The Go Blog

What's in an (Alias) Name?

Robert Griesemer
17 September 2024

This post is about generic alias types, what they are, and why we need them.

Background

Go was designed for programming at scale. Programming at scale means dealing with large amounts of data, but also large codebases, with many engineers working on those codebases over long periods of time.

Go’s organization of code into packages enables programming at scale by splitting up large codebases into smaller, more manageable pieces, often written by different people, and connected through public APIs. In Go, these APIs consist of the identifiers exported by a package: the exported constants, types, variables, and functions. This includes the exported fields of structs and methods of types.

As software projects evolve over time or requirements change, the original organization of the code into packages may turn out to be inadequate and require refactoring. Refactoring may involve moving exported identifiers and their respective declarations from an old package to a new package. This also requires that any references to the moved declarations must be updated so that they refer to the new location. In large codebases it may be unpractical or infeasible to make such a change atomically; or in other words, to do the move and update all clients in a single change. Instead, the change must happen incrementally: for instance, to “move” a function F, we add its declaration in a new package without deleting the original declaration in the old package. This way, clients can be updated incrementally, over time. Once all callers refer to F in the new package, the original declaration of F may be safely deleted (unless it must be retained indefinitely, for backward compatibility). Russ Cox describes refactoring in detail in his 2016 article on Codebase Refactoring (with help from Go).

Moving a function F from one package to another while also retaining it in the original package is easy: a wrapper function is all that’s needed. To move F from pkg1 to pkg2, pkg2 declares a new function F (the wrapper function) with the same signature as pkg1.F, and pkg2.F calls pkg1.F. New callers may call pkg2.F, old callers may call pkg1.F, yet in both cases the function eventually called is the same.

Moving constants is similarly straightforward. Variables take a bit more work: one may have to introduce a pointer to the original variable in the new package or perhaps use accessor functions. This is less ideal, but at least it is workable. The point here is that for constants, variables, and functions, existing language features exist that permit incremental refactoring as described above.

But what about moving a type?

In Go, the (qualified) identifier, or just name for short, determines the identity of types: a type T defined and exported by a package pkg1 is different from an otherwise identical type definition of a type T exported by a package pkg2. This property complicates a move of T from one package to another while retaining a copy of it in the original package. For instance, a value of type pkg2.T is not assignable to a variable of type pkg1.T because their type names and thus their type identities are different. During an incremental update phase, clients may have values and variables of both types, even though the programmer’s intent is for them to have the same type.

To solve this problem, Go 1.9 introduced the notion of a type alias. A type alias provides a new name for an existing type without introducing a new type that has a different identity.

In contrast to a regular type definition

type T T0

which declares a new type that is never identical to the type on the right-hand side of the declaration, an alias declaration

type A = T  // the "=" indicates an alias declaration

declares only a new name A for the type on the right-hand side: here, A and T denote the same and thus identical type T.

Alias declarations make it possible to provide a new name (in a new package!) for a given type while retaining type identity:

package pkg2

import "path/to/pkg1"

type T = pkg1.T

The type name has changed from pkg1.T to pkg2.T but values of type pkg2.T have the same type as variables of type pkg1.T.

Generic alias types

Go 1.18 introduced generics. Since that release, type definitions and function declarations can be customized through type parameters. For technical reasons, alias types didn’t gain the same ability at that time. Obviously, there were also no large codebases exporting generic types and requiring refactoring.

Today, generics have been around for a couple of years, and large codebases are making use of generic features. Eventually the need will arise to refactor these codebases, and with that the need to migrate generic types from one package to another.

To support incremental refactorings involving generic types, the future Go 1.24 release, planned for early February 2025, will fully support type parameters on alias types in accordance with proposal #46477. The new syntax follows the same pattern as it does for type definitions and function declarations, with an optional type parameter list following the identifier (the alias name) on the left-hand side. Before this change one could only write:

type Alias = someType

but now we can also declare type parameters with the alias declaration:

type Alias[P1 C1, P2 C2] = someType

Consider the previous example, now with generic types. The original package pkg1 declared and exported a generic type G with a type parameter P that is suitably constrained:

package pkg1

type Constraint      someConstraint
type G[P Constraint] someType

If the need arises to provide access to the same type G from a new package pkg2, a generic alias type is just the ticket (playground):

package pkg2

import "path/to/pkg1"

type Constraint      = pkg1.Constraint  // pkg1.Constraint could also be used directly in G
type G[P Constraint] = pkg1.G[P]

Note that one cannot simply write

type G = pkg1.G

for a couple of reasons:

1. Per existing spec rules, generic types must be instantiated when they are used. The right-hand side of the alias declaration uses the type pkg1.G and therefore type arguments must be provided. Not doing so would require an exception for this case, making the spec more complicated. It is not obvious that the minor convenience is worth the complication.

2. If the alias declaration doesn’t need to declare its own type parameters and instead simply “inherits” them from the aliased type pkg1.G, the declaration of G provides no indication that it is a generic type. Its type parameters and constraints would have to be retrieved from the declaration of pkg1.G (which itself might be an alias). Readability will suffer, yet readable code is one of the primary aims of the Go project.

