# Implementation of the `union!` macro.


# `union!`

`union!` - one macro to rule them all. Provides useful shortcut combinators, combines sync/async chains, transforms tuple of results in result of tuple, supports single and multi thread (sync/async) step by step execution of branches.

[![Docs][docs-badge]][docs-url]
[![Crates.io][crates-badge]][crates-url]
[![MIT licensed][mit-badge]][mit-url]

[docs-badge]: https://docs.rs/union/badge.svg
[docs-url]: https://docs.rs/union
[crates-badge]: https://img.shields.io/crates/v/union.svg
[crates-url]: https://crates.io/crates/union
[mit-badge]: https://img.shields.io/badge/license-MIT-blue.svg
[mit-url]: LICENSE

Using this macro you can write things like

```rust
#![recursion_limit = "256"]

use rand::prelude::*;
use std::sync::Arc;
use union::union_spawn;

fn generate_random_vec<T>(size: usize, max: T) -> Vec<T>
where
    T: From<u8>
        + rand::distributions::uniform::SampleUniform
        + rand::distributions::uniform::SampleBorrow<T>
        + Copy,
{
    let mut rng = rand::thread_rng();
    (0..size)
        .map(|_| rng.gen_range(T::from(0u8), max))
        .collect()
}

fn is_even<T>(value: T) -> bool
where
    T: std::ops::Rem<Output = T> + std::cmp::PartialEq + From<u8>,
{
    value % 2u8.into() == 0u8.into()
}

fn get_sqrt<T>(value: T) -> T
where
    T: Into<f64>,
    f64: Into<T>,
{
    let value_f64: f64 = value.into();
    value_f64.sqrt().into()
}

fn power2<T>(value: T) -> T
where
    T: std::ops::Mul<Output = T> + Copy,
{
    value * value
}

// Problem: generate vecs filled by random numbers in parallel, make some operations on them in parallel,
// find max of each vec in parallel and find final max of 3 vecs

// Solution:
fn main() {
    // Branches will be executed in parallel, each in its own thread
    let max = union_spawn! {
        let branch_0 =
            generate_random_vec(1000, 10000000u64)
                .into_iter()
                // Multiply every element by himself
                |> power2
                >.filter(|value| is_even(*value)).collect::<Vec<_>>()
                // Use `Arc` to share data with branch 1
                -> Arc::new
                ~-> |v: Arc<Vec<_>>| {
                    // Extract raw poiner after sharing
                    let pointer = Arc::into_raw(v);
                    unsafe {(
                        // Find max and clone its value
                        (&*pointer)
                            .iter()
                            .max()
                            .map(Clone::clone),
                        // After this we call `from_raw` to prevent memory leak
                        Arc::from_raw(pointer)
                    )}.0
                },
        generate_random_vec(10000, 100000000000000f64)
            .into_iter()
            // Extract sqrt from every element
            |> get_sqrt
            // Add index in order to compare with the values of branch 0
            >.enumerate()
            ~|> {
                // Get data from branch 0 by cloning arc
                let branch_0 = branch_0.clone();
                let len = branch_0.len();
                // Compare every element of branch 1 with element of branch 0
                // with the same index and take min
                move |(index, value)|
                    if index < len && value as u64 > branch_0[index] {
                        branch_0[index]
                    } else {
                        value as u64
                    }
            }
            >.max(),
        generate_random_vec(100000, 100000u32)
            .into_iter()
            ~>.max(),
        map => |max0, max1, max2|
            // Find final max
            *[max0, max1, max2 as u64].into_iter().max().unwrap()
    }
    .unwrap();
    println!("Max: {}", max);
}
```

