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Macro to use rust IntoIterator as a monad with haskell like syntax

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# rust-monadic

* [A monad bloc macro based on Bind and Monad as supertraits of IntoIterator (iterables)](#mdo)
* [A Reader monad bloc macro](#rdrdo)
* [A Writer monad bloc macro](#wrdo)
* [A State monad bloc macro](#stdo)
* [A ReaderT monad transformer bloc macro](#rdrt_mdo)
* [A WriterT monad transformer bloc macro](#wrt_mdo)
* [A StateT monad transformer bloc macro](#stt_mdo)

<a name="mdo" id="mdo"></a>
### The macro mdo!

A macro to write Haskell style monadic code

for [**IntoIterator**](https://doc.rust-lang.org/std/iter/trait.IntoIterator.html) (iterables) as monads

Each step monad expression is [flat_mapped](https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.flat_map) with the rest into a lazy *FlatMap* expression which implements *IntoIterator* with lambdas as [*move* closures](https://doc.rust-lang.org/1.30.0/book/first-edition/closures.html#move-closures) capturing the environment and argument as `Fn(A) -> U: IntoIterator`. The lambda body will be recursively parsed as monadic, and its type should also be an instance of *IntoIterator*.

Aside from the types that implement *IntoIterator*, all iterators also do it as [documented](https://doc.rust-lang.org/stable/core/iter/#for-loops-and-intoiterator).

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

Here is a table of patterns of the macro `mdo` where a **monadic_expression** is one of a type which must be instance of IntoIterator:

<table>
<tr><td>* to return an expression value:</td> <th>`"pure" return_expresion`</th></tr>
<tr><td>* to combine monad results:</td> <th>`"let" identifier "=" expression ";"`</th></tr>
<tr><td>* to filter results:</td> <th>`"guard" boolean_expression ";"`</th></tr>
<tr><td>* to ignore the monad result:</td> <th>`"_" "<-" monadic_expression ";"`</th></tr>
<tr><td>* to deref the item ref. of a shared iterable:</td> <th>`"&" identifier "<-" &iterable ";"`</th></tr>
<tr><td>* to lift a value and bind it:</td> <th>`identifier "<-" "pure" expression ";"`</th></tr>
<tr><td>* to bind the monad result:</td> <th>`identifier "<-" monadic_expression ";"`</th></tr>
<tr><td>* to end with a monadic expr.:</td> <th>`monadic_expression`</th></tr>
</table>

Note: *let*, within the macro, introduces only one binding.

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

```rust
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:

```bash
$ 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)

```rust
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:

```bash
$ 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

```rust
// 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);              
}
```
```bash
$ cargo run --example console_io

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

```

<a name="rdrdo" id="rdrdo"></a>
### The Reader monad macro rdrdo!

A [Reader monad](https://wiki.haskell.org/All_About_Monads#The_Reader_monad) adaptation macro example

```rust
//! 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:

```bash
$ cargo run --example reader1

result: ({"a": 1}, 9, {"b": 2, "a": 1})
```
<a name="rdrt_mdo" id="rdrt_mdo"></a>
### The ReaderT monad transformer macro rdrt_mdo!

This monad transformer is strict and works only for inner monads that implement `Monad + FromIterator + Clone`, only with Vec, LinkedList and VecDeque. You can mix instructions with either monad using `lift` since binding occurs by iterating `IntoIterator`'s through `into_iter().flat_map().collect()`.

This macro requires more type annotations, as the inner monad and the lambda argument may be undetermined.

To reduce type annotations, they are inserted with `ask()` by the macro, using `Env` as the environement type alias which must be defined.

`pure return_expression` is translated by the macro to `lift Vec::pure( return_expression)`

Example:
```rust
// examples/reader_trans1

#[allow(unused_imports)]
use monadic::{rdrt_mdo, monad::{Monad}, 
              reader_trans::{ReaderT, ask, local}};
use num::Integer;
use partial_application::partial;
use std::collections::HashMap;

/// mandatory type alias Env as it is used in the macro
/// to save you type annotations
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, _);

// example with Vec as the nested monad
  
  let bloc = rdrt_mdo!{   // possible type restriction as ReaderT<'_, Env, Vec<_>>
  
       env1 <- ask(); // the macro adds the type annotation as ReaderT<'_, Env, Vec<Env>>
       
