shapeless | Generic programming for Scala | Functional Programming library
kandi X-RAY | shapeless Summary
kandi X-RAY | shapeless Summary
shapeless is a type class and dependent type based generic programming library for Scala. It had its origins in several talks by Miles Sabin (@milessabin), given over the course of 2011, on implementing scrap your boilerplate and higher rank polymorphism in Scala. Since then it has evolved from being a resolutely experimental project into a library which, while still testing the limits of what's possible in Scala, is being used widely in production systems wherever there are arities to be abstracted over and boilerplate to be scrapped.
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QUESTION
I have been struggling for a couple days already to try to create a macro or use shapeless to create a method/function to extract field names and values as a Tuple[String, String].
Lets imagine the following case class:
...ANSWER
Answered 2021-May-14 at 20:12Based on LabelledGeneric and Keys type classes
QUESTION
I am a starter with Scala 2 Macros (before I switch to Dotty) who after trying out the shapeless type class derivation wanted to go one step beyond and write a macro that can generate a type class instances for any scala.Product without it.
(for the sake of example let's ignore nested recursive types, so my goal is flat case classes.)
My type class is an abstract class Coder[T] (e.g. trait with encode() / decode()).
So the generated code for:
...ANSWER
Answered 2021-May-07 at 13:54The way you're instantiating your class is wrong:
QUESTION
I'm trying to implement a streaming process on Kafka. The data structure that I'm using looks like this one:
...ANSWER
Answered 2021-Apr-24 at 15:15Well, case classes are Products, so if you are talking about iterating over a case class fields, and applying the same logic to them, you could do something like:
QUESTION
I'm writing macro to convert a type description into a singleton type:
...ANSWER
Answered 2021-Apr-09 at 20:16For the first question, @oleg-pyzhcov pointed out very clearly the problem: your macro is expanded when you define the class, and here the compiler can't know what actual type is used.
For the second question, @oleg-pyzhcov is right again. Indeed there is a similar example that uses implicit to solve your problem (scala 2.10.2 calling a 'macro method' with generic type not work).
So I have tried to change your code a little in order to successufly return the right type:
QUESTION
I'm trying to define a type member to be a singleton type. Here is my attempt:
...ANSWER
Answered 2021-Mar-23 at 10:40
def f(t: Test{ type Blocked = false }) = ??? // argument is parsed as t: Test{ type Blocked = Boolean }
Blocked becomes literal singleton type false; note how f(true) is rejected bellow
QUESTION
I've been trying to list all the members of a coproduct type. This answer is really close to what I want to achive:
...ANSWER
Answered 2021-Mar-17 at 19:52It's not so close. There you extract existing keys, here you create textual representation of types.
You can do
QUESTION
I have a enum that represents a container and two case classes:
...ANSWER
Answered 2021-Mar-11 at 13:45Well if you don't mind a bit of casting, you can do this:
QUESTION
Scala 3 provides polymorphic functions and Tuples similar to shapeless HList:
ANSWER
Answered 2021-Mar-11 at 12:43Shapeless map is quite a bit more magical than Scala 3 tuple map, the signature of latter being:
QUESTION
Given an arbitrary tuple, I want to extract a subset of the tuple. The signature would look something like:
...ANSWER
Answered 2021-Mar-03 at 15:11This looks like a job for typeclasses! (note that the only Scala 3-specific feature required is *:)
QUESTION
I am working on a side project, trying to implement immutable aggregates in Scala. My idea is to have base trait AggregateRoot with some common behaviors. Child classes would be actual aggregates modeled as a case classes. For now, there is one thing that I don't like, and that is that I cannot call copy method from the base trait for two reasons:
- I don't have access to
copymethod inside of the base trait - I have no idea how many parameters
copymethod will have
I have some basic knowledge of shapeless library and I thought that it might help in this case. My idea is pass list of tagged fields to the base method in the trait which will replace them and return new instance of the case class. As a step in that direction I am trying to create method that will copy one single field for the beginning using shapeless, but I am getting the same error all the time, that compiler cannot find the implicit for updater.
Here is the simplified code fragment that I am trying to use:
...ANSWER
Answered 2021-Mar-04 at 07:15As far as I understood from this great answer: How to generically update a case class field using LabelledGeneric? - you can't have Updater in general case, because Shapeless need to derive it for each particular field in case class, which means instead of having general purpose makeCopy you will need to have method for each particular field, like:
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