# Frunk [](https://crates.io/crates/frunk) [](https://github.com/lloydmeta/frunk/actions/workflows/ci.yml?query=branch%3Amaster) [](https://gitter.im/lloydmeta/frunk?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge) [](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<i32, HNil>
// 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<i32, HCons<bool, HNil>>>>
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<bool, HCons<&str, HCons<Option<{integer}>, 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<usize>,
main: usize,
secondary: Option<usize>,
}
#[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<Path!(dimensions.height), HeightIdx, TargetValue = &'a usize>
+ PathTraverser<Path!(dimensions.unit), UnitIdx, TargetValue = &'a SizeUnit>,
{
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<T, E>`) 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<String, Error> { /* elided */ }
fn get_age() -> Result<i32, Error> { /* elided */ }
fn get_street() -> Result<String, Error> { /* 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<String, String> {
Result::Err("crap name".to_owned())
}
fn get_age_faulty() -> Result<i32, String> {
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)