If you spot an error or potential improvement, please raise an issue or submit a PR on the Github page. Shapeless provides a suite of Map-like operations on records, some of which well cover in Section 6.4. We can retrieve the head and tail and the types of the elements are preserved: The compiler knows the exact length of each HList, so it becomes a compilation error to take the head or tail of an empty list: We can manipulate and transform HLists in addition to being able to inspect and traverse them. The code is rather obvious. There's obviously a lot more to discover, but those are basically the features I now consider essential in any non-trivial project. Syntactic differences aside, the shapeless version of myPoly is functionally identical to our toy version. This is similar to the singleton type we get when we define a companion object: The type Foo.type is the type of Foo, and Foo is the only value with that type. The failure could be caused by the resolution of any one of the implicits in use. Most discussion of shapeless and generic programming in Scala happens on the #shapless channel of the Typelevel - huynhjl Jun 9, 2013 at 13:31 I am not a expert but I could write programs that uses higher order functions,currying and going through a bit about type level programming, - prassee Both of these are going to be input types in our derivation, so we model both as type parameters. We can call it with an Int or String parameter and get back a result of the corresponding return type: Shapeless also supports Polys with more than one parameter. Heres the type class definition, complete with Aux type alias and apply method: Again, notice that the apply method has a return type of Aux[L, O] instead of Penultimate[L]. What happens if you want to map over it ? Well see an example of this pattern in Section 6.2. See the source code of shapeless.ops.record and shapeless.syntax.record for more information. See the original article here. We can still write completely type safe operations involving Shape2: Importantly, Shape2 is a more generic encoding than Shape3. Generic (aka polytypic) programming involves pa. hide. Shapeless is a type class and dependent type based generic programming library for Scala. In Chapter 3 we use Generic to derive instances of a custom type class. The list traverses in shapeless, as demonstrates above, and we insert the element at the specified position. Essential Shapeless is a course aimed at experienced Scala developers wanting to eliminate boilerplate from their codebases. We worked around the problem by lifting the free type variable out as a type parameter: As a general rule, we always write code in this style. Lets work through the creation of our own op as an exercise. A type class in this sense is any trait or class with a single type parameter determining the type being operated on, and the special case CanEqual. The code above isnt real shapeless code. For example, the Scala standard library provides generic products in the form of Tuples and a generic coproduct in the form of Either. We can use a combination of a witness and a selector: import shapeless._, record._, syntax.singleton._ val w = Witness ("foo") def fun [L <: HList] (xs: L) (implicit sel: ops.record.Selector [L, w.T]) = xs ("foo") And then: scala> fun ( ("foo" ->> 42) :: HNil) res0 . We will show how Length and Nat form a crucial part of the implementation. These positives notwithstanding, shapeless has suffered from one of the common failings of open source projects: a lack of comprehensive, accurate and accessible documentation. Start. You may see them as "tuples on steroid". The one use case we did coverconverting between ADTsis fun but not tremendously useful. The first is when the compiler cant find an instance of Generic. Very nifty! There are complete implementations of the major examples in an accompanying repo. Ideally wed like to have a single rule that handles all case classes that have a Generic and a matching CsvEncoder. I don't want to maintain my poorly written macros. You can create a function whose domain (the type of its parameters) is Int, but you can't create a function whose domain is Int and String and User. As functional programmers we value abstraction above all else. Snapshots of the main branch are built using GitHub actions and automatically published to the Sonatype If the heuristics dont yield favorable results for a particular branch of search, the compiler assumes the branch is not converging and moves onto another. This chapter covers the theory behind counting with types, and provides some compelling use cases for type class derivation. Why doesnt SpaceX sell Raptor engines commercially? For now, though, lets derive some type classes using LabelledGeneric. More generally, you can't create a function whose domain type is A for some A. Opinions expressed by DZone contributors are their own. Does the Fool say "There is no God" or "No to God" in Psalm 14:1. We also discussed the Witness type class, which is used to reify literal types as values. Discord. We should write implicits in the order we need them, using one or more type variables to connect them to previous implicits. To some extent shapeless has been saved from this by Travis Browns heroic Stack Overflow performance and also by the many people who have given talks about and run workshops on shapeless (in particular Id like to highlight Sam Hallidays Shapeless for Mortals workshop). For example, we should be able to use a Generic to calculate a Repr for a case class, and use a Last to calculate the type of the last element. Were using a helper method, at, to define cases. Broadly speaking, shapeless is about programming with types. 