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:

• If<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!!!
}

Compile-time guards

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

If	Returns input type if Cond can be constructed.
IfElseEqual	Returns output if left-hand-site equals to right-hand-side, otherwise returns Else.
IfElseGreater	Returns input type if Lhs is greater than Rhs, otherwise returns Else.
IfElseGreaterOrEqual	Returns input type if Lhs is greater than or equals to Rhs, otherwise returns Else.
IfElseLess	Returns input type if Lhs is less than Rhs, otherwise returns Else.
IfElseLessOrEqual	Returns input type if Lhs is less than or equals to Rhs, otherwise returns Else.
IfElsePredicate	Returns input type if Cond evaluates to True, otherwise returns Else.
IfEqual	Returns input type if Lhs equals to Rhs.
IfGreater	Returns input type if Lhs is greater than Rhs.
IfGreaterOrEqual	Returns input type if Lhs is greater than or equals to Rhs.
IfLess	Returns input type if Lhs is less than Rhs.
IfLessOrEqual	Returns input type if Lhs is less than or equals to Rhs.
IfNonZero	A type operator that checks if a typenum value implements NonZero trait.
IfNotPredicate	Returns input type if Cond evaluates to False.
IfPredicate	Returns input type if Cond evaluates to True.
IfSame	Returns input type if both Lhs and Rhs are equivalent types.
IfZero	A type operator that checks if a typenum value is either B0, Z0 or U0.

Type Definitions

IfElseEqualOutput
IfElseGreaterOrEqualOutput
IfElseGreaterOutput
IfElseLessOrEqualOutput
IfElseLessOutput
IfElsePredicateOutput
IfEqualOutput
IfGreaterOrEqualOutput
IfGreaterOutput
IfLessOrEqualOutput
IfLessOutput
IfNonZeroOutput
IfNotPredicateOutput
IfOutput
IfPredicateOutput
IfSameOutput
IfZeroOutput