spec: generics: permit type parameters on aliases

[mdempsky]

The generics proposal says "A type alias may refer to a generic type, but the type alias may not have its own parameters. This restriction exists because it is unclear how to handle a type alias with type parameters that have constraints."

I propose this should be relaxed and type aliases allowed to have their own type parameters. I think there's a clear way to handle type aliases with constrained type parameters: uses of the type alias need to satisfy the constraints, and within the underlying type expression those parameters can be used to instantiate other generic types that they satisfy.

I think it's fine to continue allowing type VectorAlias = Vector as in the proposal, but this should be considered short-hand for type VectorAlias[T any] = Vector[T]. More generally, for generic type B with type parameters [T1 C1, T2 C2, ..., Tn Cn], then type A = B would be the same as type A[T1 C1, T2 C2, ..., Tn Cn] = B[T1, T2, ..., Tn].

In particular, something like this would be an error:

type A[T comparable] int
type B[U any] = A[U]   // ERROR: U does not satisfy comparable
type C B[int]

As justification for this, analogous code in the value domain would give an error:

func F(x int) {}
func G(y interface{}) { F(y) }  // ERROR: cannot use y (type interface{}) as int
func H() { G(42) }

I suspect if TParams is moved from Named to TypeName and type instantiation is similarly changed to start from the TypeName instead of the Type, then this should work okay.

/cc @griesemer @ianlancetaylor @findleyr @bcmills

[findleyr]

If this proposal were accepted, would the following code be valid?

type A[T any] int
type B[U comparable] = A[U]

I.e. would it be possible to define an alias which tightens the constraints of the aliased type?

IMO the example in the value domain is more analogous to defining a new named type, which already behaves as expected:

type A[T comparable] int
type B[U any] A[U] // ERROR: U does not satisfy comparable

Aliasing seems more analogous to function value assignment, for which we don't redeclare parameters:

func F(x int) {}
var G = F

I think you're right about how this could be implemented, but I wonder if it is conceptually coherent. Specifically, I wonder about whether we should think of the declaration as parameterizing the type, or as defining a parameterized type, and whether it still makes sense to call the example with additional restrictions above an alias.

I'll also note that as you point out, our decisions with respect to the go/types API have real consequences for how easy it would be to relax this restriction on aliases in the future, so it is good to talk about this now. Thanks for raising this issue!

[mdempsky]

I.e. would it be possible to define an alias which tightens the constraints of the aliased type?

Yes. U (type parameter with bound comparable) satisfies the constraint any, so that's a valid type declaration in my mind. But similarly, trying to instantiate B[[]int] would be invalid, because []int does not satisfy comparable, even though it satisfies the underlying any.

I would expect that the type checker would see B[[]int], resolve B to the TypeName and check it against the type parameters, and then reject it as invalid, before proceeding to instantiating/substituting its Type with the type argument []int.

Aliasing seems more analogous to function value assignment, for which we don't redeclare parameters:

Note that var G = F is really shorthand there for var G func(int) = F. You're not allowed to write var G func(interface{}) = F, for example, even if you only ever call G with int arguments.

But this is also why I suggest still allowing type A = B as shorthand for explicitly writing out type parameters for the alias declaration.

[griesemer]

There is a reason why we didn't do this in the first place.

I don't have any principal objections to this proposal. If we accept this, I wonder whether we should still permit the type A = B form as it does deviate from the current design which requires that every use of a generic type requires an instantiation.

I'm inclined to proceed in one of two ways:
1) Disallow (not implement) the form type A = B for Go1.17. It's not crucial and we can always add it later.
2) Implement this proposal instead of permitting type A = B.

[bcmills]

I seem to recall @rogpeppe raising a similar point in various conversations.

[bcmills]

@findleyr

Aliasing seems more analogous to function value assignment, for which we don't redeclare parameters:

Note that we do allow function value assignment to strengthen (but not weaken) a type via assignability, which IMO is analogous to strengthening type constraints on a type declaration.

