need to apply runThunkFn1 function.

Similarly to ThunkFn1 there exists ThunkFn2, ThunkFn3 for each arity up to 5.

Computation with thunks

This simple examples shows how to combine computations with thunks:

readFileAndWait = do
  contents <- readFile "./README.md"
  delay 1000
  return contents

delay is a computation which results into Unit value (nothing) after some milliseconds elapsed.

To execute computation you should use runThunk function:

main = runThunk readFileAndWait handle
    where
  handle (Left err) = -- handle error
  handle (Right result) = -- handle result

runThunk function thunk as first argument and handler as second argument. Handler is supplied with either an error or a result.

Executing Eff actions inside thunk computations

There's a function liftEff which can lift Eff action into Thunk:

readFileAndPrintContents = do
  contents <- readFile "./README.md"
  liftEff (print contents)