       // run a subblock with a modified env.
       pair <- local( modify_env, rdrt_mdo!{
       
               // x <- lift (5..9).collect::<Vec<_>>();
               x <- lift_iter 5..9;
               
               guard x.is_odd();
               
               let z = x + 1;
               y <- ask();
               
               pure (z, y)   // equivalent to lift Vec::pure((z, y))
             }) ;
             
       pure (env1.clone(), pair.0, pair.1)      
    };

// applying the initial_env() to the transformer (env -> m a) 
  // returns the nested monad structure
  
  let res = bloc.initial_env( my_initial_env() );

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

```bash
$ cargo run --example reader_trans1

result: [({"a": 1}, 6, {"a": 1, "b": 2}), ({"a": 1}, 8, {"a": 1, "b": 2})]
```

<a name="wrdo" id="wrdo"></a>
### The Writer monad macro wrdo!

A [Writer monad](https://wiki.haskell.org/All_About_Monads#The_Writer_monad) adaptation macro example with String as logger, from examples/writer1.rs

```rust
//! 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;

type Log = String;

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

```bash
$ cargo run --example writer1

result: ((2, "sub"), "log1sublog2")

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

```rust
//! 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
#[allow(unused_imports)]
use monadic::{wrdo, writer::{Writer, tell, censor, listen}};
use monadic::util::concat_vec_array;
use partial_application::partial;

type Log = Vec<i32>;

fn main() {

```bash
$ cargo run --example writer2

result: ((2, [0]), [1, 2, 3, 0, 4, 5, 6])

```
<a name="wrt_mdo" id="wrt_mdo"></a>
### The WriterT monad transformer macro wrt_mdo!

Only for Vec, LinkedList or VecDeque as inner monads. You can lift expressions of either monad, since binding is done through iterate and collect.

Added macro keywords tell_str, tell_array, tell_vec, tell_string that save to type annotate the monad as the macro output do it for you. They use the `Log` type alias in macro output type annotations.

Now the keyword `pure return_expresion` lifts the return_expresion through a Vec::pure(return_expression)

Example:

```rust
//! examples/writer_trans1.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 : WriterT< _, Log > = wrdo!{...}`
//!
//! `censor(), listen() and listens()` can be used as functions or as methods of a Writer bloc

#[allow(unused_imports)]
use monadic::{wrt_mdo, monad::Monad, writer_trans::{WriterT, tell, tell_str, tell_array, censor, listen}};
use monadic::util::concat_string_str;
use partial_application::partial;
use num::Integer;

#[allow(dead_code)]
type Log = String; // used in some macro constructs

fn main() {
    
    let modify_log = partial!( concat_string_str => _, "log2");
    
    let bloc = wrt_mdo!{  // : WriterT< Vec<_>>  // type param. `log` defaults to String
    
        _ <- tell_str "log1" ;
        
        // x <- lift (5..9).collect::<Vec<_>>() ;
        x <- lift_iter 5..9 ;
        
        guard x.is_odd() ;
        let z = x + 1;
        
        // run a subbloc and modify its log afterwards
        pair <- censor( modify_log,
                        wrt_mdo!{
                            _ <- tell_str "sub";
                            pure 2
                        }.listen()
                      );
                    
        pure (z, pair.0, pair.1)
        }.listen() ;
        
    // unwrap() returns the nested monad structure       
    let res = bloc.unwrap(); 
    
    println!("result: {:?}", res); 
}
```
Execution:
```bash
$ cargo run --example writer_trans1

result: [((6, 2, "sub"), "log1sublog2"), ((8, 2, "sub"), "log1sublog2")]
```
Example with Vec as logger:

```rust
//! examples/writer_trans2.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 : WriterT< _, Vec<_>> = wrdo!{...}`
//!
//! `censor(), listen() and listens()` can be used as functions or as methods of a Writer bloc

#[allow(unused_imports)]
use monadic::{wrt_mdo, monad::Monad, writer_trans::{WriterT, tell, tell_str, tell_array, censor, listen}};
use monadic::util::concat_vec_array;
use partial_application::partial;
use num::Integer;

