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Overview

yerevan.rs is a Proof Of Concept (PoC) Rust library of Computation Expressions (CEs), aka "do-notations" in Haskell, inspired by F#'s CEs, but with changes and improvements in syntax-design.

1. This package is a PoC of the idea that CEs could be very useful in Rust.
2. The goal of this PoC is to evolve to a CEs feature proposal in Rust language, if it succeeds.

Why is this package only a PoC and not a solid solution?

1. This package's implementation of CEs uses Rust macros, which slow the Rust compiler down. Besides, CEs are meant to be used often, which will significantly increase compilation time.
2. Some syntax features of classical do-notations and CEs are not possible with Rust's macro_rules.

Docs

1. Firstly define the struct which is implementing all the neccessary functions for yerevan.rs's computation expressions. Like this:

struct SimpleBinder {}
impl SimpleBinder {
    pub fn bind<T, U>(val: Option<T>, f: &dyn Fn(T) -> Option<U>) -> Option<U> {
        match val {
            Some(v) => f(v),
                None => SimpleBinder::zero(),
        }
    }
    pub fn ret<T>(val: T) -> Option<T> {
        Some(val)
    }
    pub fn zero<T>() -> Option<T> {
        None
    }
}

2. And then use your struct as computation expression type in yer! macro. Like this:

yer! {
    SimpleBinder =>
    let! ...
    let ...
    ret ...
}

Expressions

1. $your_struct => defines the struct as the provider of computation expression functions.

2. let! $your_var = $your_expression uses the last defined struct in macros as the provider of bind function and calls the $that_struct::bind($your_expression, &\|$your_var\| { next code }) expression.

3. let $your_var = $your_expression defines the variable $your_var.

4. do! $your_expression uses the last defined struct in macros as the provider of bind function and calls the $that_struct::bind($your_expression, &\|_\| { next code }) expression.

5. do $your_expression simply runs $your_expression.

6. ret! $your_expression uses the last defined struct in macros as the provider of ret_from function and calls the $that_struct::ret_from($your_expression) expression.

7. ret $your_expression uses the last defined struct in macros as the provider of ret function and calls the $that_struct::ret($your_expression) expression.

8. yield $your_expression uses the last defined struct in macros as the provider of combine and ret_yield functions and calls the $that_struct::combine(yer!($that_struct => next code, $that_struct::ret_yield($your_expression)) expression.

9. yield! $your_expression works the same as yield but uses $your_struct::ret_yield_from instead of ret_yield.

10. If-branching:

if ( $statement ) {
    $body
} else if ( $statement ) {
    $body
} else { $body }

// or

if ( $statement ) {
    $body
} else zero;

— Uses Rust's if-statement as an expression where $body is wrapped by yer! macro, and $your_struct::zero() for else case.

11. run $your_struct => uses $your_struct::run function by providing last returned value from the CE as an argument for that function.

12. $your_struct >> is the same as run keyword.

Examples

For now examples are available in /tests directory in repository. GH-link: https://github.com/marat0n/yerevan.rs/blob/dev/tests/common.rs

Roadmap (+ mini changelog)

The linked ones are done, they are linked to the crates.io/crates/yerevan page to the version where this roadmap-point was done. Not linked points are the plan for future updates.

• 0.1
  • yer! macro:
    • some_struct => expression to create the specified structure context where all next expressions of that CE will be executed using methods of this structure (in FP that kind of structures are called monads);
    • let! expression executed by bind<T, U>: (val: T, fn: (T) -> U) -> U method in your defined struct (monad);
    • let expression (just define what you want without breaking the CE);
    • do! expression executed the same as let! but returned value from bind method is ignored;
    • do expression (just do what you want without breaking the CE);
    • ret! expression executed by ret_from<T>: (val: T) -> T method in your defined struct (monad);
    • ret expression executed by ret<T, W<T>>: (val: T) -> W<T> method in your defined struct (monad);
    • yield expression executed by combine<T, W<T>>: (val1: W<T>, val2: T) -> W<T> where val1-parameter is used for all next code in CE and val2-parameter is used for executing ret_yield<T, U>: (val: T) -> U.
  • initial tests, examples, docs.
• 0.2
  • upgrade yer! macro:
    • add implentation for methods: Run, YieldFrom, Zero from F#'s CE-types;
    • add expressions to macro: yeild!, if ... else.
  • create default CEs for Option and Result types (Railway Execution type).
• 0.3
  • upgrade yer! macro:
    • add loop-expressions to macro: for, while, loop.
• 0.4
  • upgrade yer! macro:
    • add match-expression to macro.
• 0.5
  • add more interesting and usefull CEs.