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Functional Experiment in Golang

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// repository documentation

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<h3 align="center">
 <a href="#">ƒuego - Functional Experiment in Go</a>
</h3>

## Table of content

- [Overview](#overview)
- [Type Parameters](#type-parameters)
- [Documentation](#documentation)
- [Installation](#installation)
- [Debugging](#debugging)
- [Example Stream](#example-stream)
- [Contributions](#contributions)
- [The Golden rules of the game](#the-golden-rules-of-the-game)
- [Pressure](#pressure)
- [Concept: Entry](#concept-entry)
- [Features summary](#features-summary)
  - [Concurrency](#concurrency)
- [Collectors](#collectors)
- [Golang, Receivers and Functions](#golang-receivers-and-functions)
- [Known limitations](#known-limitations)

## [Overview](#overview)

**_Making Go come to functional programming._**

This is a research project in functional programming which I hope will prove useful!

___ƒuego___ brings a few functional paradigms to Go. The intent is to save development time while promoting code readability and reduce the risk of complex bugs.

I hope you will find it useful!

Have fun!!

[(toc)](#table-of-content)

## [Type Parameters](#type-parameters)

Starting with version 11, ___ƒuego___ uses Go 1.18's [Type Parameters](https://go.googlesource.com/proposal/+/master/design/43651-type-parameters.md).

It is a drastic design change and fundamentally incompatible with previous versions of ___ƒuego___.

Use version 10 or prior if you need the pre-Go1.18 version of ___ƒuego___ that is based on interface `Entry`.

[(toc)](#table-of-content)

## [Documentation](#documentation)

The code documentation and some examples can be found on [godoc](https://pkg.go.dev/github.com/seborama/fuego/v12).

The tests form the best source of documentation. ___ƒuego___ comes with a good collection of unit tests and testable Go examples. Don't be shy, open them up and read them and tinker with them!

> **Note:**
> 
> Most tests use unbuffered channels to help detect deadlocks. In real life scenarios, it is recommended to use buffered channels for increased performance.

[(toc)](#table-of-content)

## [Installation](#installation)

### Download

```bash
go get github.com/seborama/fuego
```

Or for a specific version:

```bash
go get gopkg.in/seborama/fuego.v12
```

### Import in your code

You can import the package in the usual Go fashion.

To simplify usage, you can use an alias:

```go
package sample

import ƒ "gopkg.in/seborama/fuego.v12"
```

...or import as a blank import:

```go
package sample

import _ "gopkg.in/seborama/fuego.v12"
```

Note: dot imports should work just fine but the logger may be disabled, unless you initialised the zap global logger yourself.

[(toc)](#table-of-content)

## [Debugging](#debugging)

Set environment variable `FUEGO_LOG_LEVEL` to enable logging to the desired level.

[(toc)](#table-of-content)

## [Example Stream](#example-stream)

```go
strs := []string{
    "a",
    "b",
    "bb",
    "bb",
    "cc",
    "ddd",
}
    
Collect(
  NewStreamFromSlice[string](strs, 100).
    Filter(isString).
    Distinct(stringHash),
  GroupingBy(
    stringLength,
    Mapping(
      stringToUpper,
      Filtering(
        stringLengthGreaterThan(1),
        ToSlice[string](),
      ),
    ),
  ),
)

// result: map[1:[] 2:[BB CC] 3:[DDD]]
```

[(toc)](#table-of-content)

## [Contributions](#contributions)

Contributions and feedback are welcome.

For contributions, you must develop in TDD fashion and ideally provide Go testable examples (if meaningful).

If you have an idea to improve ___ƒuego___, please share it via an issue.

And if you like ___ƒuego___ give it a star to show your support for the project - it will put a smile on my face! :blush:

Thanks!!

[(toc)](#table-of-content)

## [The Golden rules of the game](#the-golden-rules-of-the-game)

1. Producers close their channel.

1. Consumers do not close channels.

1. Producers and consumers should be running in separate Go routines to prevent deadlocks when the channels' buffers fill up.

[(toc)](#table-of-content)

## [Pressure](#pressure)

Go channels support buffering that affects the behaviour when combining channels in a pipeline.

When the buffer of a Stream's channel of a consumer is full, the producer will not be able to send more data through to it. This protects downstream operations from overloading.

Presently, a Go channel cannot dynamically change its buffer size. This prevents from adapting the stream flexibly. Constructs that use 'select' on channels on the producer side can offer opportunities for mitigation.