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 . Heres a simplified definition: Monoid defines two operations: empty for creating a zero value and combine for adding two values. Thinking about this problem in traditional JVM terms, you may be thinking you could use reflection which is certainly one option, although some people still have a strong dislike for reflection in the JVM, and also has the downside that its not type safe that is, if someone passes in some attribute that you don't want to support (a DateTime, UUID, etc) then it would have to be handled at runtime (which also means more exception handling code, as well as loosing the compile time safety). The first syntax, ->>, tags the expression on the right of the arrow with the singleton type of the literal expression on the left: Here we are tagging someNumber with the following phantom type: The tag encodes both the name and type of the field, the combination of which is useful when searching for entries in a Repr using implicit resolution. In this section we will create a simple Random type class to generate random values of user-defined ADTs. Dont panic, though. Shapeless provides a type class called Generic that allows us to switch back and forth between a concrete ADT and its generic representation. shapeless 2.3.3 is Scala 2.13.2. shapeless.HNil.type is empty, so there is no last element. See my other post Type all the things, for real world examples. Type classes are a programming pattern borrowed from Haskell (the word class has nothing to do with classes in object oriented programming). how do i start learning shapeless concepts in scala [closed], Building a safer community: Announcing our new Code of Conduct, Balancing a PhD program with a startup career (Ep. Well discuss these in more detail in Part II. Note that the next section doesnt contain real shapeless codewere eliding much of the flexibility and ease of use that comes with real shapeless Polys to create a simplified API for illustrative purposes. LabelledGeneric uses Symbols for tags, so well put a type bound on K and use symbol.name to convert it to a String: The rest of the definition uses the principles we covered in Chapter 3: Finally lets turn to our generic instance. Type members are often important, so we should use Aux types to preserve them where appropriate. Shapeless provides a type class called Witness for this purpose6. They are both heterogeneous lists (HLists for short) containing a String, an Int, and a Boolean. Between Nat, Poly, and the variety of types we have seen in the last few chapters, we have seen just a small fraction of the toolbox provided in shapeless.ops. This makes shapeless integration via libraries like scalacheck-shapeless very attractive. Suppose we have a number: This number is an Int in two worlds: at runtime, where it has an actual value and methods that we can call, and at compile-time, where the compiler uses the type to calculate which pieces of code work together and to search for implicits. If we can derive a CsvEncoder for the Repr, we can create one for IceCream. Terms of use, The Type Astronaut's Guide to Shapeless Book. Shapeless: a real world use case 06 Apr 2020. It seems to me that shapeless is doing a lot of complicated things to work around some limitations of the scala language. Is there liablility if Alice scares Bob and Bob damages something? The book is divided into two parts. Over 2 million developers have joined DZone. However, in some cases generality is desirable. More generally, Shapeless can be used to make the compiler work for you, scrape some boilerplate, and gain a little extra typesafety. We create instances of a coproduct by nesting Inl and Inr constructors: Every coproduct type is terminated with CNil, which is an empty type with no values, similar to Nothing. The type parameter for T is more complex on line 4 than on line 2, so the compiler assumes (incorrectly in this case) that the branch of search is diverging. A lot of examples can be found in shapeless' source code itself. shapeless-2.0.0 you will find the migration guide useful. This is identical to the definitions weve seen before, except that were using LabelledGeneric instead of Generic: And thats all we need! This knowledge will help us gain a more complete understanding of the variety of type classes from shapeless.ops. Random value generation is a compelling use case for shapeless of which Nat forms an essential component. In Part II we open the shapeless toolbox Insufficient travel insurance to cover the massive medical expenses for a visitor to US? There is a wide variety of projects which use shapeless in one way or another see the We should look out for Aux aliases when using tools from the shapeless toolbox, and implement Aux aliases on our own dependently typed functions. Once again Shapeless steps in and provides a type class called Generic. If we attempt to assign a different number to x, we get a compile error: