Questions tagged [shapeless]
shapeless is (an exploration of) a type class and dependent type based generic (aka polytypic/polymorphic) programming library for Scala.
shapeless
1,037
questions
274
votes
1
answer
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views
How to use Shapeless in a Quasiquote?
I'm trying to call a Shapeless macro from inside a quasiquote with Scala and I'm not getting what I would like to get.
My macro doesn't return any errors but it doesn't expand Witness(fieldName) ...
- 21.9k
151
votes
2
answers
5k
views
Limits of Nat type in Shapeless
In shapeless, the Nat type represents a way to encode natural numbers at a type level. This is used for example for fixed size lists. You can even do calculations on type level, e.g. append a list of ...
- 12.5k
150
votes
4
answers
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Are HLists nothing more than a convoluted way of writing tuples?
I am really interested in finding out where the differences are, and more generally, to identify canonical use cases where HLists cannot be used (or rather, don't yield any benefits over regular lists)...
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113
votes
4
answers
13k
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Any reason why scala does not explicitly support dependent types?
There are path dependent types and I think it is possible to express almost all the features of such languages as Epigram or Agda in Scala, but I'm wondering why Scala does not support this more ...
74
votes
3
answers
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Can someone explain to me what the Shapeless library is for? [closed]
Can someone explain to me in simple terms what the Shapeless library is for?
Scala has generics and inheritance functionality so I'm a bit confused what Shapeless is for.
Maybe a use case to clarify ...
- 14.6k
53
votes
0
answers
1k
views
Materialize the Value for a Type that has One Inhabitant
Thanks to @MilesSabin's answer I can write a type level Fibonacci sequence:
sealed trait Digit
case object Zero extends Digit
case object One extends Digit
sealed trait Dense { type N <: Dense }
...
- 1,741
52
votes
1
answer
4k
views
Can't prove that singleton types are singleton types while generating type class instance
Suppose I've got a type class that proves that all the types in a Shapeless coproduct are singleton types:
import shapeless._
trait AllSingletons[A, C <: Coproduct] {
def values: List[A]
}
...
- 139k
49
votes
5
answers
4k
views
Testing an assertion that something must not compile
The problem
When I'm working with libraries that support type-level programming, I often find myself writing comments like the following (from an example presented by Paul Snively at Strange Loop ...
- 139k
48
votes
1
answer
2k
views
Why is the Aux technique required for type-level computations?
I'm pretty sure I'm missing something here, since I'm pretty new to Shapeless and I'm learning, but when is the Aux technique actually required? I see that it is used to expose a type statement by ...
- 1,284
43
votes
3
answers
18k
views
Use functional combinators on Scala Tuples?
'map' preserves the number of elements, so using it on a Tuple seems sensible.
My attempts so far:
scala> (3,4).map(_*2)
error: value map is not a member of (Int, Int)
(3,4).map(_*2)
...
- 8,552
42
votes
1
answer
1k
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Using the "Prolog in Scala" to find available type class instances
Considering https://speakerdeck.com/folone/theres-a-prolog-in-your-scala, I would like to "abuse" the Scala type system to find all instances of e.g. CanBuildFrom that match a given criteria. Prolog ...
- 37.5k
41
votes
1
answer
2k
views
What does `T {}` do in Scala
Browsing Shapeless code, I came across this seemingly extraneous {} here and here:
trait Witness extends Serializable {
type T
val value: T {}
}
trait SingletonOps {
import record._
type T
...
- 33.1k
30
votes
4
answers
21k
views
How to append or prepend an element to a tuple in Scala
I have a tuple and want to add an element without loosing type safety. This is what I want to achieve:
val tuple = ("", 1, 1f) // (String, Int, Float)
val newTuple:(String, Int, Float, Double) = ...
- 11.2k
30
votes
3
answers
5k
views
Can Map be performed on a Scala HList
I have done a few implementations of HList now. One based on Daniel Spiewak's High Wizardry in the Land of Scala talk and another based on a post in Apocalisp blog. The goal was to have a ...
- 1,081
25
votes
5
answers
3k
views
Type based collection partitioning in Scala
Given the following data model:
sealed trait Fruit
case class Apple(id: Int, sweetness: Int) extends Fruit
case class Pear(id: Int, color: String) extends Fruit
I've been looking to implement a ...
- 2,608
24
votes
2
answers
3k
views
Shapeless - turn a case class into another with fields in different order
I'm thinking of doing something similar to Safely copying fields between case classes of different types but with reordered fields, i.e.
case class A(foo: Int, bar: Int)
case class B(bar: Int, foo: ...