Consider this program:

package main

import "context"

func cancel() {}

type thunk func()

var f = cancel
var g context.CancelFunc = cancel

In that program, var f = cancel is shorthand for var f func() = cancel.

The declaration var g context.CancelFunc = cancel refers to the exact same function value, but with a stronger type (one that is not assignable to thunk).

[jimmyfrasche]

It looks like it could fall out of the definition but, to be explicit, partial application would also be useful:

type Named[T any] = Map[string, T]

[mdempsky]

@griesemer If we proceed with this proposal, I think it could be a nice convenience to keep type A = B as short-hand. But as it's not essential, I'd similarly be fine with just removing it altogether. We can always re-add it in the future if appropriate.

And yes, the deviating from the norm of requiring instantiation is what threw me off. I had written some code that was working under the assumption that if I only started from non-generic declarations, then I would never see a non-instantiated type. But that doesn't hold for the type A = B form. (Fortunately though, it's not hard to special case this one instance either.)

[findleyr]

@bcmills

Note that we do allow function value assignment to strengthen (but not weaken) a type via assignability, which IMO is analogous to strengthening type constraints on a type declaration.

The example from #46477 (comment) made the analogy of function parameters with type parameters (which makes sense). In that analogy, we don't allow changing function parameter types when assigning [example], i.e. we don't support covariant function assignment.

[bcmills]

In that analogy … we don't support covariant function assignment.

Sure, but pretty much the entire point of type parameters is to support variance in types. 😉

[neild]

What is the use case for permitting parameters on type aliases?

[griesemer]

@neild The same reason for which type aliases were introduced in the first place, which is to make refactoring across package boundaries easier (or possible, depending on use case).

I misread this comment. See below.

[griesemer]

Going through my notes I remember now why we didn't go this route in the first place: Note that an alias is just an alternative name for a type, it's not a new type. Introducing a smaller set of type arguments (as suggested above), or providing stronger type constraints seems counter that idea. Such changes arguably define a new type and then one should do that: declare a new defined type, i.e., leave the = away. I note that @findleyr pointed out just that in the 2nd comment on this proposal.
This would mean that the respective methods also have to be redefined (likely as forwarders) but that seems sensible if the type constraints are narrowed or partially instantiated.

In summary, I am not convinced anymore that this is such a good idea. We have explored the generics design space for the greater part of two years and the devil really is in the details. At this point we should not introduce new mechanisms until we have collected some concrete experience.

I suggest we put this on hold for the time being.

[neild]

@griesemer I don't see what the refactoring case is for changing the constraints of a type. As you say, an alias is just an alternative name for a type, but an alternative name with altered constraints is a subtler concept that I struggle to see the use for.

I may be missing something. A concrete example of when you'd use this would be useful.

[griesemer]

@neild Agreed - I misread your comment as "what is the use of allowing alias types for generic types" - my bad. See my comment just before your reply.

[mdempsky]

I don't see what the refactoring case is for changing the constraints of a type.

Under this proposal, you can do more with parameterized type aliases than just change the constraints. E.g., see #46477 (comment) for using type parameters to provide default arguments to other generic types. I called out the constraint change to clarify the semantics, not because I expect that's something people are likely to do in practice.

I anticipate analogous to how we added type aliases to facilitate large-scale refactorings while maintaining type identity, we're going to face situations where generic types need to be refactored to add, remove, or change parameters while also maintaining type identity. Having parameterized type aliases would facilitate that. I think if just "declaring a new defined type" was always an adequate solution, we could have skipped adding type aliases too.

I think it's fine though if Go 1.18 doesn't have parameterized type aliases. But I at least think we should try to ensure the go/types APIs are forward compatible with adding parameterized type aliases.

