# Let's meet the charming fold family

Today we will meet an amazing family: the fold functions!

# The well known foldRight

Lists is one of the first data structure every developer/computer scientist meet in her/his journey into programming:

sealed abstract class List[+A]
final case object Nil                              extends List[Nothing]
final case class  Cons[+A](head: A, tail: List[A]) extends List[A]


It means means values of type List[A] can be of (only) two forms:

• either Nil
• or Cons(head, tail) for some values head of type A and tail of type List[A]

For example we can define the following lists:

val empty : List[Int] = Nil
val l1 : List[Int] = Cons(61, Nil)
val l2 : List[Int] = Cons(34, Cons(61, Nil))
val l3 : List[String] = Cons("a", Cons("b", Cons("c", Nil)))


In addition, Nil and Cons can be seen as constants and functions returning List[A]:

def nil[A]: List[A] = Nil


The fold function, often called foldRight, answers the question:

What would have happened if, instead of having used Nil and Cons in the construction of a list l:List[A], we would have used another constant z:T and another function f:(A, T) => T for some type T?

Let’s illustrate this using the previous examples:

val empty : Int = 0 // z = 0
val v1 : Int = max(61, 0) // z = 0, f = max
val v2 : Int = mult(34, mult(61, 1)) // z = 1, f = mult
val v3 : String = concat("a", concat("b", concat("c", "")))
-- z = "", f = concat


The definition of foldRight illustrates well the transformation process. It deconstructs the list l:List[A] and replace Nil by z and Cons by f:

def foldList[A,T](z: T, f: (A,T) => T): List[A] => T = {
def transform(l: List[A]): T =
l match {
case Nil => z
val transformedTail = transform(tail)
}

transform _
}


# The simple cases: Enum Types

fold functions can be defined for a wide range of data structures. As a first example, let’s take this type:

sealed abstract class SingletonType
final case object SingleValue extends SingletonType


The type SingletonType admits one and only one value: SingleValue. Folding over SingletonType means, replacing SingleValue by a constant z:T for some type T :

def foldSingletonType[T](z:T): SingletonType => T = {
def transform(v: SingletonType): T =
v match {
case SingleValue => z
}

transform _
}


While SingletonType has only one value, the type Boolean have exactly two values True and False:

sealed abstract class Boolean
final case object True  extends Boolean
final case object False extends Boolean


So folding over Booleans mean, given a type T and two constants tt:T and ff:T, replacing True by tt and False by ff:

def foldBoolean[T](tt:T, ff:T): Boolean => T = {
def transform(v: Boolean): T =
v match {
case True  => tt
case False => ff
}

transform _
}


And so on for every enum type.

# Beyond enums

You may start the see general process. If values of type C are build using constructors (Nil and Cons[A] for List[A], SingleValue for SingletonType, True and False for Boolean), then folding is all about transforming values of type C into another type T by replacing each constructor of C by a constant or function on T of the same shape. Let’s consider the type Either[A,B]:

sealed abstract class Either[A,B]
final case class Left[A,B](value: A)  extends Either[A,B]
final case class Right[A,B](value: B) extends Either[A,B]


To transform values of type Either[A,B] into T we need two functions on T:

• Left being of type A => Either[A,B] we need a function f: A => T.
• Right being of type B => Either[A,B] we need a function g: B => T.

Then we can operate the transformation:

def foldEither[A,B,T](f: A => T, g: B => T): Either[A,B] => T = {
def transform(v: Either[A,B]): T =
v match {
case Left(a)  => f(a)
case Right(b) => g(b)
}

transform _
}


# Recursive Types

Folding over recursive types obey the previous rules. Recursion is handled by transforming sub-terms first. Let’s consider the type of binary trees:

sealed abstract class Tree[+A]
final case object Empty extends Tree[Nothing]
final case class  Node[+A](value:A, left: Tree[A], right: Tree[A]) extends Tree[A]


To transform values of type Tree[A] into T we need:

• Empty being a constant of type Tree[A], we need a constant z:T.
• Node being a function of type (A, Tree[A], Tree[A]) => Tree[A] we need a function f: (A, T, T) => T. Note how all occurrences of Tree[A] have been replaced by T in the type.

Then we can operate the transformation:

def foldTree[A,T](z: T, f: (A, T, T) => T): Tree[A] => T = {
def transform(v: Tree[A]): T =
v match {
case Empty => z
case Node(a,l,r) =>
val g: T = transform(l) // Transforming sub-term l
val d: T = transform(r) // Transforming sub-term r
f(a,g,d)
}

transform _
}


# Generalized Algebraic Data Types (GADT)

Instead of giving a formal definition of what Generalized Algebraic Data Types i will show you some examples.

## Type Equalities

Consider the type:

sealed abstract class EmptyOrSingleton[A]
final case object SingleValueIfAisInt extends EmptyOrSingleton[Int]


This type looks very similar to SingletonType but, while SingleValue was always a value of SingletonType, SingleValueIfAisInt is only a value of EmptyOrSingleton[Int], i.e. when A is Int. So what happens to EmptyOrSingleton[A] when A is not Int? Then there is no constructor for EmptyOrSingleton[A] so no value for SingletonIfInt[A] (excluding null which we will pretend no to exist).

GADTs are very useful to encode predicates over types. Imagine you have a value v:EmptyOrSingleton[A] for some type A (remember we pretend null does not exist). What could you say about A? The only way to get a value of type EmptyOrSingleton[A] is through SingleValueIfAisInt. Thus v is SingleValueIfAisInt which is of type EmptyOrSingleton[Int] so is v. We can conclude that A is actually Int. Not convinced? Let A be String, can you build a value of type EmptyOrSingleton[String] without using null? Try it.