And like this

```rust no_run
#![recursion_limit="1024"]

use union::union_async;
use futures::stream::{iter, Stream};
use reqwest::Client;
use futures::future::{try_join_all, ok, ready};
use failure::{format_err, Error};

fn get_urls_to_calculate_link_count() -> impl Stream<Item = &'static str> {
    iter(
        vec![
            "https://en.wikipedia.org/w/api.php?format=json&action=query&generator=random&grnnamespace=0&prop=revisions|images&rvprop=content&grnlimit=100",
            "https://github.com/explore",
            "https://twitter.com/search?f=tweets&vertical=news&q=%23news&src=unkn"
        ]
    )   
}

fn get_url_to_get_random_number() -> &'static str {
    "https://www.random.org/integers/?num=1&min=0&max=500&col=1&base=10&format=plain&rnd=new"
}

async fn read_number_from_stdin() -> Result<u16, Error> {
    use tokio::*;
    use futures::stream::StreamExt;
    
    let map_parse_error =
        |value|
            move |error|
                format_err!("Value from stdin isn't a correct `u16`: {:?}, input: {}", error, value);

    let mut result;
    let mut reader = codec::FramedRead::new(io::BufReader::new(io::stdin()), codec::LinesCodec::new());

    while {
        println!("Please, enter number (`u16`)");

        let next = reader.next();
    
        result = union_async! {
            next
                |> |value| value.ok_or(format_err!("Unexpected end of input"))
                => |result| ready(result.map_err(|err| format_err!("Failed to apply codec: {:?}", err)))
                => |value|
                    ready(
                        value
                            .parse()
                            .map_err(map_parse_error(value))
                    )
                !> |error| { eprintln!("Error: {:#?}", error); error}
        }.await;

        result.is_err()
    } {}

    result
}

#[tokio::main]
async fn main() {
    println!(
        "{} {}\n{}",
        "Hello.\nThis's is the game where winner is player, which abs(value) is closest to",
        "the max count of links (starting with `https://`) found on one of random pages.",
        "You play against random generator (0-500)."
    );

    enum GameResult {
        Won,
        Lost,
        Draw
    }

    let client = Client::new();
    
    let game = union_async! {
        // Make requests to several sites
        // and calculate count of links starting from `https://`
        get_urls_to_calculate_link_count()
            |> {
                // If pass block statement instead of fn, it will be placed before current step,
                // so it will us allow to capture some variables from context
                let ref client = client;
                move |url|
                    // `union_async!` wraps its content into `async move { }` 
                    union_async! {
                        client
                            .get(url).send()
                            => |value| value.text()
                            => |body| ok((url, body))
                    }
            }
            >.collect::<Vec<_>>()
            |> Ok
            => try_join_all
            !> |err| format_err!("Error retrieving pages to calculate links: {:#?}", err)
            => |results|
                ok(
                    results
                        .into_iter()
                        .map(|(url, body)| (url, body.matches("https://").collect::<Vec<_>>().len()))
                        .max_by_key(|(_, link_count)| link_count.clone())
                        .unwrap()
                )
            // It waits for input in stdin before log max links count
            ~?> |result| {
                result
                    .as_ref()
                    .map(
                        |(url, count)| {
                            let split = url.to_owned().split('/').collect::<Vec<_>>();
                            let domain_name = split.get(2).unwrap_or(&url);
                            println!("Max `https://` link count found on `{}`: {}", domain_name, count)
                        }
                    )
                    .unwrap_or(());
            },
        // In parallel it makes request to the site which generates random number
        get_url_to_get_random_number()
            -> ok
            => {
                // If pass block statement instead of fn, it will be placed before current step,
                // so it will allow us to capture some variables from context
                let ref client = client;
                let map_parse_error =
                    |value|
                        move |err|
                            format_err!("Failed to parse random number: {:#?}, value: {}", err, value);
                move |url|
                    union_async! {
                        client
                            .get(url)
                            .send()
                            => |value| value.text()
                            !> |err| format_err!("Error retrieving random number: {:#?}", err)
                            => |value| ok(value[..value.len() - 1].to_owned()) // remove \n from `154\n`
                            => |value|  
                                ready(
                                    value
                                        .parse::<u16>()
                                        .map_err(map_parse_error(value))
                                )
                    }
            }
            // It waits for input in stdin before log random value
            ~?> |random| {
                random
                    .as_ref()
                    .map(|number| println!("Random: {}", number))
                    .unwrap_or(());
            },
        // In parallel it reads value from stdin
        read_number_from_stdin(),
        // Finally, when we will have all results, we can decide, who is winner
        map => |(_url, link_count), random_number, number_from_stdin| {
            let random_diff = (link_count as i32 - random_number as i32).abs();
            let stdin_diff = (link_count as i32 - number_from_stdin as i32).abs();
            match () {
                _ if random_diff > stdin_diff => GameResult::Won,
                _ if random_diff < stdin_diff => GameResult::Lost,
                _ => GameResult::Draw
            }
        }    
    };

    let _ = game.await.map(
        |result|
            println!(
                "You {}",
                match result {
                    GameResult::Won => "won!",
                    GameResult::Lost => "lose...",
                    _ => "have the same result as random generator!"
                }
            )
    ).unwrap();  
}
```