[findleyr]

@bcmills

Sure, but pretty much the entire point of type parameters is to support variance in types. 😉

FWIW, I don't follow this argument. We still support variance in types no matter what we decide about this proposal, just like we allow variance in function arguments whether or not we allow covariant assignment of function values. I think we're dipping in and out of the 'meta' realm. The point I was trying to make is that if we're trying to argue by analogy with the value domain, wrapping a function is more like defining a new named type (or perhaps more correctly like struct embedding), and aliasing is more like assignment. Since we don't allow covariant assignment for functions, it's arguably a bit inconsistent to allow covariant assignment for "meta functions" (if that's how we think about generic declarations).

@griesemer

This would mean that the respective methods also have to be redefined (likely as forwarders) but that seems sensible if the type constraints are narrowed or partially instantiated.

Or use embedding, which might be more analogous to wrapping a function in the value domain.

I suggest we put this on hold for the time being.

Independent of whether we relax the restriction on aliases, this proposal indirectly makes the point that it matters whether we think of the "type" as generic or the "declaration" as generic, both in the current APIs and for future extensions of the language. For example, thinking of the type declaration as generic allows relaxing this restriction on aliases. Thinking of the function type as generic allows for generic interface methods and generic function literals. If we put this proposal on hold, we will still need to make API decisions that affect its feasibility.

[mdempsky]

[re: value vs type domain analogies]

I want to clarify that I made this analogy initially to help explain how I intuit the relationships here. Go's values and types operate sufficiently distinctly and irregularly that I think trying to read too far into the analogy is going to hit rough edges and become more philosophical than actionable. E.g., the value domain has no analog to defined types and type identity, because it's impossible to create a copy of a Go value that's distinguishable from the original. (Emphasis: I'm talking specifically about values here, not variables.)

Certainly we should revisit these discussions when it comes time to add dependent types to Go 3 though. :)

[rogpeppe]

Note that an alias is just an alternative name for a type, it's not a new type.

I'm not sure that this is entirely true. What about this, which is currently allowed?

type S1[V any] struct { .... }

type S2 = S1[int]

S2 neither an alternative name for an existing type nor an entirely new type. More of a composite type, perhaps. Also, it does have some identity of its own (its name is used when it's embedded)

Introducing a smaller set of type arguments (as suggested above), or providing stronger type constraints seems counter that idea. Such changes arguably define a new type and then one should do that: declare a new defined type, i.e., leave the = away

Sometimes defining a new type isn't possible. For example, if a type is specifically mentioned in a type signature, it's not possible to use a new type - you have to use the same type as the original. Also, the fact that all methods are lost when you define a new type is a real problem and embedding doesn't always work either.

For non-generic code, it might usually be possible to define a fully-qualified type alias like S2 above, but in generic code that's often not possible because a type parameter might be free.

An example:

Say some package defines an OrderedMap container that allows an arbitrary comparison operation for keys:

package orderedmap

type Map[K any, V any, Cmp Comparer[K]] struct {
    ...
}

func (m *Map[K, V, Cmp]) Clone() *Map[K, V, Cmp]

func (m *Map[K, V, Cmp]) Get(k K) (V, bool)

type Comparer[K any] interface {
    Cmp(k1, k2 K) int
}

I want to implement a higher level container in terms of orderedmap.Map. In my implementation, only the value type is generic:

package foo

type Container[V any] struct {
}

func NewContainer[V any]() *Container[V] {
    ...
   var m *orderedmap.Map[internalKey, V, keyComparer]
}

type internalKey struct {
    ...
}

type keyComparer struct{}

func (keyComparer) Cmp(k1, k2 internalKey) int {
    ...
}

In the above code, whenever I wish to pass around the orderedmap.Map[internalKey, V, keyComparer] type, I have to do so explicitly in full. This could end up very tedious (and annoying to change when refactoring the code). It would be nice to be able to do:

type internalMap[V any] = orderedmap.Map[internalKey, V, keyComparer]

Then we can avoid duplicating the type parameters everywhere.

Defining a new type wouldn't be great here - you'd either have to explicitly forward all the methods (if you did type internalMap[V any] orderedmap.Map[...]) or reimplement some of the methods (if you did type internalMap[V any] struct {orderedmap.Map[...]}).