[(toc)](#table-of-content)

## [Features summary](#features-summary)

Streams:

- Stream:
  - Filter
  - Map / FlatMap
  - Reduce
  - GroupBy
  - All/Any/None -Match
  - Intersperse
  - Distinct
  - Head* / Last* / Take* / Drop*
  - StartsWith / EndsWith
  - ForEach / Peek
  - ...
- ComparableStream
- MathableStream

Functional Types:

- Optional
- Predicate

Functions:

- Consumer / BiConsumer
- Function / BiFunction
- StreamFunction

Collectors:

- GroupingBy
- Mapping
- FlatMapping
- Filtering
- Reducing
- ToSlice
- ToMap*

Check the [godoc](https://pkg.go.dev/github.com/seborama/fuego/v12) for full details.

[(toc)](#table-of-content)

### Concurrency

As of v8.0.0, a new concurrent model offers to process a stream concurrently while preserving order.

This is not possible yet with all Stream methods but it is available with e.g. `Stream.Map`.

#### Notes on concurrency

Concurrent streams are challenging to implement owing to ordering issues in parallel processing. At the moment, the view is that the most sensible approach is to delegate control to users. Multiple ___ƒuego___ streams can be created and data distributed across as desired. This empowers users of ___ƒuego___ to implement the desired behaviour of their pipelines.

`Stream` has some methods that fan out (e.g. `ForEachC`). See the [godoc](https://pkg.go.dev/github.com/seborama/fuego/v12) for further information and limitations.

I recommend Rob Pike's slides on Go concurrency patterns:

- [Go Concurrency Patterns, Rob Pike, 2012](https://talks.golang.org/2012/concurrency.slide#1)

As a proof of concept and for facilitation, ___ƒuego___ has a `CStream` implementation to manage concurrently a collection of Streams.

[(toc)](#table-of-content)

## [Collectors](#collectors)

A `Collector` is a mutable reduction operation, optionally transforming the accumulated result.

Collectors can be combined to express complex operations in a concise manner.
 
Simply put, a collector allows the creation of bespoke actions on a Stream.

___ƒuego___ exposes a number of functional methods such as `MapToInt`, `Head`, `LastN`, `Filter`, etc...
 
Collectors also provide a few functional methods.

But... what if you need something else? And it is not straightforward or readable when combining the existing methods ___ƒuego___ offers?

Enters `Collector`: implement your own requirement functionally!
 
Focus on _**what**_ needs doing in your streams (and delegate the details of the _**how**_ to the implementation of your `Collector`).

[(toc)](#table-of-content)

## [Golang, Receivers and Functions](#golang-receivers-and-functions)

Some tests (e.g. `TestCollector_Filtering`) are using receiver Getter methods in guise of `Function[T, R any]`. Here is the explanation how this is possible.

`Function[T, R any]` is defined as `func(T) R`.

A method Getter is typically defined as `func (T) Property() R {...}`.

With `t` as the receiver, Go allows `t.Property()` be called as `T.Property(t)`. This is a `func(T) R` and hence a `Function[T, R any]`.

Example - `TestCollector_Filtering`:

`employee.Department(employees[0])` is the same as `employees[0].Department()`, and of course, they both return a `string`.

The first syntax has one advantage for our purpose though: it is a `Function[T, R any]`.

[(toc)](#table-of-content)

## [Known limitations](#known-limitations)

- several operations may be memory intensive or poorly performing.

### No parameterised method in Go

Go 1.18 brings typed parameters. However, parameterised methods are not allowed.

This prevents the Map() method of `Stream` from mapping to, and from returning, a new typed parameter.

To circumvent this, we need to use a decorator function to re-map the `Stream`.

This can lead to a leftward-growing chain of decorator function calls that makes the intent opaque:

```go
ReStream(
  ReStream(is, Stream[int]{}).Map(float2int),
  Stream[string]{}).Map(int2string)
// This is actually performing: Stream.Map(float2int).Map(int2string)
```
___ƒuego___ includes a set of casting functions that reduce the visually leftward-growing chain of decorators while preserving a natural functional flow expression:

```go
C(C(C(
  s.
    Map(float2int_), Int).
    Map(int2string_), String).
    Map(string2int_), Int).
    ForEach(print[int])
// This is actually performing: s.Map(float2int).Map(int2string).Map(string2int).ForEach(print)
```

While not perfect, this is the best workable compromise I have obtained thus far.

[(toc)](#table-of-content)

### Performance issues when using numerous parameterised methods in Go 1.18

As a result of this [issue](https://github.com/golang/go/issues/51957), an experiment to add `MapTo<native_type>() Stream[<native_type>]` is disabled.

Instead, use function `CC` (ComparableStream Cast) to access Min(), Max(), and `MC` (MathableStream Cast) to access Sum().

[(toc)](#table-of-content)

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