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https://github.com/lloydmeta/frunk

Funktional generic type-level programming in Rust: HList, Coproduct, Generic, LabelledGeneric, Validated, Monoid and friends.
https://github.com/lloydmeta/frunk

coproduct datatype-generic-programming fp generic generic-programming hlist labelled-generic lenses rust type-level type-level-programming validated

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Funktional generic type-level programming in Rust: HList, Coproduct, Generic, LabelledGeneric, Validated, Monoid and friends.

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# Frunk [![Crates.io](https://img.shields.io/crates/v/frunk.svg)](https://crates.io/crates/frunk) [![Continuous integration](https://github.com/lloydmeta/frunk/actions/workflows/ci.yml/badge.svg?branch=master)](https://github.com/lloydmeta/frunk/actions/workflows/ci.yml?query=branch%3Amaster) [![Gitter](https://badges.gitter.im/lloydmeta/frunk.svg)](https://gitter.im/lloydmeta/frunk?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge) [![Frunk](https://docs.rs/frunk/badge.svg)](https://docs.rs/frunk)



> **frunk** *frəNGk*

>  * Functional programming toolbelt in Rust.

>  * Might seem funky at first, but you'll like it.

>  * Comes from: funktional (German) + Rust → Frunk



The general idea is to make things easier by providing FP tools in Rust to allow for stuff like this:



```rust

use frunk::monoid::combine_all;



let v = vec![Some(1), Some(3)];

assert_eq!(combine_all(&v), Some(4));



// Slightly more magical

let t1 =       (1, 2.5f32,                String::from("hi"),  Some(3));

let t2 =       (1, 2.5f32,            String::from(" world"),     None);

let t3 =       (1, 2.5f32,         String::from(", goodbye"), Some(10));

let tuples = vec![t1, t2, t3];



let expected = (3, 7.5f32, String::from("hi world, goodbye"), Some(13));

assert_eq!(combine_all(&tuples), expected);

```



For a deep dive, RustDocs are available for:

* Code on [Master](https://beachape.com/frunk)

* Latest [published release](https://docs.rs/frunk)



## Table of Contents



1. [HList](#hlist)

2. [Generic](#generic)

    * 2.1 [LabelledGeneric](#labelledgeneric)

       * 2.1.2 [Path (Lenses)](#path)

3. [Coproduct](#coproduct)

4. [Validated](#validated)

5. [Semigroup](#semigroup)

6. [Monoid](#monoid)

7. [Features](#features)

8. [Benchmarks](#benchmarks)

9. [Todo](#todo)

10. [Contributing](#contributing)

11. [Inspirations](#inspirations)

12. [Maintainers](#maintainers)



## Examples



### HList



Statically typed heterogeneous lists.



First, let's enable `hlist`:

```rust

use frunk::{HNil, HCons, hlist};

```



Some basics:

```rust

let h = hlist![1];

// Type annotations for HList are optional. Here we let the compiler infer it for us

// h has a static type of: HCons



// HLists have a head and tail

assert_eq!(hlist![1].head, 1);

assert_eq!(hlist![1].tail, HNil);



// You can convert a tuple to an HList and vice-versa

let h2 = hlist![ 42f32, true, "hello" ];

let t: (f32, bool, &str) = h2.into();

assert_eq!(t, (42f32, true, "hello"));



let t3 = (999, false, "world");

let h3: HList![ isize, bool, &str ] = t3.into();

assert_eq!(h3, hlist![ 999, false, "world" ]);

```



HLists have a `hlist_pat!` macro for pattern matching;

```rust

let h: HList!(&str, &str, i32, bool) = hlist!["Joe", "Blow", 30, true];

// We use the HList! type macro to make it easier to write

// a type signature for HLists, which is a series of nested HCons

// h has an expanded static type of: HCons<&str, HCons<&str, HCons>>>



let hlist_pat!(f_name, l_name, age, is_admin) = h;

assert_eq!(f_name, "Joe");

assert_eq!(l_name, "Blow");

assert_eq!(age, 30);

assert_eq!(is_admin, true);



// You can also use into_tuple2() to turn the hlist into a nested pair

```



To traverse or build lists, you can also prepend/or pop elements at the front:

```rust

let list = hlist![true, "hello", Some(41)];

// h has a static type of: HCons, HNil>>>

let (head1, tail1) = list.pop();

assert_eq!(head1, true);

assert_eq!(tail1, hlist!["hello", Some(41)]);

let list1 = tail1.prepend(head1);

assert_eq!(list, list1);



// or using macro sugar:

let hlist_pat![head2, ...tail2] = list; // equivalent to pop

let list2 = hlist![head2, ...tail2];    // equivalent to prepend

assert_eq!(list, list2);