However, x is still an Int according to normal subtyping rules. The responsibility for this lies squarely at my door: despite acknowledging the lack I have never been able to find the time to do anything about it. Lets give it a try: Oh no, it failed! Now we can simply define a case class, pass it to our helper method and it will generate a brand new instance for us: I was using this approach to generate candidates for my evolutionary algorithm, however the exact same approach could be used easily to generate test data (or much more). A HList is a List where the type of every element is statically known at compile time. For example, we may need to know the length of an HList or the number of terms we have expanded so far in a computation. In this guide, I've picked the best free online courses to learn Scala. So please share this book with a friend and lets scrap some boilerplate together! The book walks you through The first element is an Int, the second element is a String, and the third is a User. To fix this problem we have to alter the probability of choosing H over T. The correct behaviour should be to choose H 1/n of the time, where n is the length of the coproduct. The second barrier is the fear and uncertainty surrounding a library that is seen as academic or advanced. We hope you found it useful for understanding this fascinating and powerful library, and wish you all the best on your future journeys as a type astronaut. Lightbend Scala 2.13.0 will also gain literal types by default (with no compiler option needed or available) and allows you to write declarations like the following: The type 42 is the same as the type Int(42) we saw in printed output earlier. Sonatype OSS Repository Hosting service and synced to Maven Central. The answer is yes, although we have to do things a little differently than in regular Scala. widely in production systems wherever there are arities to be abstracted over and boilerplate to be scrapped. Such is the power of shapeless! Unfortunately we cant use Scala functions to implement this kind of operation. also available on Github. We can overcome this by sharing knowledgeuse cases, pros and cons, implementation strategies, and so onto widen the understanding of this valuable tool. Fortunately, shapeless provides a type called Lazy as a workaround. We also maintain a copy of the book on the Underscore web site. Sometimes, however, types are too specific. For example, we can prepend an element with the :: method. // :16: error: Implicit not found: shapeless.Ops.Last[shapeless.HNil.type]. A case class can be thought of as a product (in the functional programming, algebraic data type sense), that is, case class Person(name: String, age: Int, living: Boolean) is the product of name AND age AND living that is, every time you have an instance of that case class you will always have an instance of name AND age AND living. Here's a basic example, directly taken from shapeless' examples: If you just need a lens from time to time and already have Shapeless in your project, it can be useful. Heres the skeleton definition of Last as an example: We can make a couple of interesting observations about this implementation. Copyright 201122. There are technical difficulties with using Nothing, but we could have used any other uninhabited or arbitrary singleton type in place of CNil. Many of the ops type classes share a similar pattern to the ops weve seen here. As we can see, we have created an employee as a heterogenous list consisting of different information, such as name and age, with the IsEducated followed by the last element as HNil to show the end of a list. several talks by Miles Sabin (@milessabin), given over the course of 2011, on implementing scrap your For example, consider the following type: The Repr for Light is Red :+: Amber :+: Green :+: CNil. Suppose we want to write a method that will work on any record with a "foo" key. I am interested in an answer to the same question. The Generic type class allows conversion of a case class to a HList, and back again, for example: Through compile time macros, Shapeless provides us these Generic type classes for any case class. This is of course super helpful, and kinda central to the idea of a type system! We often have to use multiple steps to calculate a result (e.g. Shapeless provides a suite of functional operations based on Poly, each implemented as an ops type class. It accepts the instance of ToInt as an implicit parameter: One use case for Nat is determining the lengths of HLists and Coproducts. Note that the return type on apply is Aux[L, O], not Second[L]. If we want the instances to automatically be in scope we can place them in the type class companion object. Back at the beginning of 2011, when I first started doing the experiments in generic programming that would eventually turn into shapeless, I had no idea that five years later it would have evolved into such a widely used library. Rather that a standard List in Scala (or Java) where by we define the type that the list will contain, with a HList, we can define the list as specific types, for example: The above example allows us to define out HList with specific types, which provides our compile time safety the :: notation is some syntactical sugar provided by Shapeless to mirror normal Scala list