- 8,534
24
votes
1
answer
3k
views
Shapeless: Generic.Aux
I'm trying to understand how Generic works (and TypeClass too). The github wiki is very sparse on examples and documentation. Is there a canonical blog post / documentation page describing Generic and ...
- 667
24
votes
1
answer
6k
views
how do i start learning shapeless concepts in scala [closed]
I would like to learn about polytypic concepts in Scala, I came across shapeless library what would be the best starting point for learning and applying shapeless.
- 3,641
22
votes
2
answers
2k
views
Weird behavior trying to convert case classes to heterogeneous lists recursively with Shapeless
I stayed up way too late last night trying to figure out this Shapeless issue and I'm afraid it's going to eat my evening if I don't get it off my chest, so here goes.
In this minimized version I'm ...
- 139k
21
votes
1
answer
8k
views
Getting subclasses of a sealed trait
Is it possible (via macros, some form of Shapeless automagic or otherwise) to obtain a list of the subclasses of a sealed trait:
At compile time?
At runtime?
- 966
20
votes
1
answer
4k
views
Converting Map[String,Any] to a case class using Shapeless
The question here asks about mapping a case class to a Map[String,Any]. I was wondering what would be the other way around, converting Map[String,Any] to a case class. Given the following map:
val mp ...
- 1,969
19
votes
5
answers
10k
views
Converting a tuple of options to an option of tuple with Scalaz or Shapeless
Having
(Some(1), Some(2))
I expect to get
Some((1, 2))
and having
(Some(1), None)
I expect to get
None
- 43.1k
19
votes
2
answers
3k
views
How to define a function whose output type depends on the input type
Given the following classes:
case class AddRequest(x: Int, y: Int)
case class AddResponse(sum: Int)
case class ToUppercaseRequest(str: String)
case class ToUppercaseResponse(upper: String)
How do I ...
- 1,538
19
votes
2
answers
4k
views
Converting nested case classes to nested Maps using Shapeless
I am trying to solve [this][1] question using Shapeless, in summary it's about converting a nested case class to Map[String,Any], here is the example:
case class Person(name:String, address:Address)
...
- 1,969
19
votes
1
answer
4k
views
Extract label values from a LabelledGeneric instance
Consider the following example:
import shapeless._
case class Foo(bar: String, baz: Boolean)
val labl = LabelledGeneric[Foo]
Now, the type of labl is (prettified)
LabelledGeneric[Foo] {
type ...
- 107k
18
votes
7
answers
15k
views
Different types in Map Scala
I need a Map where I put different types of values (Double, String, Int,...) in it, key can be String.
Is there a way to do this, so that I get the correct type with map.apply(k) like
val map: Map[...
- 2,132
18
votes
2
answers
4k
views
Understanding the Aux pattern in Scala Type System
This question may be asked and answered before, but I would like to understand this with an example and I could not reason out where the Aux pattern might be helpful! So here is the trait:
trait Foo[...
- 14.6k
18
votes
2
answers
3k
views
Passing a Shapeless Extensible Record to a Function
I am trying to learn Shapeless (using version 2.10.2). I have created a very simple extensible record:
val rec1 = ("foo" ->> 42) :: HNil
According to the REPL, this has type
shapeless.::[Int ...
- 8,372
18
votes
2
answers
2k
views
Sequencing an HList
Given a Shapeless HList where every list element shares the same type constructor, how can the HList be sequenced?
For example:
def some[A](a: A): Option[A] = Some(a)
def none[A]: Option[A] = None
...
- 3,855
17
votes
1
answer
945
views
What are the important features of the shapeless API (in Scala), and what do they do?
I'm trying to learn shapeless (2.0.0). It seems like an amazing tool and I'm very excited about it, but I am having problems moving forward. Because there is not yet much documentation, I've been ...
- 4,011
16
votes
2
answers
2k
views
Refined and existential types for runtime values
Suppose I want to map between some strings and integer identifiers, and I want my types to make it impossible to get a runtime failure because someone tried to look up an id that was out of range. ...
- 139k
15
votes
2
answers
4k
views
Pattern matching with shapeless coproduct
Can I use pattern matching with shapeless coproducts?
import shapeless.{CNil, :+:}
type ListOrString = List[Int] :+: String :+: CNil
def f(a: ListOrString): Int = a match {
case 0 :: second :: ...
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15
votes
1
answer
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Scala Function.tupled and Function.untupled equivalent for variable arity, or, calling variable arity function with tuple
I was trying to do some stuff last night around accepting and calling a generic function (i.e. the type is known at the call site, but potentially varies across call sites, so the definition should be ...
