rust-monadic

• A monad bloc macro based on Bind and Monad as supertraits of IntoIterator (iterables)
• A Reader monad bloc macro
• A Writer monad bloc macro
• A State monad bloc macro

The macro mdo!

A macro to write Haskell style monadic code

for IntoIterator (iterables) as monads

Each step monad expression is flat_mapped with the rest into a lazy FlatMap expression which implements IntoIterator with lambdas as move closures capturing the environment and argument. The lambda body will be recursively parsed as monadic, and its type should also be an instance of IntoIterator.

The traits Bind and Monad are defined in module monad as supertraits of IntoIterator.

Here is a table where a monadic_expression is one of a type instance of IntoIterator:

Note: let, within the macro, introduces an expression, not a Haskell block.

Example1: monadic comprehensions à la Haskell (file: examples/comprehension.rs)

use monadic::{mdo, monad::{Bind, Monad}};
use num::Integer;

fn main() {
    let xs = mdo!{ 
    
        x <- 1..7;
        y <- 1..x;
        guard (&y).is_odd() ;
        let z = match x.is_even() { 
                    true => &y + 1,
                    _ => &y - 1,
                };
        pure (x, z)
        
    }.collect::<Vec<_>>();
    
    println!("result: {:?}", xs); 
}

Execution:

$ cargo run --example comprehension

result: [(2, 2), (3, 0), (4, 2), (4, 4), (5, 0), (5, 2), (6, 2), (6, 4), (6, 6)]

Example2: variation with references to container and lambda argument position (file: examples/comprehension2.rs)

use monadic::{mdo, monad::{Bind, Monad}};
use num::Integer;

fn main() {
    let xs = mdo!{ 
    
        &x <- &vec![1,2,3,4];   // with item ref pattern (&x) in the lambda argument position
        guard x.is_odd() ;
        let z = x + 1 ;
        pure (x, z)
        
    }.collect::<Vec<_>>();
    
    println!("result: {:?}", xs); 
}

Execution:

$ cargo run --example comprehension2

result: [(1, 2), (3, 4)]

Example: console io. If you want to return String variables, you may do it through cloning

// example console io

use monadic::{mdo, monad::{Bind, Monad}, 
                   mio::{read_line, print_str, stdout_flush}};

fn main() {
    let res =mdo!{
    
                x <- pure 1;
                let y = x + 1;
                
                _ <- print_str("enter integer i32>");
                _ <- stdout_flush();
                
                li1 <- read_line();
                z <- li1.trim().parse::<i32>() ;
                
                pure (y, z, li1.clone())
                
              }.collect::<Vec<_>>();

    println!("result: {:?}", res);              
}

$ cargo run --example console_io

enter integer i32>10
result: [(2, 10, "10")]

The Reader monad macro rdrdo!

A Reader monad adaptation macro example

//! examples/reader1
//!
//! You must specify in a type restriction the type of the environment of the Reader bloc
//!
//! `local` can be used as a function or as a method

use monadic::{rdrdo, reader::{Reader, ask, local}};
use partial_application::partial;
use std::collections::HashMap;

type Env = HashMap<String, i32>;

fn immutable_insert( k_slice: &str, v: i32, dict: Env) -> Env {
   let mut dict1 = dict.clone();
   dict1.insert( String::from(k_slice), v);
   dict1
}

fn my_initial_env() -> Env {
   immutable_insert( "a", 1, HashMap::new())
}   


fn main() {

  let modify_env = partial!(immutable_insert => "b", 2, _);
  
  let bloc1: Reader<'_, Env, _>  = rdrdo!{
  
       env1 <- ask();
       
       // run a subbloc with a modified environment
       pair <- local( modify_env, rdrdo!{ 
       
               x <- pure 9;
               y <- ask();
               pure (x, y)
             }) ;
             
       pure (env1.clone(), pair.0, pair.1)      
    };


  let res = bloc1.initial_env( my_initial_env() );

  println!("result: {:?}", res);  
}

Execution:

$ cargo run --example reader1

result: ({"a": 1}, 9, {"b": 2, "a": 1})

The Writer monad macro wrdo!

