`std::num::Checked{Add,Sub,Mul,Div}` should have type parameters

[cuviper]

From @steveklabnik on February 11, 2015 1:35

Issue by lifthrasiir
Monday Apr 14, 2014 at 12:33 GMT

For earlier discussion, see rust-lang/rust#13510

This issue was labelled with: A-libs in the Rust repository

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The corresponding std::ops traits have both the type of right hand side operand and the type of result, while Checked* traits do not (and always assume that both operands and result have the same type). There is no real reason to make Checked* traits behave differently from std::ops traits, and having them as is hampers advanced uses of operator overloading: for example, if the scalar type (e.g. DateTime) and difference type (e.g. Duration) are distinct from each other then the current scheme wouldn't work at all.

Copied from original issue: rust-num/num#59

[cuviper]

From @clarcharr on September 30, 2017 21:40

Adding onto this, there should be checked versions of Rem, Shl, and Shr as well.

[clarfonthey]

IMHO these traits should also take things by value for consistency with the original traits. Object safety is already broken because they still rely on the original not-object-safe traits.

[cuviper]

I've decided to limit the clutter of breaking change issues, so they will now be tracked collectively in #47. I'm closing this issue for now as postponed but we can still discuss its merits here as needed.

[regexident]

For sake of completeness it's worth mentioning that not just the Checked{Add,Sub,Mul} set of traits should have type parameters, but all of these, too:

• Wrapping{Add,Sub,Mul}
• Saturating{Add,Sub,Mul}
• Overflowing{Add,Sub,Mul}

Same for Unchecked{Add,Sub,Mul}, should they get added to num-traits in the future.

[regexident]

To add another motivation for adding type parameters consider vector-scaling arithmetic:

impl CheckedMul<Scalar> for Vector {
    fn checked_mul(&self, v: &Self) -> Option<Self> {
                           // ~~~~
                           // not possible as we would require `Scalar` here,
                           // not `Self` (problem caused by `Self: Mul<Self, Output = _>`)
                                                                       ~~~~
    }
}

impl CheckedMul<Vector> for Scalar {
    fn checked_mul(&self, v: &Self) -> Option<Self> {
                           // ~~~~            ~~~~
                           // not possible either as we would require `Vector` here,
                           // not `Self` (problem caused by `Self: Mul<Self, Output = Self>`)
                                                                       ~~~~           ~~~~
    }
}

[regexident]

Since this issue doesn't actually mention what the desired re-definitions would actually look like I'd like to take the opportunity to provide one such recommendation here:

Checked{Add,Sub,Mul,Div}:

pub trait Checked…<Rhs = Self> {
    type Output;

    #[must_use]
    fn checked_…(self, rhs: Rhs) -> Option<Self::Output>;
}

Unchecked{Add,Sub,Mul,Div}:

pub trait UncheckedAdd<Rhs = Self> {
    type Output;

    #[must_use]
    fn unchecked_add(self, rhs: Rhs) -> Self::Output;
}

Saturating{Add,Sub,Mul,Div}:

pub trait Saturating…<Rhs = Self> {
    type Output;

    #[must_use]
    fn saturating_…(self, rhs: Rhs) -> Self::Output;
}

Wrapping{Add,Sub,Mul,Div}:

pub trait Wrapping…<Rhs = Self> {
    type Output;

    #[must_use]
    fn wrapping_…(self, rhs: Rhs) -> Self::Output;
}

Overflowing{Add,Sub,Mul,Div}:

pub trait Overflowing…<Rhs = Self> {
    type Output;

    #[must_use]
    fn overflowing_…(self, rhs: Rhs) -> (Self::Output, bool);
}