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https://github.com/hadronized/do-notation

The Haskell’s do notation brought to Rust
https://github.com/hadronized/do-notation

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The Haskell’s do notation brought to Rust

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# `do-notation`, the monadic `do` notation brought to Rust.



This crate provides the `m!` macro, which provides the Haskell monadic syntactic sugar `do`.



> Note: it is not possible to use the `do!` syntax as `do` is a reserved keyword in Rust.



The syntax is very similar to what you find in Haskell:



- You use the `m!` macro; in Haskell, you use the `do` keyword.

- The `<-` syntactic sugar binds its left hand side to the monadic right hand side

  by _entering_ the right side via a closure.

- Like almost any statement in Rust, you must end your statement with a semicolon (`;`).

- The last line must be absent of `;` or contains the `return` keyword.

- You can use `return` nowhere but on the last line.

- A line containing a single expression with a semicolon is a valid statement and has the same effect as `_ <- expr`.

- `let` bindings are allowed in the form `let  = ;` and have the regular Rust meaning.

- The `do` notation syntax does not extend into inner code blocks; however, it can have its own `m!` block. For example:

  `m! { outer_do... if exp { m! { inner_do... } } else { ... } ... }`.



## How do I make my monad works with `m!`?



Because monads are higher-kinded types, it is not possible to define the monadic do-notation in a fully type-system

elegant way. However, this crate is based on the rebindable concept in Haskell (i.e. you can change what the `>>=`

operator’s types are), so `m!` has one type-system requirement and one syntactic requirement.



First, you have to implement one trait: [`Lift`], which allows to _lift_ a value `A` into a _monadic structure of

`A`_. For instance, lifting a `A` into the `Option` monad yields an `Option`.



Then, you have to provide an `and_then` method, which is akin to Haskell’s `>>=` operator. The choice of using

`and_then` and not a proper name like `flat_map` or `bind` is due to the current state of the standard-library —

monads like `Option` and `Result<_, E>` don’t have `flat_map` defined on them but have `and_then`. The type signature

is not enforced, but:



- `and_then` must be a binary function taking a type `A`, a closure `A -> Monad` and returns `Monad`, where

  `Monad` is the monad you are adding `and_then` for. For instance, if you are implementing it for `Option`,

  `and_then` takes an `A`, a closure `A -> Option` and returns an `Option`.

- `and_then` must move its first argument, which has to be `self`. The type of `Self` is not enforced.

- `and_then`’s closure must take `A` with a `FnOnce` closure.



## Meaning of the `<-` operator



The `<-` syntactic sugar is not strictly speaking an operator: it’s not valid vanilla Rust. Instead, it’s a trick

defined in the `m!` allowing to use both [`Lift::lift`] and `and_then`. When you look at code inside a do-notation

block, every monadic statements (separated with `;` in this crate) can be imagined as a new level of nesting inside

a closure — the one passed to `and_then`, indeed.



## First example: fallible code



One of the first monadic application that people learn is the _fallible_ effect — `Maybe` in Haskell.

In `Rust`, it’s `Option`. `Option` is an interesting monad as it allows you to fail early.



```rust

use do_notation::m;



let r = m! {

  x <- Some("Hello, world!");

  y <- Some(3);

  Some(x.len() * y)

};



assert_eq!(r, Some(39));

```



The `binding <- expr` syntax unwraps the right part and binds it to `binding`, making it available to

next calls — remember, nested closures. The final line re-enters the structure (here, `Option`) explicitly.



Note that it is possible to re-enter the structure without having to specify how / knowing the structure

(with `Option`, you re-enter with `Some`). You can use the `return` keyword, that will automatically lift the

value into the right structure:



```rust

use do_notation::m;



let r = m! {

  x <- Some(1);

  y <- Some(2);

  z <- Some(3);

  return [x, y, z];

};



assert_eq!(r, Some([1, 2, 3]));

```