Comment by typical182

7 hours ago

It's not a great example.

I think it's trying to show a mapping operation for a generic container where the container values are of one type and the mapping function is allowed to return a container with values of a different type.

Without generics, something along the lines of the following (with runnable example at https://go.dev/play/p/KHBI1uAhbO0):

  type MySlice []int

  // Map maps from a slice of ints to a slice of float64s.
  func (s MySlice) Map(f func(int) float64) []float64 {
     var out []float64
     for i := range s {
         out = append(out, f(s[i]))
     }
     return out
  }

From a quick search, this seems to be better explanation of this new 1.27 feature:

https://www.gopherguides.com/articles/golang-generic-methods

(That uses an example that seems similar in spirit to the Interactive Tour's example, but with a more useful type of a Stack[T] and corresponding explanation seem clearer.)

1 comment

typical182

typical182 6 hours ago

And to answer the second half of your request, here is that exact same code as above, but now using the 1.27 generic methods feature (with a runnable example using tip at https://go.dev/play/p/1YK62tGetsm?v=gotip):

  // Map maps from a slice containing type In to a slice containing type Out.
  func (s MySlice[In]) Map[Out any](f func(In) Out) []Out {
      var out []Out
      for i := range s {
          out = append(out, f(s[i]))
      }
      return out
  }

In short, you could always have methods on a generic type since Go first introduced generics in Go 1.18, but with 1.27, the methods on the generic type can also introduce their own additional type parameters.

(Previously, you could achieve the same net effect with a top-level generic function, but then the code would not be grouped as nicely as hanging it off of the type, and arguably it now can have slightly better ergonomics in some cases. You can see more of the rationale from Robert Griesemer at https://github.com/golang/go/issues/77273.)