# CHR-based Constraint Systems
This sub-module implements a collection of CHR-based constraint
systems, and a generic framework for abstracting Rust ASTs using
constraints.
## Overview
The end user interface to this module is captured through two main elements:
- *LocalConstraint* - a trait that your custom constraint should implement
```rust
pub trait LocalConstraint : Any + Display + Clone {
const CHR_RULES: &'static str;
fn parse(s: &str) -> nom::IResult<&str, Self>;
fn collect<'a>(fun: &Annotated<'a, syn::ItemFn>) -> Vec<Self>;
}
```
- *ConstraintManager* - used to represent a collection of constraint systems, encapsulating the process of running each individual constraint
```rust
let mut constraint_manager : ConstraintManager = Default::default();
constraint_manager.add_constraint::<ArrayConstraint>();
constraint_manager.add_constraint::<MutabilityConstraint>();
constraint_manager.analyze(fun);
let arr_constraints : Vec<ArrayConstraint> = constraint_manager.get_constraints::<ArrayConstraint>();
let mut_constraints : Vec<MutabilityConstraint> = constraint_manager.get_constraints::<MutabilityConstraint>();
```
## Project structure
```
./src/
├── constraint.rs
├── annotation.rs
├── chr.rs
├── common.rs
└── lib.rs
1 directory, 10 files
```
The sub-component is composed of the following modules:
- constraint.rs -- defines the core generic constraint collection framework
- annotation.rs -- defines a visitor to label the syn ASTs
- chr.rs -- defines a wrapper around SwiPL to run CHR rules over a list of constraints
- common.rs -- defines a few instantiations of the constraint system for mutability and arrays
Parser should be moved to utils, and constraint specific parts moved into common
Solver should be moved to rewrite