To find how to fold EmptyOrSingleton[A] into T, let’s apply the technique we used in the previous sections. EmptyOrSingleton[A] has only one constructor, SingleValueIfAisInt, so we need a constant z:T. But SingleValueIfAisInt is not of type EmptyOrSingleton[A] but EmptyOrSingleton[Int]. The argument A matters so let T depend on A: we want to transform values of type EmptyOrSingleton[A] into T[A].

• SingleValueIfAisInt being of type EmptyOrSingleton[Int] we need a constant z:T[Int]

Then we can operate the transformation:

def foldEmptyOrSingleton[A, T[_]](z: T[Int]): EmptyOrSingleton[A] => T[A] = {
def transform(v: EmptyOrSingleton[A]): T[A] =
v match {
case SingleValueIfAisInt => z // Because we know A = Int
}

transform _
}


foldEmptyOrSingleton means that, for some T[_], if you have a value z:T[Int] then you can transform any value EmptyOrSingleton[A] into T[A]. For example, let’s take

type T[X] = X =:= Int
val z:T[Int] = implicitly[Int =:= Int]


Then foldEmptyOrSingleton[A,T](z) gives us, for any value v:EmptyOrSingleton[A] a proof that A =:= Int. Another important use case is asserting type equality:

sealed abstract class Eq[A,B]
final case class Refl[X]() extends Eq[X,X]


Any non-null value v:Eq[A,B] must be a Refl[X]() : Eq[X,X] for some X, then Eq[A,B] = Eq[X,X] proving that A = X = B. To transform a value of type Eq[A,B] into T[A,B] we need:

• Refl[X]() is essentially a constant of type Eq[X,X] for all type X (note: Scala write this type [X]Eq[X,X]). We need a constant z:T[X,X] for all type X (so the type [X]T[X,X]). Scala does not support transparent higher-ranked types, we need to emulate them with a trait:
trait ElimRefl[T[_,_]] {
def apply[X]: T[X,X]
}


Then we could have hoped to be able to operate the transformation like previous section. But given a value v:Eq[A,B], convincing Scala that A = B is a bit tough. Instead we can write the fold as a method:

sealed abstract class Eq[A,B] {
def fold[T[_,_]](z: ElimRefl[T]): T[A,B]
}
final case class Refl[X]() extends Eq[X,X] {
def fold[T[_,_]](z: ElimRefl[T]): T[X,X] = z[X]
}

def foldEq[A, B, T[_,_]](z: ElimRefl[T]): Eq[A,B] => T[A,B] =
(v:Eq[A,B]) => v.fold[T](z)


Ingenious definition of T[_,_] leads to interesting results:

trait C[X]

type T1[A,B] = C[A] =:= C[B]

val z1: ElimRefl[T1] =
new ElimRefl[T1] {
def apply[X]: T1[X,X] = implicitly[C[X] =:= C[X]]
}

def transform[A,B]: Eq[A,B] => C[A] =:= C[B] =
foldEq[A,B,T1](z1)


## Existential Quantification

GADTs not only provide useful type equalities, they also offer existential quantification!

sealed abstract class Ex[F[_]] {
type hidden
val value: hidden
val evidence: F[hidden]
}
final case class MakeEx[F[_],A](value: A, evidence: F[A]) extends Ex[F] {
type hidden = A
}


Any value v:Ex[F] has to be an instance of MakeEx[F,A] for some type A. Which means we have a value, v.value, of type A and an instance of the type-class F for A (for example an instance of Monoid[A] with F[X] = Monoid[X]).

To transform values of type Ex[F] into T we need:

• MakeEx[F[_],?] being of type [A](A, F[A]) => Ex[F] meaning: For_all_type A, (A, F[A]) => Ex[F], we need a function f of type [A](A, F[A]) => T. Scala still does not support transparent higher ranked types, we need to emulate them with another trait:
trait ElimMakeEx[F[_],T] {
def apply[A](value: A, evidence: F[A]): T
}


Then we can operate the transformation:

def foldEx[F[_], T](f: ElimMakeEx[F, T]): Ex[F] => T = {
def transform(v: Ex[F]): T =
v match {
case w@MakeEx(value, evidence) => f[w.hidden](value, evidence)
}

transform _
}


# Duality

In this post we have deduced the fold functions from the definition of each type. It is possible to do the opposite: each constructor can be derived from the fold function of its type. For example:

trait List[+A] {
def fold[T](z:T, f: (A,T) => T): T
}

def nil[A]: List[A] =
new List[A] {
def fold[T](z:T, f: (A,T) => T): T = z
}

def cons[A](head:A, tail: List[A]): List[A] =
new List[A] {
def fold[T](z:T, f: (A,T) => T): T = f(head, tail.fold(z,f))
}

def equality[A](l1: List[A], l2:List[A]): Boolean = ??? // Difficult but worthy exercice


# Conclusion

I hope i convinced you folds are immensely useful. First, they let us write simply complex transform functions. But this not the most interesting property. It is sometimes easier to define a type by its fold function. Java, for example, does not have support for neither sealed classes nor pattern-matching. How could we define the List type so that Nil and Cons are the two only cases? The fold function forces any instance of List to fit into the desired shape (if some rules are obeyed like no null and no runtime-reflection). It can also happen that type-inference is not smart enough, fold function provide an alternative way which is often easier for the Scala type-checker.