4558ef6 Mar 31, 2015
@fthomas @pchiusano @jedesah @waywardmonkeys
158 lines (111 sloc) 4.85 KB
package examples
import org.scalacheck._
import Prop._
import scalaz.concurrent.Task
import scodec.bits.ByteVector
object StartHere extends Properties("examples.StartHere") {
Let's start by looking at a reasonably complete example.
This program opens a file `fahrenheit.txt`, containing
temperatures in degrees fahrenheit, one per line, and
converts each temperature to celsius, incrementally
writing to the file `celsius.txt`. Both files will be
closed, regardless of whether any errors occur.
property("simple file I/O") = secure {
val converter: Task[Unit] =
.filter(s => !s.trim.isEmpty && !s.startsWith("//"))
.map(line => fahrenheitToCelsius(line.toDouble).toString)
Let's dissect this line by line -
property("dissected simple file I/O") = secure {
`Process[Task, String]` is a stream of `String` values which may
periodically evaluate a `scalaz.concurrent.Task` in order to
produce additional values. `Process` is the core data type of
``. It is parameterized on a type constructor (here, `Task`)
which defines what sort of external requests it can make, and
an output type (here, `String`), which defines what type of
values it _emits_.
Many operations on `Process` are defined for any choice of
type constructor, not just `Task`.
`` has a number of helper functions for
constructing or working with streams that talk to the outside world.
`linesR` creates a stream of lines from a filename. It encapsulates
the logic for opening and closing the file, so that users of this
stream do not need to remember to close the file when they are done
or in the event of exceptions during processing of the stream.
val src: Process[Task, String] =
Many of the functions defined for `List` are defined for
`Process` as well, for instance `filter` and `map`.
Note that no side effects occur when we call `filter` or `map`.
`Process` is a purely functional value which can _describe_
a streaming computation that interacts with the outside
world. Nothing will occur until we interpret this description,
and `Process` values are thread-safe and can be shared freely.
val filtered: Process[Task, String] =
src.filter(s => !s.trim.isEmpty && !s.startsWith("//"))
val mapped: Process[Task, String] = => fahrenheitToCelsius(line.toDouble).toString)
/* Adds a newline between emitted strings of `mapped` */
val withNewlines: Process[Task, String] =
We can pipe a `Process`-based stream through a stream transducer,
called a `Process1`. This allows for stateful transformations
of the input stream, for instance, if we want to emit a running
count of the values seen so far.
val encoder: Process1[String, ByteVector] = text.utf8Encode
val chunks: Process[Task, ByteVector] =
val chunks2: Process[Task, ByteVector] = // alternate 'spelling' of `pipe`
withNewlines |> encoder
We can also represent sinks and effectful channels with `Process`.
A sink is represented as a source of effectful functions.
`fileChunkW` will open the file and close it when it is finished
receiving values, or in the event of an error.
val sink: Process[Task, ByteVector => Task[Unit]] =
val sink2: Sink[Task, ByteVector] = sink // `Sink` is a type alias for the above
The `to` function sends an effectful stream to some `Sink`. Again,
nothing happens when we call it, we are just constructing a
description of a computation that, when interpreted, will
incrementally consume the stream, sending values to the given
Likewise, calling `` just returns a pure `Task` value.
Only when we finally call `` will this have the side effect
of opening `fahrenheit.txt` and `celsius.txt`, incrementally
transforming lines from `fahrenheit.txt` and sending them
to `celsius.txt`, and then closing both files. If we do ``
again, the entire computation will be repeated, from opening the
files to closing them.
val pipeline: Process[Task, Unit] =
val task: Task[Unit] =
/* This is the only place we actually have a side effect */
val result: Unit =
def fahrenheitToCelsius(f: Double): Double =
(f - 32.0) * (5.0/9.0)