```



You can reverse, map, and fold over them too:



```rust

// Reverse

let h1 = hlist![true, "hi"];

assert_eq!(h1.into_reverse(), hlist!["hi", true]);



// Fold (foldl and foldr exist)

let h2 = hlist![1, false, 42f32];

let folded = h2.foldr(

    hlist![

        |acc, i| i + acc,

        |acc, _| if acc > 42f32 { 9000 } else { 0 },

        |acc, f| f + acc

    ],

    1f32

);

assert_eq!(folded, 9001)



// Map

let h3 = hlist![9000, "joe", 41f32];

let mapped = h3.map(hlist![

    |n| n + 1,

    |s| s,

    |f| f + 1f32]);

assert_eq!(mapped, hlist![9001, "joe", 42f32]);

```



You can pluck a type out of an `HList` using `pluck()`, which also gives you back the remainder after plucking that type

out. This method is checked at compile-time to make sure that the type you ask for *can* be extracted.



```rust

let h = hlist![1, "hello", true, 42f32];

let (t, remainder): (bool, _) = h.pluck();

assert!(t);

assert_eq!(remainder, hlist![1, "hello", 42f32])

```



Similarly, you can re-shape, or sculpt, an `Hlist`, there is a `sculpt()` method, which allows you to re-organise and/or

cull the elements by type. Like `pluck()`, `sculpt()` gives you back your target with the remainder data in a pair. This

method is also checked at compile time to make sure that it won't fail at runtime (the types in your requested target shape

must be a subset of the types in the original `HList`.



```rust

let h = hlist![9000, "joe", 41f32, true];

let (reshaped, remainder): (HList![f32, i32, &str], _) = h.sculpt();

assert_eq!(reshaped, hlist![41f32, 9000, "joe"]);

assert_eq!(remainder, hlist![true]);

```



### Generic



`Generic` is a way of representing a type in ... a generic way. By coding around `Generic`, you can to write functions

that abstract over types and arity, but still have the ability to recover your original type afterwards. This can be a fairly powerful thing.



#### Setup



In order to derive the trait `Generic` (or `LabelledGeneric`) you will have to add `frunk_core` dependency



```toml

[dependencies]

frunk_core = { version = "$version" }

```



Frunk comes out of the box with a nice custom `Generic` derivation so that boilerplate is kept to a minimum.



Here are some examples:



#### HList ⇄ Struct



```rust

#[derive(Generic, Debug, PartialEq)]

struct Person<'a> {

    first_name: &'a str,

    last_name: &'a str,

    age: usize,

}



let h = hlist!("Joe", "Blow", 30);

let p: Person = frunk::from_generic(h);

assert_eq!(p,

           Person {

               first_name: "Joe",

               last_name: "Blow",

               age: 30,

           });

```



This also works the other way too; just pass a struct to `into_generic` and get its generic representation.



#### Converting between Structs



Sometimes you may have 2 different types that are structurally the same (e.g. different domains but the same data). Use cases include:



 * You have a models for deserialising from an external API and equivalents for your app logic

 * You want to represent different stages of the same data using types (see [this question on StackOverflow](http://stackoverflow.com/questions/31949455/transform-one-case-class-into-another-when-the-argument-list-is-the-same))



Generic comes with a handy `convert_from` method that helps make this painless:



```rust

// Assume we have all the imports needed

#[derive(Generic)]

struct ApiPerson<'a> {

    FirstName: &'a str,

    LastName: &'a str,

    Age: usize,

}



#[derive(Generic)]

struct DomainPerson<'a> {

    first_name: &'a str,

    last_name: &'a str,

    age: usize,

}



let a_person = ApiPerson {

                   FirstName: "Joe",

                   LastName: "Blow",

                   Age: 30,

};

let d_person: DomainPerson = frunk::convert_from(a_person); // done

```



#### LabelledGeneric



In addition to `Generic`, there is also `LabelledGeneric`, which, as the name implies, relies on a generic representation

that is _labelled_. This means that if two structs derive `LabelledGeneric`, you can convert between them only if their

field names match!



