On Thu, 2022-01-20 at 19:05 -0800, Mandolyte wrote:
> 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.)
> >
> > IanI saw a post at JBD's blog about making this easier: https://rakyll.org/generics-facilititators/ -- 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/7c8eba6ce27db310daec8c3a34d53cb8c233ddd6.camel%40kortschak.io.