## Combinators

- Map: `|>` expr - `value`.map(`expr`)

- AndThen: `=>` expr - `value`.and_then(`expr`),

- Then: `->` expr - `expr`(`value`)

- Dot: `>.` expr - `value`.`expr`

- Or: `<|` expr - `value`.or(`expr`)

- OrElse: `<=` expr - `value`.or_else(`expr`)  

- MapErr: `!>` expr - `value`.map_err(`expr`)

- Inspect: `?>` expr - (|`value`| { `expr`(&`value`); `value` })(`value`) for sync chains and (|`value`| `value`.inspect(`expr`))(`value`) for futures

where `value` is the previous value.

Every combinator prefixed by `~` will act as deferred action (all actions will wait until completion in every step and only after move to the next one).

## Handler

might be one of

- `map` => will act as results.map(|(result0, result1, ..)| handler(result0, result1, ..))

- `and_then` => will act as results.and_then(|(result0, result1, ..)| handler(result0, result1, ..))

- `then` => will act as handler(result0, result1, ..)

or not specified - then Result<(result0, result1, ..), Error> or Option<(result0, result1, ..)> will be returned.

## Custom futures crate path

You can specify custom path (`futures_crate_path`) at the beginning of macro call

```rust
use union::union_async;
use futures::future::ok;

#[tokio::main]
async fn main() {
    let value = union_async! {
        futures_crate_path(::futures)
        ok::<_,u8>(2u16)
    }.await.unwrap();
    
    println!("{}", value);
}
```

## Single thread combinations

### Simple results combination

Converts input in series of chained results and joins them step by step.

```rust

use std::error::Error;
use union::union;

type Result<T> = std::result::Result<T, Box<dyn Error>>;

fn action_1() -> Result<u16> {
    Ok(1)
}

fn action_2() -> Result<u8> {
    Ok(2)
}

fn main() {
    let sum = union! {
        action_1(),
        action_2().map(|v| v as u16),
        action_2().map(|v| v as u16 + 1).and_then(|v| Ok(v * 4)),
        action_1().and_then(|_| Err("5".into())).or(Ok(2)),
        map => |a, b, c, d| a + b + c + d
    }.expect("Failed to calculate sum");

    println!("Calculated: {}", sum);
}
```

### Futures combination

Each branch will represent chain of tasks. All branches will be joined using `::futures::join!` macro and `union_async!` will return `unpolled` future.

```rust
#![recursion_limit="256"]

use std::error::Error;
use union::union_async;
use futures::future::{ok, err};

type Result<T> = std::result::Result<T, Box<dyn Error>>;

async fn action_1() -> Result<u16> {
    Ok(1)
}
async fn action_2() -> Result<u8> {
    Ok(2)
}

#[tokio::main]
async fn main() {
    let sum = union_async! {
        action_1(),
        action_2().and_then(|v| ok(v as u16)),
        action_2().map(|v| v.map(|v| v as u16 + 1)).and_then(|v| ok(v * 4u16)),
        action_1().and_then(|_| err("5".into())).or_else(|_| ok(2u16)),
        and_then => |a, b, c, d| ok(a + b + c + d)
    }.await.expect("Failed to calculate sum");

    println!("Calculated: {}", sum);
}
```

## Multi-thread combinations

To execute several tasks in parallel you could use `union_spawn!` (`spawn!`) for sync tasks
and `union_async_spawn!` (`async_spawn!`) for futures. Since `union_async` already provides parallel futures execution in one thread, `union_async_spawn!` spawns every branch into `tokio` executor so they will be evaluated in multi-threaded executor.