Here's an example:



```rust

// Suppose that again, we have different User types representing the same data

// in different stages in our application logic.



#[derive(LabelledGeneric)]

struct NewUser<'a> {

    first_name: &'a str,

    last_name: &'a str,

    age: usize,

}



#[derive(LabelledGeneric)]

struct SavedUser<'a> {

    first_name: &'a str,

    last_name: &'a str,

    age: usize,

}



let n_user = NewUser {

    first_name: "Joe",

    last_name: "Blow",

    age: 30

};



// Convert from a NewUser to a Saved using LabelledGeneric

//

// This will fail if the fields of the types converted to and from do not

// have the same names or do not line up properly :)

//

// Also note that we're using a helper method to avoid having to use universal

// function call syntax

let s_user: SavedUser = frunk::labelled_convert_from(n_user);



assert_eq!(s_user.first_name, "Joe");

assert_eq!(s_user.last_name, "Blow");

assert_eq!(s_user.age, 30);



// Uh-oh ! last_name and first_name have been flipped!

#[derive(LabelledGeneric)]

struct DeletedUser<'a> {

    last_name: &'a str,

    first_name: &'a str,

    age: usize,

}



//  This would fail at compile time :)

let d_user: DeletedUser = frunk::labelled_convert_from(s_user);



// This will, however, work, because we make use of the Sculptor type-class

// to type-safely reshape the representations to align/match each other.

let d_user: DeletedUser = frunk::transform_from(s_user);

```



##### Transmogrifying



Sometimes you need might have one data type that is "similar in shape" to another data type, but it

is similar _recursively_ (e.g. it has fields that are structs that have fields that are a superset of

the fields in the target type, so they are transformable recursively).  `.transform_from` can't help you

there because it doesn't deal with recursion, but the `Transmogrifier` can help if both are `LabelledGeneric`

by `transmogrify()`ing from one to the other.



What is "transmogrifying"? In this context, it means to recursively tranform some data of type A into data

of type B, in a typesafe way, as long as A and B are "similarly-shaped".  In other words, as long as B's

fields and their subfields are subsets of A's fields and their respective subfields, then A can be turned

into B.



As usual, the goal with Frunk is to do this:

* Using stable (so no specialisation, which would have been helpful, methinks)

* Typesafe

* No usage of `unsafe`



Here is an example:



```rust

use frunk::labelled::Transmogrifier;



#[derive(LabelledGeneric)]

struct InternalPhoneNumber {

    emergency: Option,

    main: usize,

    secondary: Option,

}



#[derive(LabelledGeneric)]

struct InternalAddress<'a> {

    is_whitelisted: bool,

    name: &'a str,

    phone: InternalPhoneNumber,

}



#[derive(LabelledGeneric)]

struct InternalUser<'a> {

    name: &'a str,

    age: usize,

    address: InternalAddress<'a>,

    is_banned: bool,

}



#[derive(LabelledGeneric, PartialEq, Debug)]

struct ExternalPhoneNumber {

    main: usize,

}



#[derive(LabelledGeneric, PartialEq, Debug)]

struct ExternalAddress<'a> {

    name: &'a str,

    phone: ExternalPhoneNumber,

}



#[derive(LabelledGeneric, PartialEq, Debug)]

struct ExternalUser<'a> {

    age: usize,

    address: ExternalAddress<'a>,

    name: &'a str,

}



let internal_user = InternalUser {

    name: "John",

    age: 10,

    address: InternalAddress {

        is_whitelisted: true,

        name: "somewhere out there",

        phone: InternalPhoneNumber {

            main: 1234,

            secondary: None,

            emergency: Some(5678),

        },

    },

    is_banned: true,

};



/// Boilerplate-free conversion of a top-level InternalUser into an

/// ExternalUser, taking care of subfield conversions as well.

let external_user: ExternalUser = internal_user.transmogrify();



let expected_external_user = ExternalUser {

    name: "John",

    age: 10,

    address: ExternalAddress {

        name: "somewhere out there",

        phone: ExternalPhoneNumber {

            main: 1234,

        },

    }

};



assert_eq!(external_user, expected_external_user);

```



Note that as of writing, there are a couple of known limitations with `transmogrify()`,

some of which may be addressed in the future:



* If one of the fields is an identical type **and** derives `LabelledGeneric`,

  the compiler will tell you that it can't "infer an index" for `transmogrify()`; this

  is because `impl`s of the `Transmogrifier` trait will clash. This may or may not

  change in the future (perhaps if we move to a pure procedural macro powered way of doing

  things?)

* For types that contain many multiple deeply-nested fields that require `transmogfiy()`ing,

  using this technique will likely increase your compile time.

* If you've balked at the the compile-time errors with `transform_from` when a transform is deemed

  impossible (e.g. missing field), the errors for `transmogrify()` are worse to the degree that

  recursive `transmogrify()` is required for your types.



