A cocategory enumeration library based on path semantics

### 6 releases(breaking)

 0.5.0 Dec 25, 2022 Sep 14, 2020 Sep 14, 2020 Sep 13, 2020 Sep 13, 2020

#1 in #iter

MIT/Apache

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# Path-Iter

A cocategory enumeration library based on path semantics

Implementation based on paper Cocategory Enumeration.

For an introduction to Path Semantics, see this paper.

### Sub-types in Path Semantics

In normal Path Semantics, one uses normal paths in theorem proving. Normal paths is a derivation from functions with sub-types.

This library focuses on sub-types, not on the more general case of normal paths.

A sub-type in Path Semantics is written in this form:

``````x : [f] a
``````

Where `x` is some input, `f` is a function and `a` is the output of `f`.

This library is for enumerating such sub-types efficiently.

### Example: AND

The `path!` macro is used to write in the standard notation of Path Semantics. It constructs a type using `Path` that implements `IntoIterator`:

``````use path_iter::*;

fn main() {
for a in path!([And] true) {
// Prints `(true, true)`
println!("{:?}", a);
}
}
``````

It prints `(true, true)` because that is the only input value to `And` which produces `true` as output.

### Example: AND 2

You can decide the output value at runtime:

``````use path_iter::*;

fn main() {
for &b in &[false, true] {
for a in path!([And] b) {
println!("{:?}", a);
}
println!("");
}
}
``````

This prints:

``````(false, false)
(false, true)
(true, false)

(true, true)
``````

### Example: AND-NOT

You can chain path sub-types together:

``````use path_iter::*;

fn main() {
for a in path!([And] [Not] true) {
println!("{:?}", a);
}
}
``````

### Example: Partial Application

Partial application is a technique where a function reduces to another function when calling it with fewer arguments than the signature.

For example, `And(true)` reduces to `Idb`.

``````use path_iter::*;

fn main() {
for a in path!([And(true)] true) {
println!("{:?}", a);
}
}
``````

This should not be confused with function currying, which is extensionally equal to partial application, but captures the underlying function in a closure.

The `path!` macro expands to partial application automatically, but it is very limited. Outside the macro `path!` or for complex cases, one must use `PApp::papp`.

### Example: AND 3

The standard notation for composing paths is not very friendly with Rust macros. Therefore, one can use a single bracket `[]` with functions separated by commas:

``````use path_iter::*;

fn main() {
for a in path!([((And, And), (And, And)), (And, And), And] true) {
println!("{:?}", a);
}
}
``````