Description
Note: Discussion is now at #47319
This proposal is for use with #43651. The proposal document for that issue mentions adding a constraints package to define some standard useful constraints. Here we propose a specific API to define. If this proposal is accepted, the new package will be included with the first release of Go that implements #43651 (we currently expect that that will be Go 1.18).
This description below is focused on the API, not the implementation. In general the implementation will be straightforward.
The intent is that if we add any new predeclared types, such as int128 and uint128, those types will be supported by these constraints as appropriate based on the constraint descriptions. To make this work, code should follow these guidelines: 1) given a type parameter whose constraint is taken from the constraints package, don't use that type parameter to instantiate a type/function whose constraint is not any and is not taken from the constraints package; 2) don't write a type switch to handle all types when using a constraint taken from the constraints package. We can add vet checks to detect cases where these guidelines are not followed.
// Package constraints defines a set of useful constraints to be used with type parameters.
package constraints
// Signed is a constraint that permits any signed integer type.
type Signed interface { ... }
// Unsigned is a constraint that permits any unsigned integer type.
type Unsigned interface { ... }
// Integer is a constraint that permits any integer type.
type Integer interface { ... }
// Float is a constraint that permits any floating-point type.
type Float interface { ... }
// Complex is a constraint that permits any complex numeric type.
type Complex interface { ... }
// Ordered is a constraint that permits any ordered type: any type that supports the operators < <= >= >.
type Ordered interface { ... }
// Slice is a constraint that matches slices of any element type.
type Slice[Elem any] interface { ~[]Elem }
// Map is a constraint that matches maps of any element and value type.
type Map[Key comparable, Val any] interface { ~map[Key]Val }
// Chan is a constraint that matches channels of any element type.
type Chan[Elem any] interface { ~chan Elem }