A Writer monad adaptation macro example with String as logger, from examples/writer1.rs

//! examples/writer1.rs
//!
//! you may set the logger type 
//! by beginning with a `tell...` function within the macro `wrdo` 
//! or by declaring it as the result type 
//! where String is the default if omitted
//! as in `let res : Writer< _, String > = wrdo!{...}`
//!
//! `censor(), listen() and listens()` can be used as functions or as methods of a Writer bloc

#[allow(unused_imports)]
use monadic::{wrdo, writer::{Writer, tell, tell_str, censor, listen}};
use monadic::util::concat_string_str;
use partial_application::partial;

fn main() {
    
    let modify_log = partial!( concat_string_str => _, "log2");
    
    let res : Writer< _, String> = wrdo!{ 
    
        _ <- tell_str( "log1") ;
        
        // run a subbloc and modify the log afterwards
        censor( modify_log,
                   wrdo!{
                        listen( Writer::pure( 2))
                    })
        }.listen() ;
    
    println!("result: {:?}", res.unwrap()); 
}

Exec:

$ cargo run --example writer1

result: ((2, ""), "log1log2")

Example 2 with Vec as logger from examples/writer2.rs

//! examples/writer2.rs
//! 
//! you may set the logger type 
//! by beginning with a `tell...` function within the macro `wrdo` 
//! or by declaring it as the result type 
//! where String is the default if omitted
//! as in `let res : Writer< _, Vec<_> > = wrdo!{...}`
//!
//! `censor(), listen() and listens()` can be used as functions or as methods of a Writer bloc

use monadic::{wrdo, writer::{Writer, tell, censor, listen}};
use monadic::util::concat_vec_array;
use partial_application::partial;


fn main() {

    let modify_log = partial!( concat_vec_array => _, &[4,5,6]);
    
    let res : Writer< _, Vec<_>> = wrdo!{ 
    
        _ <- tell( vec![1,2,3]) ;
        
        // run a subbloc and modify the log afterwards
        censor( modify_log,
                   wrdo!{
                        listen( Writer::pure( 2))
                    })
        }.listen() ;
    
    println!("result: {:?}", res.unwrap()); 
}

$ cargo run --example writer2

result: ((2, []), [1, 2, 3, 4, 5, 6])

The State monad macro stdo!

A State monad adaptation macro example from examples/state1.rs

//! examples/state1.rs
//!
//! You may specify in a type restriction the type of the State bloc
//! or apply it directly to an initial_state without the type restriction

use monadic::{stdo, state::{State, get, put}};

fn main() {
  let bloc: State<'_, i32, _> = stdo!{
  
       x <- pure 9;
       y <- get();
       _ <- put( 1);
       z <- get(); 
       pure (x, y, z) 
       
    };
    
  let res = bloc.initial_state(0);  

  println!("result: {:?}", res);  
}

$ cargo run --example state1

result: ((9, 0, 1), 1)

Some tests:

$ cargo test
running 10 tests
test monad::tests::prop_monad_comprehension_vs_iteration ... ok
test monad::tests::prop_option_mplus_associative ... ok
test monad::tests::prop_option_mplus_left_catch ... ok
test monad::tests::prop_option_mplus_left_identity ... ok
test monad::tests::prop_option_mplus_left_zero ... ok
test monad::tests::prop_option_mplus_right_identity ... ok
test monad::tests::prop_vec_mplus_associative ... ok
test monad::tests::prop_vec_mplus_left_identity ... ok
test monad::tests::prop_vec_mplus_left_distribution ... ok
test monad::tests::prop_vec_mplus_right_identity ... ok

test result: ok. 10 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

Changes: v. 0.4.2: added MonadPlus with quickcheck tests

v. 0.4.1: console_io example showing String return through cloning

v. 0.4.0:

• renamed writer function censor_do as censor
• added writer function listen() and listens()
• renamed local_do() as local()
• removed intoiter module as it duplicates functionality without added applicability, use module monad's mdo macro instead

v. 0.3.14: added writer function censor_do

v. 0.3.13: added reader function local_do

v. 0.3.12: example reader1 simplification.

v. 0.3.11: suppressed the form "&v <- ..." from Writer and State monads.

v. 0.3.10: Added the Reader macro. It runs good over clonable environments e.g. HashMap. The State macro has been updated, using a non static lifetime for the boxed closure

v. 0.3.9: Added (<-) rhs pure.