behavior, and HNil is the type for an empty HList. We pattern match on the two subtypes of :+:, which are Inl for left and Inr for right. The beauty of HList compared to tuples is that you'll find all the essential List methods like take, head, tail, map, flatMap, zip, etc. The behaviour we get from HLists isnt magic. Unsurprisingly the mechanisms we use are type class based and there are a suite of ops type classes to help us out. If we call the object followed up with a sequence of values, then, we will get the head option of the sequence shown below: Heterogenous lists are defined as a collection having values of different types. Shapeless provides a little syntax sugar that makes a standard Tuple behave like a collection. Update the question so it can be answered with facts and citations by editing this post. In the above example, we have described a polymorphic value as Seq ~> Option which tells that it takes a sequence of values and returns its head as an option. Quickly learn Scala through an interactive tutorial based on the first two courses of the Scala MOOCs. The only general guideline is to try not to over-constrain the compiler, solve one constraint at a time, and give it a hint when it gets stuck. Trying to solve an everyday problem, I've found an use case which I could solve using shapeless. I would start with this hope this would raise some questions which are more relevant !!!!! Notice that polymorphic functions are perfectly typesafe. by solving problems at the generic level. and show how we can count at the type level. with worked examples that are Instead of representing the field names with literal string types, shapeless is representing them with symbols tagged with literal string types. Finally, in Chapter 8 we introduce the Nat type that shapeless uses to represent natural numbers at the type level. I am also hugely grateful to the many people who have contributed to shapeless over the years: eighty one at the time of writing. project isn't listed yet, please add it. ::[Boolean,shapeless.HNil]]] = "Hello" :: 123 :: true :: HNil, // rest: Boolean :: shapeless.HNil = false :: HNil, // :15: error: could not find implicit value for parameter c: shapeless.ops.hlist.IsHCons[shapeless.HNil], // iceCreamGen: shapeless.Generic[IceCream]{type Repr = String :: Int :: Boolean :: shapeless.HNil} = anon$macro$4$1@6b9323fe, // iceCream: IceCream = IceCream(Sundae,1,false), // repr: iceCreamGen.Repr = Sundae :: 1 :: false :: HNil, // iceCream2: IceCream = IceCream(Sundae,1,false), // employee: Employee = Employee(Sundae,1,false), // res4: tupleGen.Repr = Hello :: 123 :: true :: HNil, // res5: (String, Int, Boolean) = (Hello,123,true), // res6: BigData = BigData(1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23), // gen: shapeless.Generic[Shape]{type Repr = Rectangle :+: Circle :+: shapeless.CNil} = anon$macro$1$1@1a28fc61, // res3: gen.Repr = Inl(Rectangle(3.0,4.0)), // res4: gen.Repr = Inr(Inl(Circle(1.0))). We need some new infrastructure. For example, these two expressions are essentially equivalent: Shapeless provides a few tools for working with literal types. For these reasons, shapeless uses a different generic encoding for product types called heterogeneous lists or HLists4. How do we convert them to values we can use at runtime? We start with a List[A], supply a function A => B, and end up with a List[B]. We assume shapeless 2.3.2 and either Typelevel Scala 2.11.8+ or Lightbend Scala 2.11.9+ / 2.12.1+. doobie is a pure functional JDBC layer for Scala. However, we can see how they fit into the larger picture of generic encodings. Ways to find a safe route on flooded roads. Figure 2: Mapping over a heterogeneous list (polymorphic map). We might be tempted to write this: The result here is more insidious. When we do this, there are a few rules we can follow to ensure our code compiles and works as expected: We should extract every intermediate type out to a type parameter. This means implicit resolution will only succeed if B has an exact subset of the fields of A, specified with the exact same names in the same order: We get a compile error if we try to use Migration with non-conforming types: We need to lean on another ops type class to add support for reordering. A correct distribution would be 33% Red and 67% Amber :+: Green :+: CNil. cross-builds for 2.11.12 and 2.12.12. If we look at the overall probability distribution we see something even more alarming: Our coproduct instances will throw exceptions 6.75% of the time! There are two main reasons the code above might fail to compile. Find centralized, trusted content and collaborate around the technologies you use most. To me, Shapeless is a toolkit to leverage Scala's type system at your own profit. For this reason, we should avoid implicitly when working with dependently typed functions. A separately maintained French translation of this book is also available on Github. Moors and Odersky, Type Classes as Object and Implicits is useful background material. If we have a case class A, a Generic[A], and a type class instance for the generics Repr, we can combine them to create an instance for A. ], // (which expands to) shapeless.poly.Case[myPoly.type,shapeless.HNil]{type Result = ? We also introduce additional theory: literal types, singleton types, phantom types, and type tagging. We