Huangzj
223 lines
6.9 KiB
Markdown
223 lines
6.9 KiB
Markdown
# go-linq [](https://godoc.org/github.com/ahmetalpbalkan/go-linq) [](https://travis-ci.org/ahmetalpbalkan/go-linq) [](https://coveralls.io/github/ahmetalpbalkan/go-linq?branch=master) [](https://goreportcard.com/report/github.com/ahmetalpbalkan/go-linq)
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A powerful language integrated query (LINQ) library for Go.
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* Written in vanilla Go, no dependencies!
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* Complete lazy evaluation with iterator pattern
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* Safe for concurrent use
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* Supports generic functions to make your code cleaner and free of type assertions
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* Supports arrays, slices, maps, strings, channels and custom collections
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## Installation
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$ go get github.com/ahmetalpbalkan/go-linq
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We recommend using a dependency manager (e.g. [govendor][govendor] or
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[godep][godep]) to maintain a local copy of this package in your project.
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[govendor]: https://github.com/kardianos/govendor
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[godep]: https://github.com/tools/godep/
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> :warning: :warning: `go-linq` has recently introduced _breaking API changes_
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> with v2.0.0. See [release notes](#release-notes) for details. v2.0.0 comes with
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> a refined interface, dramatically increased performance and memory efficiency,
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> and new features such as lazy evaluation ([read more](http://kalan.rocks/2016/07/16/manipulating-data-with-iterators-in-go/)).
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>
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> The old version is still available in `archive/0.9` branch and tagged as `0.9`
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> as well. If you are using `go-linq`, please vendor a copy of it in your
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> source tree to avoid getting broken by upstream changes.
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## Quickstart
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Usage is as easy as chaining methods like:
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`From(slice)` `.Where(predicate)` `.Select(selector)` `.Union(data)`
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**Example 1: Find all owners of cars manufactured after 2015**
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```go
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import . "github.com/ahmetalpbalkan/go-linq"
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type Car struct {
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year int
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owner, model string
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}
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...
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var owners []string
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From(cars).Where(func(c interface{}) bool {
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return c.(Car).year >= 2015
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}).Select(func(c interface{}) interface{} {
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return c.(Car).owner
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}).ToSlice(&owners)
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```
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Or, you can use generic functions, like `WhereT` and `SelectT` to simplify your code
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(at a performance penalty):
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```go
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var owners []string
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From(cars).WhereT(func(c Car) bool {
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return c.year >= 2015
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}).SelectT(func(c Car) string {
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return c.owner
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}).ToSlice(&owners)
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```
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**Example 2: Find the author who has written the most books**
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```go
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import . "github.com/ahmetalpbalkan/go-linq"
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type Book struct {
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id int
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title string
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authors []string
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}
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author := From(books).SelectMany( // make a flat array of authors
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func(book interface{}) Query {
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return From(book.(Book).authors)
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}).GroupBy( // group by author
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func(author interface{}) interface{} {
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return author // author as key
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}, func(author interface{}) interface{} {
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return author // author as value
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}).OrderByDescending( // sort groups by its length
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func(group interface{}) interface{} {
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return len(group.(Group).Group)
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}).Select( // get authors out of groups
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func(group interface{}) interface{} {
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return group.(Group).Key
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}).First() // take the first author
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```
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**Example 3: Implement a custom method that leaves only values greater than the specified threshold**
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```go
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type MyQuery Query
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func (q MyQuery) GreaterThan(threshold int) Query {
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return Query{
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Iterate: func() Iterator {
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next := q.Iterate()
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return func() (item interface{}, ok bool) {
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for item, ok = next(); ok; item, ok = next() {
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if item.(int) > threshold {
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return
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}
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}
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return
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}
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},
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}
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}
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result := MyQuery(Range(1,10)).GreaterThan(5).Results()
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```
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## Generic Functions
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Although Go doesn't implement generics, with some reflection tricks, you can use go-linq without
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typing `interface{}`s and type assertions. This will introduce a performance penalty (5x-10x slower)
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but will yield in a cleaner and more readable code.
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Methods with `T` suffix (such as `WhereT`) accept functions with generic types. So instead of
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.Select(func(v interface{}) interface{} {...})
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you can type:
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.SelectT(func(v YourType) YourOtherType {...})
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This will make your code free of `interface{}` and type assertions.
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**Example 4: "MapReduce" in a slice of string sentences to list the top 5 most used words using generic functions**
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```go
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var results []string
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From(sentences).
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// split sentences to words
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SelectManyT(func(sentence string) Query {
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return From(strings.Split(sentence, " "))
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}).
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// group the words
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GroupByT(
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func(word string) string { return word },
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func(word string) string { return word },
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).
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// order by count
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OrderByDescendingT(func(wordGroup Group) int {
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return len(wordGroup.Group)
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}).
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// order by the word
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ThenByT(func(wordGroup Group) string {
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return wordGroup.Key.(string)
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}).
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Take(5). // take the top 5
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// project the words using the index as rank
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SelectIndexedT(func(index int, wordGroup Group) string {
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return fmt.Sprintf("Rank: #%d, Word: %s, Counts: %d", index+1, wordGroup.Key, len(wordGroup.Group))
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}).
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ToSlice(&results)
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```
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**More examples** can be found in the [documentation](https://godoc.org/github.com/ahmetalpbalkan/go-linq).
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## Release Notes
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~~~
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v3.0.0 (2017-01-10)
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* Breaking change: ToSlice() now overwrites existing slice starting
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from index 0 and grows/reslices it as needed.
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* Generic methods support (thanks @cleitonmarx!)
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- Accepting parametrized functions was originally proposed in #26
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- You can now avoid type assertions and interface{}s
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- Functions with generic methods are named as "MethodNameT" and
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signature for the existing LINQ methods are unchanged.
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* Added ForEach(), ForEachIndexed() and AggregateWithSeedBy().
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v2.0.0 (2016-09-02)
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* IMPORTANT: This release is a BREAKING CHANGE. The old version
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is archived at the 'archive/0.9' branch or the 0.9 tags.
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* A COMPLETE REWRITE of go-linq with better performance and memory
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efficiency. (thanks @kalaninja!)
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* API has significantly changed. Most notably:
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- linq.T removed in favor of interface{}
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- library methods no longer return errors
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- PLINQ removed for now (see channels support)
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- support for channels, custom collections and comparables
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v0.9-rc4
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* GroupBy()
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v0.9-rc3.2
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* bugfix: All() iterating over values instead of indices
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v0.9-rc3.1
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* bugfix: modifying result slice affects subsequent query methods
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v0.9-rc3
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* removed FirstOrNil, LastOrNil, ElementAtOrNil methods
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v0.9-rc2.5
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* slice-accepting methods accept slices of any type with reflections
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v0.9-rc2
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* parallel linq (plinq) implemented
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* Queryable separated into Query & ParallelQuery
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* fixed early termination for All
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v0.9-rc1
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* many linq methods are implemented
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* methods have error handling support
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* type assertion limitations are unresolved
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* travis-ci.org build integrated
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* open sourced on github, master & dev branches
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~~~
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