On Thu, Oct 19, 2023 at 5:14 PM Jason E. Aten <j.e.a...@gmail.com> wrote:
>
> Analogous to
>
> type IntSlice []int
>
> func (p IntSlice) Len() int { return len(p) }
> func (p IntSlice) Less(i, j int) bool { return p[i].val < p[j].val }
> func (p IntSlice) Swap(i, j int) { p[i], p[j] = p[j], p[i] }
>
> and then calling sort.Sort() on an IntSlice, I figured I
> could create a type that is a new name for an instantiated generic type and
> then call methods on that new type... but no?
>
> For example:
> https://go.dev/play/p/K5FGtNi6hLM
>
> package main
>
> import "fmt"
>
> type Addable interface {
> ~complex128 | ~complex64 | ~float64 | ~float32 |
> ~byte | ~uint16 | ~uint32 | ~uint64 |
> ~int8 | ~int16 | ~int32 | ~int64 | ~int
> }
>
> type Matrix[T Addable] struct {
> Nrow int
> Ncol int
> Dat []T
> }
>
> func (m *Matrix[T]) Col(j int) (res []T) {
> for i := 0; i < m.Nrow; i++ {
> res = append(res, m.At(i, j))
> }
> return
> }
>
> // At reads out the [i,j]-th element.
> func (m *Matrix[T]) At(i, j int) T {
> return m.Dat[i*m.Ncol+j]
> }
>
> func main() {
> m := &Matrix[float64]{
> Nrow: 2,
> Ncol: 3,
> Dat: make([]float64, 2*3),
> }
> fmt.Printf("m.At(1,2) = '%v'", m.At(1, 2)) // this is fine, of course.
> }
>
> // I assumed this would work... but go 1.21 rejects the call to At() below.
> type MatrixF64 Matrix[float64]
>
> func UseMatrixF64(m *MatrixF64) {
> // so why won't this compile? if the same line in main() works?
> fmt.Printf("m.At(1,2) = '%v'", m.At(1, 2)) // compile error: m.At undefined
> (type *MatrixF64 has no field or method At)
> }
Using "type MatrixF64 ..." defines a new type that doesn't inherit any
of the method of the old type. That's how type definitions work in
Go. It will work as you expect if you use a type alias: "type
MatrixF64 = ...".
Ian
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