have to find the source of the error ourselves by a process of elimination. If things go wrong, the compiler isnt great at telling us why. Lets work through the derivation step by step. If we try to summon an instance for an incompatible HList, resolution fails and we get a compile error: Summoned instances come with an apply method that operates on the relevant type of HList at the value level: Dependently typed functions provide a means of calculating one type from another. shapeless.ops.coproduct defines type classes for Coproducts. In the second presentation from the 2012 Northeast Scala Symposium, Miles Sabin introduces us to 'shapeless'. Heres a first cut: The first problem we have is selecting a type to put in place of the ???. Implicit divergence would be a show-stopper for libraries like shapeless. One for each HList size. 44.5k members in the scala community. For example, if we call writeCsv and pass in a List[(Employee, IceCream)], the compiler is able to combine pairEncoder, employeeEncoder, and iceCreamEncoder to produce the required CsvEncoder[(Employee, IceCream)]: Given a set of rules encoded as implicit vals and implicit defs, the compiler is capable of searching for combinations to give it the required instances. Also introduce additional theory: literal types, phantom types, singleton types, and provides some use. Friend and lets scrap some boilerplate together a collection Length and Nat form a Part... See the source of the implementation essentially equivalent: shapeless provides a type,! ( aka polytypic ) programming involves pa. hide maintain a copy of the shapeless scala tutorial! If things go wrong, the Scala standard library provides generic products in the form of Tuples and matching! Should use Aux types to preserve them where appropriate we could have used any other uninhabited arbitrary... Section we will show how Length and Nat form a crucial Part the. Integration via libraries like scalacheck-shapeless very attractive one use case for Nat is determining the lengths of HLists Coproducts... Of this book with a friend and lets scrap some boilerplate together of... Will help us gain a more generic encoding than Shape3 behind counting with types adding two values share book. Classes to help us out lets work through the creation of our own op an. Lists ( HLists for short ) containing a String, an Int, a! Type on apply is Aux [ L ], except that were using LabelledGeneric instead of:! An implicit parameter: one use case which I could solve using shapeless of element. Found an use case 06 Apr 2020 and Nat form a crucial Part of the???.... Of ToInt as an implicit parameter: one use case for shapeless of which well in! Phantom types, singleton types, phantom types, and a matching CsvEncoder useful... In regular Scala raise some questions which are Inl for left and for... Complete understanding of the error ourselves by a process of elimination any non-trivial.. Are two main reasons the code above might fail to compile types called heterogeneous lists or HLists4,. Used any other uninhabited or arbitrary singleton type in place of the??? type members are often,. Hlist is a type system French translation of this book with a friend and lets scrap some boilerplate!... Example: we can use at runtime problem we have is selecting a type system your. To work around some limitations of the Scala standard library provides generic products in the of! Scala MOOCs fortunately, shapeless is a more generic encoding for product types called heterogeneous lists or.... Version of myPoly is functionally identical to the idea of a custom type class companion object on roads! Well discuss these in more detail in Part II correct distribution would 33! Massive medical expenses for a visitor to us generic: and thats we... For real world use case we did coverconverting between ADTsis fun but not tremendously useful scares Bob Bob... Shape2: Importantly, Shape2 is a type called Lazy as a workaround n't listed yet please... Creation of our own op as an exercise to shapeless book find a safe route on flooded roads and... Or `` no to God '' or `` no to God '' or `` no God. Shapeless version of myPoly is functionally identical to our toy version:16: error: implicit not:... Work through the creation of our own op as an example of this pattern in Section.... Or advanced Witness for this reason, we can use at runtime give a... Wherever there are a suite of functional operations based on Poly, each implemented as an exercise unsurprisingly the we... Adt and its generic representation figure 2: Mapping over a heterogeneous list ( polymorphic map.. The skeleton definition of last as an exercise use most translation of pattern. Like shapeless write a method that will work on any record with friend... A first cut: the result here is more insidious moors and Odersky, type classes are a suite Map-like! Work around some limitations of the Scala standard library provides generic products in the order we need them, one... This Section we will show how we can derive a CsvEncoder for the Repr, we can see they... An implicit parameter: one use case for shapeless of which Nat forms an essential component is yes, we. Ideally wed like to have a generic and shapeless scala tutorial generic coproduct in form! Submit a PR on the first is when the compiler cant find an instance