For more information how Generic and Field work, check out their respective Rustdocs:

  * [Generic](https://beachape.com/frunk/frunk_core/generic/index.html)

  * [Labelled](https://beachape.com/frunk/frunk_core/labelled/index.html)



#### Path



One of the other things that `LabelledGeneric`-deriving structs can do is be generically traversed

using `Path` and its companion trait `PathTraverser`. In some circles, this functionality is also

called a Lens.



`Path`-based traversals are

* Easy to use through the procedural macro `path!` (`frunk_proc_macros`)

  * Traversing multiple levels is familiar; just use dot `.` syntax (`path!(nested.attribute.value)`)

* Compile-time safe

* Composable (add one to the other using `+`)

* Allows you to get by value, get by reference or get by mutable reference, depending on the type

  of thing you pass it.



```rust

#[derive(LabelledGeneric)]

struct Dog<'a> {

    name: &'a str,

    dimensions: Dimensions,

}



#[derive(LabelledGeneric)]

struct Cat<'a> {

    name: &'a str,

    dimensions: Dimensions,

}



#[derive(LabelledGeneric)]

struct Dimensions {

    height: usize,

    width: usize,

    unit: SizeUnit,

}



#[derive(Debug, Eq, PartialEq)]

enum SizeUnit {

    Cm,

    Inch,

}



let mut dog = Dog {

    name: "Joe",

    dimensions: Dimensions {

        height: 10,

        width: 5,

        unit: SizeUnit::Inch,

    },

};



let cat = Cat {

    name: "Schmoe",

    dimensions: Dimensions {

        height: 7,

        width: 3,

        unit: SizeUnit::Cm,

    },

};



// generic, re-usable, compsable paths

let dimensions_lens = path!(dimensions);

let height_lens = dimensions_lens + path!(height); // compose multiple

let unit_lens = path!(dimensions.unit); // dot syntax to just do the whole thing at once



assert_eq!(*height_lens.get(&dog), 10);

assert_eq!(*height_lens.get(&cat), 7);

assert_eq!(*unit_lens.get(&dog), SizeUnit::Inch);

assert_eq!(*unit_lens.get(&cat), SizeUnit::Cm);



// modify by passing a &mut

*height_lens.get(&mut dog) = 13;

assert_eq!(*height_lens.get(&dog), 13);

```



There's also a `Path!` type-level macro for declaring shape-constraints. This allows you to write adhoc shape-dependent

functions for `LabelledGeneric` types.



```rust

// Prints height as long as `A` has the right "shape" (e.g.

// has `dimensions.height: usize` and `dimension.unit: SizeUnit)

fn print_height<'a, A, HeightIdx, UnitIdx>(obj: &'a A) -> ()

where

    &'a A: PathTraverser

        + PathTraverser,

{

    println!(

        "Height [{} {:?}]",

        path!(dimensions.height).get(obj),

        path!(dimensions.unit).get(obj)

    );

}

```



See `examples/paths.rs` to see how this works.



### Coproduct



If you've ever wanted to have an adhoc union / sum type of types that you do not control, you may want

to take a look at `Coproduct`. In Rust, thanks to `enum`, you could potentially declare one every time you

want a sum type to do this, but there is a light-weight way of doing it through Frunk:



```rust

use frunk::prelude::*; // for the fold method



// Declare the types we want in our Coproduct

type I32F32Bool = Coprod!(i32, f32, bool);



let co1 = I32F32Bool::inject(3);

let get_from_1a: Option<&i32> = co1.get();

let get_from_1b: Option<&bool> = co1.get();



assert_eq!(get_from_1a, Some(&3));

// None because co1 does not contain a bool, it contains an i32

assert_eq!(get_from_1b, None);



// This will fail at compile time because i8 is not in our Coproduct type

let nope_get_from_1b: Option<&i8> = co1.get(); // <-- will fail

// It's also impossible to inject something into a coproduct that is of the wrong type

// (not contained in the coproduct type)

let nope_co = I32F32Bool::inject(42f64); // <-- will fail



// We can fold our Coproduct into a single value by handling all types in it

assert_eq!(

    co1.fold(hlist![|i| format!("int {}", i),

                    |f| format!("float {}", f),

                    |b| (if b { "t" } else { "f" }).to_string()]),

    "int 3".to_string());

```



For more information, check out the [docs for Coproduct](https://beachape.com/frunk/frunk/coproduct/index.html)



### Validated



`Validated` is a way of running a bunch of operations that can go wrong (for example,

functions returning `Result`) and, in the case of one or more things going wrong,

having all the errors returned to you all at once. In the case that everything went well, you get

an `HList` of all your results.



Mapping (and otherwise working with plain) `Result`s is different because it will

stop at the first error, which can be annoying in the very common case (outlined

best by [the Cats project](http://typelevel.org/cats/datatypes/validated.html)).