Writing down an explicit type parameter list may seem like an unnecessary burden at first, but it also provides additional flexibility. For one, the number of type parameters declared by the alias type doesn’t have to match the number of type parameters of the aliased type. Consider a generic map type:

type Map[K comparable, V any] mapImplementation

If uses of Map as sets are common, the alias

type Set[K comparable] = Map[K, bool]

might be useful (playground). Because it is an alias, types such as Set[int] and Map[int, bool] are identical. This would not be the case if Set were a defined (non-alias) type.

Furthermore, the type constraints of a generic alias type don’t have to match the constraints of the aliased type, they only have to satisfy them. For instance, reusing the set example above, one could define an IntSet as follows:

type integers interface{ ~int | ~int8 | ~int16 | ~int32 | ~int64 }
type IntSet[K integers] = Set[K]

This map can be instantiated with any key type that satisfies the integers constraint (playground). Because integers satisfies comparable, the type parameter K may be used as type argument for the K parameter of Set, following the usual instantiation rules.

Finally, because an alias may also denote a type literal, parameterized aliases make it possible to create generic type literals (playground):

type Point3D[E any] = struct{ x, y, z E }

To be clear, none of these examples are “special cases” or somehow require additional rules in the spec. They follow directly from the application of the existing rules put in place for generics. The only thing that changed in the spec is the ability to declare type parameters in an alias declaration.

An interlude about type names

Before the introduction of alias types, Go had only one form of type declarations:

type TypeName existingType

This declaration creates a new and different type from an existing type and gives that new type a name. It was natural to call such types named types as they have a type name in contrast to unnamed type literals such as struct{ x, y int }.

With the introduction of alias types in Go 1.9 it became possible to give a name (an alias) to type literals, too. For instance, consider:

type Point2D = struct{ x, y int }

Suddenly, the notion of a named type describing something that is different from a type literal didn’t make that much sense anymore, since an alias name clearly is a name for a type, and thus the denoted type (which might be a type literal, not a type name!) arguably could be called a “named type”.

Because (proper) named types have special properties (one can bind methods to them, they follow different assignment rules, etc.), it seemed prudent to use a new term in order to avoid confusions. Thus, since Go 1.9, the spec calls the types formerly called named types defined types: only defined types have properties (methods, assignability restrictions, etc) that are tied to their names. Defined types are introduced through type definitions, and alias types are introduced through alias declarations. In both cases, names are given to types.

The introduction of generics in Go 1.18 made things more complicated. Type parameters are types, too, they have a name, and they share rules with defined types. For instance, like defined types, two differently named type parameters denote different types. In other words, type parameters are named types, and furthermore, they behave similarly to Go’s original named types in some ways.

To top things off, Go’s predeclared types (int, string and so on) can only be accessed through their names, and like defined types and type parameters, are different if their names are different (ignoring for a moment the byte and rune alias types). The predeclared types truly are named types.

Therefore, with Go 1.18, the spec came full circle and formally re-introduced the notion of a named type which now comprises “predeclared types, defined types, and type parameters”. To correct for alias types denoting type literals the spec says: “An alias denotes a named type if the type given in the alias declaration is a named type.”

Stepping back and outside the box of Go nomenclature for a moment, the correct technical term for a named type in Go is probably nominal type. A nominal type’s identity is explicitly tied to its name which is exactly what Go’s named types (now using the 1.18 terminology) are all about. A nominal type’s behavior is in contrast to a structural type which has behavior that only depends on its structure and not its name (if it has one in the first place). Putting it all together, Go’s predeclared, defined, and type parameter types are all nominal types, while Go’s type literals and aliases denoting type literals are structural types. Both nominal and structural types can have names, but having a name doesn’t mean the type is nominal, it just means it is named.

None of this matters for day-to-day use of Go and in practice the details can safely be ignored. But precise terminology matters in the spec because it makes it easier to describe the rules governing the language. So should the spec change its terminology one more time? It is probably not worth the churn: it is not just the spec that would need to be updated, but also a lot of supporting documentation. A fair number of books written on Go might become inaccurate. Furthermore, “named”, while less precise, is probably intuitively clearer than “nominal” for most people. It also matches the original terminology used in the spec, even if it now requires an exception for alias types denoting type literals.

Availability

Implementing generic type aliases has taken longer than expected: the necessary changes required adding a new exported Alias type to go/types and then adding the ability to record type parameters with that type. On the compiler side, the analogous changes also required modifications to the export data format, the file format that describes a package’s exports, which now needs to be able to describe type parameters for aliases. The impact of these changes is not confined to the compiler, but affects clients of go/types and thus many third-party packages. This was very much a change affecting a large code base; to avoid breaking things, an incremental roll-out over several releases was necessary.

After all this work, generic alias types will finally be available by default in Go 1.24.

To allow third-party clients to get their code ready, starting with Go 1.23, support for generic type aliases can be enabled by setting GOEXPERIMENT=aliastypeparams when invoking the go tool. However, be aware that support for exported generic aliases is still missing for that version.

Full support (including export) is implemented at tip, and the default setting for GOEXPERIMENT will soon be switched so that generic type aliases are enabled by default. Thus, another option is to experiement with the latest version of Go at tip.

As always, please let us know if you encounter any problems by filing an issue; the better we test a new feature, the smoother the general roll-out.

Thanks and happy refactoring!

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