Projection

[jamesgarfield]

Projection

Projection is a reference to both the deeper level of inference that is involved (projecting what will be needed) and to one of the key uses of the technique (projecting a set of one type into a set of another)

In that it differs greatly from mere implementation, it may require a separate sub-command for write (goast write project)

Consider the following User and Users definitions. It could reasonably be inferred that one might want to project a given Users into []string (slice of names) or []int (slice of ages)

type User struct {
    Name string
    Age  int
}
type Users []*User

With projection the following generic code could be transformed into multiple, distinct operations on Users

type X interface{}
type T struct {
    _ X
}
type Slice []*T

func (s Slice) MapTo_(fn func(T) X) (result []X) {
    for _, v := range s {
        result = append(result, fn(v))
    }
    return
}

Becomes

func (s Users) MapToString(fn func(*User) string) (result []string) {
    for _, v := range s {
        result = append(result, fn(v))
    }
    return
}

func (s Users) MapToInt(fn func(*User) int) (result []int) {
    for _, v := range s {
        result = append(result, fn(v))
    }
    return
}