In short, I'm fairly sure that generic type aliases are going to be a much requested feature when people start using generics in seriousness, and that they're definitely worth considering now even if they're not implemented, so that the type checker isn't implemented in a way that makes it hard to add them later.

[rsc]

This proposal has been added to the active column of the proposals project
and will now be reviewed at the weekly proposal review meetings.
— rsc for the proposal review group

[beoran]

I think we have to consider what exactly an alias is in Go language:

Alias declarations
An alias declaration binds an identifier to the given type.
AliasDecl = identifier "=" Type .
Within the scope of the identifier, it serves as an alias for the type.

Since an alias is an identifier, that is, in essence, a name, it does not make sense for such a name to have type parameters. An alias is simply a name for a type, not a type in itself. And names should not be parameterizable.

If it is desirable to define a new generic type based on an other generic type, this should be done using different syntax than a type alias.

Therefore I, respectfully oppose this proposal.

[mdempsky]

I think we have to consider what exactly an alias is in Go language:

We're discussing amending the Go language spec here, so I think referring to the current wording is somewhat begging the question. If we decide to amend the spec to allow type-parameterized aliases, then I think it's within scope to amend those sentences too.

If it is desirable to define a new generic type based on an other generic type, this should be done using different syntax than a type alias.

We have existing syntax for type aliases and for type parameters, and conveniently they're compatible. I don't see why we'd want to use a new syntax for type-parameterized aliases.

[beoran]

Well, the reason I refer to the current spec is because that explains what the current concept of am alias is. If we were to change the spec, the concept of what an alias is will also change quite radically. An alias will not be just a name for a type anymore. And I feel this will make Go quite a bit harder to learn.

Furthermore, I would say that the changing concept of an alias as a name to something else is at least not conceptually backwards compatible. With this proposal an alias is not just a name any more, but also a way to define types. Since the concept of this proposal is different, i feel the syntax should be different as well. Maybe just using := in stead of = for example.

[jimmyfrasche]

This would not let you define any new types: it would let you give a name to a subfamily of a family of types.

[rsc]

What is the concrete benefit that this would bring?
And is it necessary to have in Go 1.18, or should we wait until a future release?

[DmitriyMV]

This could be extremely useful for signatures like type Type[A] = GenericType[A,*A] which you need for methods defined on pointer receivers.

[stephenafamo]

What are the chances this will make it into Go1.21?

[griesemer]

@stephenafamo We have been concentrating on improved type inference this dev. cycle. At the moment improved aliases seem somewhat unlikely to happen for 1.21.

[adonovan]

One case to consider: would this be legal or not?

type M[K comparable, V any] struct{ ... }
type A[K comparable, V any] = M[K, V]
func (A) Get(K) V { ... }

The spec says "the method receivers must declare the same number of type parameters as present in the generic type definition".

[mdempsky]

@alandonovan I think you mean func (A[K, V]) Get...?

I'd lean towards not allowing that. I argued for allowing type aliases in receiver parameters originally, but now I mostly regret that from an implementation complexity point of view

[griesemer]

@adonovan I've been thinking about that, too. I'm not sure yet what the right approach is. Consider situations where the alias type has fewer type parameters then the aliased type. And what it we allow more type parameters in the alias than in the aliased type. There are lots of open questions here.

[gopherbot]

Change https://go.dev/cl/521956 mentions this issue: go/types, types2: introduce _Alias type node

[findleyr]

We've been talking about this on the tools team, and think it would be good to land the support for Alias nodes in go/types one release before supporting parameterized aliases. This will give tools time to opt-in and handle the new node type (a potentially large change), before also supporting parameterized aliases (a likely smaller change). So we could land the new go/types node in 1.22, and parameterized aliases in 1.23.

[DeedleFake]

Any chance of it going into 1.22 behind a GOEXPERIMENT or a GODEBUG?

[griesemer]

This will be part of 1.23. We won't get this done for 1.22.