### Multi-thread sync branches

`union_spawn` spawns one `::std::thread` per each step of each branch (number of branches is the max thread count at the time).

```rust

use std::error::Error;
use union::union_spawn;

type Result<T> = std::result::Result<T, Box<dyn Error + Send + Sync>>;

fn action_1() -> Result<usize> {
    Ok(1)
}

fn action_2() -> Result<u16> {
    Ok(2)
}

fn main() {
    // Branches will be executed in parallel
    let sum = union_spawn! {
        action_1(),
        action_2().map(|v| v as usize),
        action_2().map(|v| v as usize + 1).and_then(|v| Ok(v * 4)),
        action_1().and_then(|_| Err("5".into())).or(Ok(2)),
        map => |a, b, c, d| a + b + c + d
    }.expect("Failed to calculate sum");

    println!("Calculated: {}", sum);
}
```

`union_async_spawn!` uses `::tokio::spawn` function to spawn tasks so it should be done inside `tokio` runtime
(number of branches is the max count of `tokio` tasks at the time).

### Multi-thread futures

```rust
#![recursion_limit="256"]

use std::error::Error;
use union::union_async_spawn;
use futures::future::{ok, err};

type Result<T> = std::result::Result<T, Box<dyn Error + Send + Sync>>;

async fn action_1() -> Result<u16> {
    Ok(1)
}

async fn action_2() -> Result<u8> {
    Ok(2)
}

#[tokio::main]
async fn main() {
    let sum = union_async_spawn! {
        action_1(),
        action_2().and_then(|v| ok(v as u16)),
        action_2().map(|v| v.map(|v| v as u16 + 1)).and_then(|v| ok(v * 4u16)),
        action_1().and_then(|_| err("5".into())).or_else(|_| ok(2u16)),
        and_then => |a, b, c, d| ok(a + b + c + d)
    }.await.expect("Failed to calculate sum");

    println!("Calculated: {}", sum);
}
```

Using combinators we can rewrite first sync example like

```rust

use std::error::Error;
use union::union;

type Result<T> = std::result::Result<T, Box<dyn Error>>;

fn action_1() -> Result<u16> {
    Ok(1)
}

fn action_2() -> Result<u8> {
    Ok(2)
}

fn main() {
    let sum = union! {
        action_1(),
        action_2() |> |v| v as u16,
        action_2() |> |v| v as u16 + 1 => |v| Ok(v * 4),
        action_1() => |_| Err("5".into()) <| Ok(2),
        map => |a, b, c, d| a + b + c + d
    }.expect("Failed to calculate sum");

    println!("Calculated: {}", sum);
}
```

By separating chain in actions, you will make actions wait for completion of all of them in current step before go to the next step.

```rust
#![recursion_limit="256"]

use std::error::Error;
use union::union;

type Result<T> = std::result::Result<T, Box<dyn Error + Send + Sync>>;

fn action_1() -> Result<u16> {
    Ok(1)
}

fn action_2() -> Result<u8> {
    Ok(2)
}

fn main() {
    let sum = union! {
        action_1(),
        let result_1 = action_2() ~|> |v| v as u16 + 1,
        action_2() ~|> {
            let result_1 = result_1.as_ref().ok().map(Clone::clone);
            move |v| {
                // `result_1` now is the result of `action_2()` [Ok(1u8)]
                if result_1.is_some() {
                    v as u16 + 1
                } else {
                    unreachable!()
                }
            }
        } ~=> {
            let result_1 = result_1.as_ref().ok().map(Clone::clone);
            move |v| {
                // `result_1` now is the result of `|v| v as u16 + 1` [Ok(2u16)]
                if let Some(result_1) = result_1 {
                    Ok(v * 4 + result_1)
                } else {
                    unreachable!()
                }
            }
        },
        action_1() ~=> |_| Err("5".into()) <| Ok(2),
        map => |a, b, c, d| a + b + c + d
    }.expect("Failed to calculate sum");
    println!("Calculated: {}", sum);
}
```