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
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