[−][src]Module type_freak::control

Compile-time guards and static assertions.

Overview

Most type operators in this module copies the input type to Self::Output when certain conditions are met. We have these categories of operators:

IfOutput<Output, Type>: Asserts Type can be constructed.
IfPredicate<Output, Predicate>: Asserts Predicate derives to True.
IfSame<Output, Lhs, Rhs>: Asserts Lhs and Rhs are of the same type.
IfLess, IfLessOrEqual, IfGreater, IfGreaterOrEqual, IfEqual: Asserts two typenum numbers follows the order.

By convention, IfSameOutput<Output, Lhs, Rhs> is type alias of <Output as IfSame<Lhs, Rhs>>::Output trait cast, and others follow. Only IfOutput<Output, Type> has no corresponding trait.

Static assertions

We can make use of If*Output aliases to build compile time assertions. For example, IfLessOutput asserts LHS is less than RHS.

use typenum::consts::*;
use type_freak::control::IfLessOutput;

type Out1 = IfLessOutput<usize, U3, U5>;  // U3 < U5 is true, thus Out1 ~= usize
type Out2 = IfLessOutput<usize, U5, U3>;  // U5 < U5 is false

fn assert() {
   let _: Out1 = 0;  // Goes fine here.
   let _: Out2 = 0;  // Compile error!!!
}

By placing If* trait bounds in where block. we can build compile-time guarded functions. For example, we add IfSame trait bound to assert two function generic parameters have identical types. The same trick applies to guarded structs, traits and impl blocks.

use type_freak::control::IfSame;

fn guarded_function<Lhs, Rhs>() -> String
where
    Lhs: IfSame<Lhs, Rhs>
{
    "Yeeeeeee!".to_owned()
}

fn comile_me() {
    let _ = guarded_function::<String, String>();  // fine
    let _ = guarded_function::<String, u8>();      // Compile error!!!
}

Traits

IfElsePredicate	A type operator that returns output depending Boolean condition.
IfEqual	A type operator that checks if left-hand-site equals to right-hand-side.
IfGreater	A type operator that checks if left-hand-site is greater than right-hand-side.
IfGreaterOrEqual	A type operator that checks if left-hand-site is greater than or equals to right-hand-side.
IfLess	A type operator that checks if left-hand-site is less than right-hand-side.
IfLessOrEqual	A type operator that checks if left-hand-site is less than or equals to right-hand-side.
IfNotPredicate	A type operator that checks if condition is False.
IfPredicate	A type operator that checks if condition is True.
IfSame	A type operator that checks you both types are equivalent.

Type Definitions

IfElsePredicateOutput
IfEqualOutput
IfGreaterOrEqualOutput
IfGreaterOutput
IfLessOrEqualOutput
IfLessOutput
IfNotPredicateOutput
IfOutput	A type alias that checks if type can be constructed.
IfPredicateOutput
IfSameOutput