- 1,633
14
votes
1
answer
2k
views
Extractor for a shapeless HList that mimics parser concatenation `~`
Question
Is it somehow possible to create an extractor for shapeless' HList that looks like the following.
val a ~ _ ~ b = 4 :: "so" :: 4.5 :: HNil
=> a == 4 && b == 4.5
Replace :...
- 28.5k
14
votes
3
answers
2k
views
What is "at" in shapeless (scala)?
I've seen an object (probably a function) called "at" sprinkled throughout the shapeless source and in code that uses shapeless. In particular, it is used in the answer to this other question. Here ...
- 4,011
14
votes
2
answers
909
views
How to represent a partial update on case classe in Scala ?
I have the following case class :
case class PropositionContent(title:String,content:String)
And I would like to represent a partial modification of it as Data.
One way would be to create the ...
- 1,749
14
votes
1
answer
1k
views
Creating an HList of all pairs from two HLists
I'm using shapeless in Scala, and I'd like to write a function allPairs that will take two HLists and return an HList of all pairs of elements. For example:
import shapeless._
val list1 = 1 :: "one" :...
- 3,590
14
votes
2
answers
1k
views
Dynamically creating Akka Stream Flows at Runtime
I'm currently trying to dynamically create Akka Stream graph definitions at runtime. The idea being that users will be able to define flows interactively and attach them to existing/running ...
- 151
13
votes
2
answers
4k
views
Type casting using type parameter
Given is a Java method that returns java.lang.Objects for a given string. I'd like to wrap this method in a Scala method that converts the returned instances to some type T. If the conversion fails, ...
- 4,823
13
votes
1
answer
1k
views
Explain the `LowPriorityImplicits` pattern used in Scala type-level programming
When looking at the source of some Scala libraries, e.g. shapeless, I often find traits named LowPriorityImplicits.
Can you please explain this pattern? What is the problem that is solved, and how ...
- 28.5k
13
votes
2
answers
3k
views
Safely copying fields between case classes of different types
Assuming you have case classes like the following
case class Test1(a:String,b:Int,c:Char)
case class Test2(a:String,b:Int)
And you instantiate the classes with the following variables
val test1 = ...
- 1,889
13
votes
1
answer
489
views
Proving associativity of natural number addition using Scala shapeless
The following code is Idris:
natAssociative : (a : Nat) -> (b : Nat) -> (c : Nat) -> (a + b) + c = a + (b + c)
natAssociative Z b c = the (b + c = b + c) refl
natAssociative (S k) b c = ...
- 45.9k
13
votes
1
answer
966
views
shapeless HList to TupleN where the tuple shape need not exactly match the HList shape
I would like to create the equivalent of:
def toTupleN[A1, ..., AN, L <: HList](l: L): TupleN[A1, ..., AN]
Code using toTupleN should compile only when there is exactly one N combination of ...
- 13.3k
12
votes
2
answers
2k
views
How to get the name of a case class field as a string/symbol at compile time using shapeless?
I'd like to somehow get at compile time the name of a field of a case class in a val (possibly a singleton-typed string or symbol?).
Something like the following:
import shapeless._
case class ...
- 5,496
12
votes
1
answer
1k
views
Using Slick with shapeless HList
Slick's support for HList is generally a great thing. Unfortunately, it comes with its own implementation that does barely provide any useful operations. I'd therefore like to use the shapeless HList ...
- 6,998
12
votes
1
answer
2k
views
Mapping over Shapeless record
In a Play application I'm working on, I'm trying to improve our system for processing flags, some of which are meant to be persistent options as a user navigates our app via links. I'd like to use ...
- 35.6k
11
votes
2
answers
2k
views
How to shapeless case classes with attributes and typeclasses?
I am currently implementing a library to serialize and deserialize to and from XML-RPC messages. It's almost done but now I am trying to remove the boilerplate of my current asProduct method using ...
- 784
11
votes
2
answers
898
views
What is the purpose of the emptyCoproduct and coproduct methods of the TypeClass trait in Shapeless
It is not fully clear to me what is the purpose of the emptyCoProduct and coproduct methods of the TypeClass trait in Shapeless.
When would one use the TypeClass trait instead of the ProductTypeClass?...
- 3,013
11
votes
3
answers
2k
views
Shapeless: map from coproduct to different coproduct
In the following, I'm trying to make a polymorphic function to convert a RawFeatureValue into a RefinedFeatureValue.
import shapeless._
object test {
type RawFeatureValue = Int :+: Double :+: ...
- 8,534
11
votes
2
answers
2k
views
Update case class from incomplete JSON with Argonaut or Circe
I need to create an updated instance from a case class instance (with any needed DecodeJsons implicitly derived), given an incomplete json (some fields missing). How can this be accomplished with ...
- 811