Or perhaps a hybrid, where the methods call generic functions... On Thursday, January 20, 2022 at 7:23:37 PM UTC-5 Ian Lance Taylor wrote:
> On Thu, Jan 20, 2022 at 3:30 AM Travis Keep <kee...@gmail.com> wrote: > > > > I am working on a Pipeline[T, U any] type which receives any number of T > values and in turn emits U values. Pipelines are used to read from > databases. > > > > For instance, this code fetches the ages of 5 people under 50. > > > > int[] ages > > PeopleOrderedByName(Start[Person](). > > Map(func(p Person) int { return p.Age }). > > Filter(func(age int) bool { return age < 50 }). > > Slice(0, 5). > > AppendTo(&ages)) > > > > // Output: {49, 19, 25, 34, 42} > > fmt.Println(ages) > > > > Some notes about this code: > > > > 1. Start[Person]() returns a Pipeline[Person, Person] that emits each > Person value it receives. > > 2. the Map method on Pipeline is parameterized. What Map returns depends > on the return value of the mapper function passed in. In the example above, > Map() returns a Pipeline[Person, int] > > 3. AppendTo(&ages) returns a Consumer[Person] which takes Person values > without emitting anything. As a side effect, this Consumer[Person] appends > the ages of people under 50 to the ages slice. > > 4. Even if there are millions of people in the database, this code will > read just enough people to get 5 ages under 50. > > 5. This code won't work in 1.18 because methods in GO on a parameterized > type like Pipeline cannot be further parameterized, but in this API, the > Map() method has to be parameterized. > > > > To get around this limitation, I revised my API to look like this: > > > > int[] ages > > PeopleOrderedByName( > > SendTo[Person, int]( > > Start(). > > Map(func(person interface{}) interface{} { return person.(Person).Age }). > > Filter(func(age interface{}) bool { return age.(int) < 50 }). > > Slice(0, 5), > > AppendTo(&ages)) > > > > // Output: {49, 19, 25, 34, 42} > > fmt.Println(ages) > > > > Notes about this API: > > > > 1. Pipeline is no longer a parameterized type. Pipeline instances > receive interface{} values and emit interface{} values. > > 2. AppendTo(&ages) returns a Consumer[int] that appends all the values > it receives to the ages slice. > > 3. SendTo[Person, int](....) returns a Consumer[Person] that applies a > pipeline to the Person values it receives and then sends the emitted values > to the Consumer[int] passed as a second value. > > 4. This API suffers all the disadvantages that generics aims to solve. > The pipeline code is clunkier because values have to be constantly > converted from interface{}, Problems with type mismatches become runtime > errors instead of compile time errors, storing a Person value in an > interface results in an extra memory allocation etc. > > > > Is there a more elegant way around the limitation that methods on > parameteried types can't be parameterized? > > > I don't know about "more elegant" but I would suggest not trying to > use a fluent API. Use a function based API instead. Maybe it would > look like > > p := pipelinePeopleOrderedByName(Start[Person]() > p = pipeline.Map(p, func(p Person) int { return p.Age }) > p = pipeline.Filter(p, func(age int) bool { return age < 50 }) > p = pipeline.Slice(p, 0, 5) > ages = append(ages, pipeline.Results(p)) > > (Of course I've omitted the error handling, but your example did as well.) > > Ian > -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/golang-nuts/163d0bd5-a50f-45d0-96bb-1129685a5037n%40googlegroups.com.