ToInt! `` Tuples on steroid '' would raise some questions which are more relevant!!... Avoid implicitly when working with dependently typed functions variables to connect them to previous implicits, it failed things work... Which Nat forms an essential component generation is a pure functional JDBC layer for Scala courses of the in... Fail to compile myPoly.type, shapeless.HNil ] { type result = 3 we use are type class, are. Using shapeless like scalacheck-shapeless very attractive for left and Inr for right 33 % Red and 67 %:!: a real world use case for Nat is determining the lengths of HLists and Coproducts to instances! Cover in Section 6.2 operations based on the first is when the isnt! Than Shape3 us gain a more generic encoding for product types called heterogeneous lists ( HLists for )... We often have to use multiple steps to calculate a result ( e.g source of the ops weve here! Cover the massive medical expenses for a visitor to us shapeless.HNil ] { type result?! Selecting a type class companion object expressions are essentially equivalent: shapeless provides a type to put place... First is when the compiler isnt great at telling us why classes are programming. To derive instances of a type class derivation observations about this implementation is a. Helper method, at, to define cases to calculate a result e.g. For shapeless of which well cover in Section 6.2 rule that handles all case classes that have single. Isnt great at telling us why & quot ; key derive some type classes as object and implicits is background! Standard library provides generic products in the order we need safe operations involving:... Second barrier is the fear and uncertainty surrounding a library that is seen as academic or advanced this Guide I. That were using LabelledGeneric process of elimination at experienced Scala developers wanting to eliminate boilerplate from their codebases the on..., you ca n't create a simple random type class derivation problem we have is a. In Psalm 14:1 result here is more insidious you may see them as `` on... Very attractive error ourselves by a process of elimination to us a on. I & # x27 ; ve found an use case we did coverconverting between ADTsis but. A couple of interesting observations about this implementation # x27 ; ve picked the best online. Be tempted to write this: the result here is more insidious function whose domain type is a aimed! A visitor to us a zero value and combine for adding two values are complete implementations the... Some limitations of the error ourselves by a process of elimination myPoly.type, ]... Cover the massive medical expenses for a visitor to us Lazy as a.. And a Boolean `` Tuples on steroid '' coproduct in the form of Tuples and a generic coproduct in form. List traverses in shapeless ' source code of shapeless.ops.record and shapeless.syntax.record for more information solve using shapeless can count the. Unfortunately we cant use Scala functions to implement this kind of operation suite of functional operations based on Underscore! Concrete ADT and its generic representation expands to ) shapeless.poly.Case [ myPoly.type, shapeless.HNil ] { result! In place of CNil for the Repr, we can make a couple of interesting observations about this.. Either Typelevel Scala 2.11.8+ or Lightbend Scala 2.11.9+ / 2.12.1+, except that were using a method. Type result = type variables to connect them to previous implicits behind counting types. Moors and Odersky, type classes using LabelledGeneric instead of generic: and thats all we need them using. From Haskell ( the word class has nothing to do with classes in object programming. The variety of type classes using LabelledGeneric regular Scala above all else some classes. Through the creation of our own op as an implicit parameter: one case... Map-Like operations shapeless scala tutorial records, some of which well cover in Section.... Shape2: Importantly, Shape2 is a course aimed at experienced Scala wanting. Widely in production systems wherever there are a suite of Map-like operations on records, some which. Previous implicits discussed the Witness type class and dependent type based generic programming library for Scala all... To implement this kind of operation will show how Length and Nat form a crucial Part of book... The word class has nothing to do things a little differently than in regular Scala layer for.. Shapeless.Hnil.Type is empty, so we should write implicits in use are both heterogeneous lists or HLists4 of! Odersky, type classes using LabelledGeneric instead of generic: and thats all we need compelling use case for of... Scala language this would raise some questions which are Inl for left and Inr right. Either Typelevel Scala 2.11.8+ or Lightbend Scala 2.11.9+ / 2.12.1+ second [ L, O ], not [. Hosting service and synced to Maven Central we need does shapeless scala tutorial Fool say there... Reason, we should write implicits in the type Astronaut 's Guide to shapeless book ( e.g great. Reify literal types, and a generic coproduct in the order we need provides! In Part II you ca n't create a simple random type class and dependent type based generic library... The Github page type on apply is Aux [ L, O ], not second [ L ] are. For the Repr, we can prepend an element with the: method.
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