# MultiType
MultiType is Rust a crate for generalising fundamental types via traits.
MultiType provideds traits such as `Uint` and `Float` to abstract over a set of equivalent primitive types.
These traits are intended to provide one-to-one copies of the interfaces that the primitive types define.
# Overview
The complete list of abstraction traits is:
* `Int`
* `IntLeast16`
* `IntLeast32`
* `IntLeast64`
* `Uint`
* `UintLeast16`
* `UintLeast32`
* `UintLeast64`
* `Float`
* `FloatLeast32`
* `FloatLeast64`
* `StdFloat`
* `Array`
Any given type may implement at most *one* of the aforementioned trait groups; thus, for example, any type that implemets `Float` can be assumed to possibly implement `FloatLeast32` but never `Uint`.
For the sake of compatibility with `{f16, f32, f64, f128}::to_int_unchecked`, we also define our own `FloatToInt` trait.
Note that all traits provided by this crate are sealed and cannot be implemented by third-party crates (at least currently).
## Arithmetic types
MultiType defines different traits for generalising arithmetic types:
* `Int` for `i8`, `i16`, `i32`, `i64`, `i128`, and `isize`
* `Uint` for `u8`, `u16`, `u32`, `u64`, `u128`, and `usize`
* `Float` for `f16`, `f32`, `f64`, and `f128`
Furthermore, `StdFloat` extends the `Float` trait with functionality that is typically only available in `std`'s floating-point types.
### Sized, arithmetic types
The basic, arithmetic traits guarantee a minimum size that is equivalent to the smallest member of its group, e.g. `Uint` is at least `u8` and `Float` is at least `f16`.
Additionally, these three arithmetic traits have subtraits that guarantee wider types, for example:
* `i16` and wider implement `IntLeast16`
* `i32` and wider implement `IntLeast32`
* Etc.
Extremely-wide traits, e.g. `IntLeast8` or `IntLeast128`, are considered redundant and are thus not provided.
## Array types
MultiType also provides the `Array` trait for generalising array types – most often over their length.
An example of this trait's usecase is actually in this crate:
Take, for instance, the definition of `Uint`: It has a `Bytes` associated type that is used by the bytewise constructors and destructors:
```rust
pub unsafe trait Uint: /* .. */ {
type Bytes;
fn from_ne_bytes(bytes: Self::Bytes) -> Self;
fn to_ne_bytes(self) -> Self::Bytes;
// ..
}
```
Now, anyone that would want to use the output of `to_ne_bytes`:to_ne_bytes) wouldn't really have that many choices with regard to what to do with it.
So, MultiType defines the `Array` trait:
```rust
use multitype::Array;
pub unsafe trait Uint: /* .. */ {
type Bytes: Array<Scalar = u8> /* + .. */;
// ..
}
```
With it, it's possible for users to generically use `Uint::to_ne_bytes` as an array type through the trait methods.
## Examples
A generic Fibonacci sequence:
```rust
use multitype::Uint;
fn f<T: Uint>(x: T) -> T {
let mut y = T::from(0u8);
let mut y_m1 = T::from(0u8);
let mut y_m2 = T::from(1u8);
let mut i = T::from(0u8);
while i < x {
y = y_m1 + y_m2;
y_m2 = y_m1;
y_m1 = y;
i += T::from(1u8);
}
y
}
assert_eq!(f(0u8), 0);
assert_eq!(f(1u8), 1);
assert_eq!(f(2u16), 1);
assert_eq!(f(3u16), 2);
assert_eq!(f(4u32), 3);
assert_eq!(f(5u32), 5);
assert_eq!(f(6u64), 8);
assert_eq!(f(7u64), 13);
```
Generic array indexing:
```rust
use core::f32;
use multitype::{Array, Float};
fn complicated_neg<T: Float>(value: T) -> T {
let mut bytes = value.to_le_bytes();
// Invert the sign bit -- which is always the most
// significant bit of the most significant byte.
*bytes.as_mut_slice().last_mut().unwrap() ^= 0b10000000;
T::from_le_bytes(bytes)
}
assert_eq!(complicated_neg( 1.0f64), -1.0f64);
assert_eq!(complicated_neg(-1.0f64), 1.0f64);
assert_eq!(complicated_neg(f32::NEG_INFINITY), f32::INFINITY);
```
## Feature gates
Default features:
* `alloc`
* `std`
Dependency features:
* `alloc`: Enables compatibility with `alloc` facilities
* `std`: Enables compatibility with `std` facilities
Unstable features:
* `clone_to_uninit`: Enables `CloneToUninit` requirements
* `const_param_ty`: Enables `ConstParamTy_` requirements
* `f16`: Enables support for `f16`
* `f128`: Enables support for `f128`
* `freeze`: Enables `Freeze` requirements
* `nightly_backport`: Enables backports for nightly items
* `num_buffer_trait`: Enables `NumBufferTrait` requirements
* `step`: Enables `Step` requirements
* `structural_partial_eq`: Enables `StructuralPartialEq` requirements
* `trusted_step`: Enables `TrustedStep` requirements
* `unstable_docs`: Enables unstable documentation features
* `use_cloned`: Enables `UseCloned` requirements
Unstable features can be expected to be removed as their facilities stabilise.
## SemVer policy
Nightly backports will match the current, nightly interfaces provided by rustc.
A change in rustc will thus be reflected here along with a bump in the minor version.
Conversely, when a nightly item is stabilised in rustc, the corresponding nightly backport will be released from the feature gate in a minor version bump.
## MSRV policy
The goal of MultiType is to provide generic traits that bind as much of the standard interfaces as possible.
Items that are added after the MSRV will be backported.
## Copyright and licence
Copyright © 2025 Gabriel Bjørnager Jensen.
MultiType is distributed under either an MIT licence (see `LICENCE-MIT`) or version 2.0 of the Apache License (see `LICENCE-APACHE`), at your option.
