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.
You can use:
* to return an expression value: pure return_expresion
* to end with a monadic expr. monadic_expression
* to use the monad result: v <- monadic_expression
* to ignore the monad result: _ <- monadic_expression
* to combine monad results: let z = expression
* to filter results: guard boolean_expression
Note: let, within the macro, introduces an expression, unlike Haskell.
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 refs (&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. There is a problem capturing string variables because String does not implement Copy, but it works using variables in the same closure, just in the line that follows.
```rust // examples/console_io.rs
use monadic::{mdo, monad::{Bind, Monad}, mio::{readline, printstr, stdout_flush}};
fn main() { let res =mdo!{
x <- pure 1;
let y = x + 1;
_ <- print_str("enter i32>");
_ <- stdout_flush();
li1 <- read_line();
z <- li1.trim().parse::<i32>() ;
pure (y, z)
}.collect::<Vec<_>>();
println!("result: {:?}", res);
} ```
Same functionality as mdo using IntoIterator and Iterator methods directly, avoiding intermixed Bind and Monad traits definitions.
Here is example1 using it:
```rust use monadic::monadic; use num::Integer;
fn main() { let xs = monadic!{
x <- 1..7;
y <- 1..x;
guard (&y).is_odd() ;
let z = match x.is_even() {
true => &y + 1,
_ => &y - 1,
};
w <- pure 5; // (<-) rhs pure
pure (x, z, w)
}.collect::<Vec<_>>();
println!("result: {:?}", xs);
}
```
A Reader monad adaptation macro example
```rust // examples/reader1 use monadic::{rdrdo, reader::{Reader, ask, localdo}}; use partialapplication::partial; use std::collections::HashMap;
type Env = HashMap
fn immutableadd( kslice: &str, v: i32, dict: Env) -> Env { let mut dict1 = dict.clone(); dict1.insert( String::from(k_slice), v); dict1 }
fn myinitialenv() -> Env { immutable_add( "a", 1, HashMap::new()) }
fn main() {
let myenvtoenv = partial!(immutableadd => "b", 2, _);
let bloc: Reader<'_, Env, _> = rdrdo!{
env1 <- ask();
pair <- local_do( my_env_to_env, rdrdo!{
x <- pure 9;
y <- ask();
pure (x, y)
}) ;
pure (env1.clone(), pair)
};
let res = bloc.initialenv( myinitial_env() );
println!("result: {:?}", res);
}
```
Execution:
```bash $ cargo run --example reader1
result: ({"a": 1}, (9, {"a": 1, "b": 2})) ```
A Writer monad adaptation macro example with String as logger, from examples/writer1.rs
``rust
//! you may set the logger type
//! by beginning with atell...function within the macrowrdo
//! or by declaring it as the result type
//! where String is the default if omitted
//! as inlet res : Writer< (i32, i32), String > = wrdo!{...}`
use monadic::{wrdo, writer::{Writer, tell, tellstr}}; use monadic::util::concatstringstr; use partialapplication::partial;
fn main() {
let res = wrdo!{
_ <- tell_str( "log1") ; // either `tell(String::from("log1"))`
x <- pure 1 ;
let z = x+1;
pure (x, z)
}.censor( partial!( concat_string_str => _, "log2")
).listen() ;
println!("result: {:?}", res.unwrap());
} ``` Exec:
```bash $ cargo run --example writer1
result: ((1, 2), "log1log2")
``` Example 2 with Vec as logger from examples/writer2.rs
```rust use monadic::{wrdo, writer::{Writer, tell}}; use monadic::util::concatvecarray; use partial_application::partial;
fn main() {
let res = wrdo!{
_ <- tell( vec![1,2,3]) ;
x <- pure 1 ;
let z = x+1;
pure (x, z)
}.censor( partial!( concat_vec_array => _, &[4,5,6])
).listen() ;
println!("result: {:?}", res.unwrap());
} ```
```bash $ cargo run --example writer2
result: ((1, 2), [1, 2, 3, 4, 5, 6])
```
A State monad adaptation macro example from examples/state1.rs
```rust use monadic::{stdo, state::{State, get, put}};
fn main() { let res = stdo!{
x <- pure 9;
y <- get();
_ <- put( 1);
z <- get();
pure (x, y, z)
}.initial_state( 0);
println!("result: {:?}", res);
}
```
```bash $ cargo run --example state1
result: ((9, 0, 1), 1)
```
Changes:
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.