The developers of Go have release a new draft proposal for type parameters. The biggest change is the replacing the concept of constraints, which complicated the proposal somewhat, and replaced it with interfaces to express the same thing. You can read the proposal here latest proposal here.

I think they’re starting to reach a great balance between what currently exists in the language, and the features required to make a useful type parameter system. The use of interfaces to constrain the type, that is declare the operators that a type must implement in order to be used as a type parameter for a function or struct, makes total sense. It also makes moving to type parameters in some areas of the standard library trivial. For example, the sort.Sort function prototype:

func Sort(data Interface)

can simply be written as:

func Sort(type T Interface)(data T)

I do have some minor concerns though. The biggest one is the use of interfaces to express constraints related to operators, which are expressed as type lists. I think listing the types that a particular type parameter can instantiate makes sense. It dramatically simplifies the process of expressing a constraint based on the operators a particular type supports. However, using the concept of interfaces for this purpose seems a little strange, especially so when these interfaces cannot be used in areas other than type constraints. To be fair, it seems like they recognise this, and I’m suspecting that in practice these interfaces will be defined in a package that can simply be used, thereby not requiring us to deal with them directly unless we need to.

But all in all, this looks promising. It is starting to feel like the design is coming together, with the rough edges starting to be smoothed out. I appreciate the level of careful consideration the core Go developers are exhibiting in this process. This is after all a large change to the language, and they only have one real chance at this. Sure we have to wait for it, but especially in language design, mistakes are forever.