To use `Validated`, first:

```rust

use frunk::prelude::*; // for Result::into_validated

```



Assuming we have a `Person` struct defined

```rust

#[derive(PartialEq, Eq, Debug)]

struct Person {

    age: i32,

    name: String,

    street: String,

}

```



Here is an example of how it can be used in the case that everything goes smoothly.



```rust

fn get_name() -> Result { /* elided */ }

fn get_age() -> Result { /* elided */ }

fn get_street() -> Result { /* elided */ }



// Build up a `Validated` by adding in any number of `Result`s

let validation = get_name().into_validated() + get_age() + get_street();

// When needed, turn the `Validated` back into a Result and map as usual

let try_person = validation.into_result()

                           // Destructure our hlist

                           .map(|hlist_pat!(name, age, street)| {

                               Person {

                                   name: name,

                                   age: age,

                                   street: street,

                               }

                           });



assert_eq!(try_person.unwrap(),

           Person {

               name: "James".to_owned(),

               age: 32,

               street: "Main".to_owned(),

           }));

}

```



If, on the other hand, our `Result`s are faulty:

```rust

/// This next pair of functions always return Recover::Err

fn get_name_faulty() -> Result {

    Result::Err("crap name".to_owned())

}



fn get_age_faulty() -> Result {

    Result::Err("crap age".to_owned())

}



let validation2 = get_name_faulty().into_validated() + get_age_faulty();

let try_person2 = validation2.into_result()

                             .map(|_| unimplemented!());



// Notice that we have an accumulated list of errors!

assert_eq!(try_person2.unwrap_err(),

           vec!["crap name".to_owned(), "crap age".to_owned()]);

```



### Semigroup



Things that can be combined.



```rust

use frunk::Semigroup;

use frunk::semigroup::All;



assert_eq!(Some(1).combine(&Some(2)), Some(3));



assert_eq!(All(3).combine(&All(5)), All(1)); // bit-wise &&

assert_eq!(All(true).combine(&All(false)), All(false));

```



### Monoid



Things that can be combined *and* have an empty/id value.



```rust

use frunk::monoid::combine_all;



let t1 = (1, 2.5f32, String::from("hi"), Some(3));

let t2 = (1, 2.5f32, String::from(" world"), None);

let t3 = (1, 2.5f32, String::from(", goodbye"), Some(10));

let tuples = vec![t1, t2, t3];



let expected = (3, 7.5f32, String::from("hi world, goodbye"), Some(13));

assert_eq!(combine_all(&tuples), expected)



let product_nums = vec![Product(2), Product(3), Product(4)];

assert_eq!(combine_all(&product_nums), Product(24))

```



### Features



Frunk comes with support for deriving [serde](https://github.com/serde-rs/serde) serializer/deserializers for its core

data structures. This can be enabled by adding the `serde` feature flag.



For example, if you'd like to use just `frunk_core` with serde



```toml

[dependencies]

frunk_core = { version = "$version", features = ["serde"] }

```



Or, if you'd like to use `frunk` with serde, you need to explicitly include `frunk_core` as well



```toml

[dependencies]

frunk = { version = "$version", features = ["serde"] }

frunk_core = { version = "$version", features = ["serde"] }

```



### Benchmarks



Benchmarks are available in `./benches` and can be run with:



`$ rustup run nightly cargo bench`



Benchmarks on `master` are also [auto-generated, uploaded and available online](https://beachape.com/frunk/dev/bench).



## Todo



### Stabilise interface, general cleanup



Before a 1.0 release, would be best to revisit the design of the interfaces

and do some general code (and test cleanup).



### Not yet implemented



Given that Rust has no support for Higher Kinded Types, I'm not sure if these

are even possible to implement. In addition, Rustaceans are used to calling `iter()`

on collections to get a lazy view, manipulating their elements with `map`

or `and_then`, and then doing a `collect()` at the end to keep things

efficient. The usefulness of these following structures maybe limited in that context.



0. `Functor`

1. `Monad`

2. `Apply`

3. `Applicative`



## Contributing



Yes please !



The following are considered important, in keeping with the spirit of Rust and functional programming:



- Safety (type and memory)

- Efficiency

- Correctness



## Inspirations



Scalaz, Shapeless, Cats, Haskell, the usual suspects ;)



## Maintainers



A.k.a. people whom you can bug/tag/@ on Gitter :D



1. [lloydmeta](https://github.com/lloydmeta)

2. [Centril](https://github.com/centril)

3. [ExpHP](https://github.com/ExpHP)