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.
Aside from the types that implement IntoIterator, all iterators also do it as documented.
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 which must be instance of IntoIterator:
| * 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 lift a value and bind it: | `v <- pure 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 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::{readline, printstr, 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 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
fn immutableinsert( kslice: &str, v: i32, dict: Env) -> Env { let mut dict1 = dict.clone(); dict1.insert( String::from(k_slice), v); dict1 }
fn myinitialenv() -> Env { immutable_insert( "a", 1, HashMap::new()) }
fn main() {
let modifyenv = partial!(immutableinsert => "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.initialenv( myinitial_env() );
println!("result: {:?}", res);
}
```
Execution:
```bash $ cargo run --example reader1
result: ({"a": 1}, 9, {"b": 2, "a": 1}) ```
A 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 atell...function within the macrowrdo
//! or by declaring it as the result type
//! where String is the default if omitted
//! as inlet res : Writer< _, String > = wrdo!{...}
//!
//!censor(), listen() and listens()` can be used as functions or as methods of a Writer bloc
use monadic::{wrdo, writer::{Writer, tell, tellstr, censor, listen}}; use monadic::util::concatstringstr; use partialapplication::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:
```bash $ cargo run --example writer1
result: ((2, ""), "log1log2")
``` Example 2 with Vec as logger from examples/writer2.rs
``rust
//! examples/writer2.rs
//!
//! 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< _, 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::concatvecarray; 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());
} ```
```bash $ cargo run --example writer2
result: ((2, []), [1, 2, 3, 4, 5, 6])
```
A 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}};
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);
}
```
```bash $ cargo run --example state1
result: ((9, 0, 1), 1)
``` Some tests:
```bash $ cargo test running 10 tests test monad::tests::propmonadcomprehensionvsiteration ... ok test monad::tests::propoptionmplusassociative ... ok test monad::tests::propoptionmplusleftcatch ... ok test monad::tests::propoptionmplusleftidentity ... ok test monad::tests::propoptionmplusleftzero ... ok test monad::tests::propoptionmplusrightidentity ... ok test monad::tests::propvecmplusassociative ... ok test monad::tests::propvecmplusleftidentity ... ok test monad::tests::propvecmplusleftdistribution ... ok test monad::tests::propvecmplusrightidentity ... ok
test result: ok. 10 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out ```
Changes:
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.