Streams - Higher Order Functions for Streams

Functions for creating and using Streams. Good for performing map / filter / reduce operations on datasets you can't keep entirely in memory.

Purpose

I was messing around with large datasets, and I couldn't find a lot of good tools out there. I found some good design patterns for making iterators with goroutines and channels, but nothing on using those streams. This library is designed to help you manipulate large data sets using well understood components that you've seen in other languages. I borrowed a lot of nomenclature from Java's Streams API because that's what I am familiar with.

Features

streams

This is the main package. It contains the key functions for manipulating streams: Filter, Map, Reduce, Collect, ForEach, and ForEachThen. It also has several functions for creating a Streams object: FromCollection FromStream, and FromScanner

mappers

This package contains some common helpful mappers. Mappers are functions that match the streams.Mapper function signature and can be passed to the streams.Map function. This is also a good place to look if you're trying to understand how to write your own Mapper function.

collectors

This package contains some common helpful collectors. Collectors are structs that implement the streams.Collector interface and can be used in the streams.Collect function. This is also a good place to look if you're trying to understand how to write your own Collector.

filters

This package has one helpful filter function. Filter functions are functions that match the streams.Predicate function signature and can be passed to streams.Filter to filter a stream. Since filters are easy to write, and often task specific, this package is very sparse. This is also a good place to look if you're trying to understand how to write your own Predicate.

consumers

This package contains some helpful consumers. Consumers are functions that match the Consumer function signature; they can be passed to streams.ForEach and streams.ForEachThen. This is also a good place to look if you're trying to understand how to write your own Consumer.

Examples

I strongly recommend you look at the unit and integration tests, as those are examples that you can trust to compile and work. In addition to the tests, here are some examples to get you going.

Basic Trivial Use Case

This example builds a stream from a collection of ints, filters out the ints not divisible by 2, then multiplies them each 5 five, and sums the elements together. The example is designed to be compact and readable so you can quickly understand what this library does.

import (
	"log"
	"github.com/Luke-Sikina/streams"
)

func TrivialExample() {
    stream := streams.FromCollection([]interface{}{1,2,3,4})
	someSum := stream.
		Filter(DivisibleByTwo).
		Map(TimesFive).
		Reduce(0, Sum)

	log.Printf("Aaah! Numbers! %v", someSum)
}

func DivisibleByTwo(subject interface{}) bool {
	return subject.(int)%2 == 0
}

func TimesFive(element interface{}) interface{} {
	return element.(int)*5
}

func Sum(first, second interface{}) interface{} {
	return first.(int) + second.(int)
}

Read from File, Write to Different File

This example reads a stream of numbers from a file, filters out the positive even numbers, then writes the results to a second file. There's a lot of boilerplate opening files and setting up bufio objects, so if you're looking for the meat of the example, start at stream := streams.FromScanner(scanner, 1023).

import (
	"bufio"
	"log"
	"os"

	"github.com/Luke-Sikina/streams"
	"github.com/Luke-Sikina/streams/consumers"
	"github.com/Luke-Sikina/streams/mappers"
)

func main() {
	// open the input and output files
	input, err := os.Open("input.txt")
	defer closeFileLogErr(input)
	if err != nil {
		log.Fatalf("Error opening file: %v", err)
	}

	output, err := os.Create("output.txt")
	defer closeFileLogErr(output)
	if err != nil {
		log.Fatalf("Error opening file: %v", err)
	}

	// set up the scanner and writers
	scanner := bufio.NewScanner(input)
	stream := streams.FromScanner(scanner, 1023)
	writer := bufio.NewWriter(output)

	// run the stream logic
	stream.
		Map(mappers.StringToIntMapper).
		Filter(func(e interface{}) bool {return e.(int) % 2 == 1}).
		Map(mappers.IntToStringMapper).
		ForEach(consumers.ConsumeWithDelimitedWriter(writer, "\n"))

	// make sure to flush the writer so you actually get output
	err = writer.Flush()
	if err != nil {
		log.Fatalf("Error flushing buffer: %v", err)
	}
}

func closeFileLogErr(f *os.File) {
	err := f.Close()
	if err != nil {
		log.Printf("Error closing file: %v", err)
	}
}