/// mandatory type alias Log only if it is not the default
/// as it is used in the macro
/// to save you type annotations
type Log = Vec<i32>;

fn main() {
    
    let modify_log = partial!( concat_vec_array => _, &[4,5,6]);
    
    let bloc = wrt_mdo!{  // : WriterT< Vec<_>, Log>
    
        _ <- tell_array &[1,2,3] ;
        
        x <- lift (5..9).collect::<Vec<_>>() ;
        
        guard x.is_odd() ;
        let z = x + 1;
        
        // run a subbloc and modify its log afterwards
        pair <- censor( modify_log,
                        wrt_mdo!{
                            _ <- tell_array &[0];
                            pure 2
                        }.listen()
                      );
                    
        pure (z, pair.0, pair.1)            
        }.listen() ;
        
    // unwrap() returns the nested monad structure       
    let res = bloc.unwrap(); 
    
    println!("result: {:?}", res); 
}
```

```bash
$ cargo run --example writer_trans2

result: [((6, 2, [0]), [1, 2, 3, 0, 4, 5, 6]), ((8, 2, [0]), [1, 2, 3, 0, 4, 5, 6])]
```

<a name="stdo" id="stdo"></a>
### The State monad macro stdo!

A [State monad](https://wiki.haskell.org/All_About_Monads#The_State_monad) adaptation macro example from examples/state1.rs

```rust
//! 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}};

type St = i32;

fn main() {

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

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

```bash
$ cargo run --example state1

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

```

<a name="stt_mdo" id="stt_mdo"></a>
### The StateT monad transformer macro stt_mdo!

```rust
use monadic::{stt_mdo, state_trans::{StateT, get, put}};
use num::Integer;

// mandatory type alias as it is used within the macro for type annotations
type St = i32;

fn main() {
  let bloc = stt_mdo!{ // : StateT<'_, St, Vec<_>, _>    // StateT<'a, St, Monad, A>
  
       // x <- lift (5..9).collect::<Vec<_>>() ;
       x <- lift_iter 5..9 ;                        // lift_iter iterator
       guard x.is_odd();
       
       y <- get() ; 
       
       _ <- put( 1) ; 
       z <- get() ;
       
       let v = x +1 ; 
       
       pure (v, y, z)
    };
  
  // returns the monad within the transformer boxed function (s -> m (a,s))
  let res = bloc.initial_state( 0);

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

```
Exec:

```bash
$ cargo run --example state_trans1

result: [((6, 0, 1), 1), ((8, 0, 1), 1)]

```

<a name="tests" id="tests"></a>
### Some tests

```bash
$ cargo test
running 1 test
test monad::tests::prop_monad_comprehension_vs_iteration ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
```

Changes:

v. 0.5.4: readme typos

v. 0.5.3: feature based conditional compilation, features ["reader", "reader_trans", "writer", "writer_trans", "state", "state_trans"]. Default is all compiled. The module Monad with its macro "mdo" is unconditionally compiled.

[dependencies.monadic]
    version = "~0.5"
    default-features = false
    features = ["reader_trans"]    # pick the modules of your interest.

v. 0.5.2: added lift_iter to ReaderT and WriterT, plus the deref bind pattern (&v) in macros

v. 0.5.1: StateT transformer macro

v. 0.5.0: updates on ReaderT and WriterT transformer macros to reduce the number of type annotations

* the macro production "pure" $expr translates to lift(Vec::pure($exp))
* the ReaderT macro production "$v <- ask()" generates a type annotation in its output using the type alias Env.
* the WriterT macro productions "_ <- tell_.. $expr" generates a type annotation in its output using the type allias Log.

v. 0.4.10: added let bindings to the ReaderT and WriterT transformers macro

v. 0.4.9: readme correction.

v. 0.4.8: added the WriterT transformer for (Vec, LinkedList, VecDeque) as nested monads

v. 0.4.7: added the ReaderT transformer for (Vec, LinkedList, VecDeque) as nested monads

v. 0.4.5 and 0.4.6: doc cleaning

v. 0.4.4: doc cleaning of old intoiter macro refs. Suppressed experimental MonadPlus, which is not ready.

v. 0.4.3: readme typos.

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`.

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