This is the fourth episode of the series about types. If you have not read previous episodes yet, i strongly recommend doing so before diving into this one.
CoProducts, often called sum types, discriminated unions or disjoint unions, are a convenient way to express an alternative between different types. The coproduct of n types (with n being 0, 1, 2, etc) A_1, A_2, …, A_n is precisely the type whose values are (i, a_i) where i is a number, called the tag, between 1 and n both included (1 ≤ i ≤ n) and a_i is a value of type A_i (the actual type then depends on the value of i). Is is often written Either[A_1, ..., A_n] in programming and A_1 + ... + A_n in Type Theory.
Let’s take, as an example, n = 4, A_1 = Char, A_2 = Boolean, A_3 = Char and A_4 = Double. The following values are all valid and distinct values of type Either4[Char, Boolean, Char, Double]: (1, 'C'), (2, true), (3, 'D'), (4, 3.2), (3, 'C'). Note that (1, 'C') and (3, 'C') are different values because the tag is different.
Instead of writing the tag explicitly, programming languages often let us write n constructors: Inj_i: A_i => Either_n[A_1, ..., A_n] such that
$$\forall i\in{1,\dots,n},\quad \texttt{Inj}_i : \texttt{A}_i \rightarrow \texttt{Either}_n[\texttt{A}_1,\dots,\texttt{A}_n]$$
$$\forall i\in{1,\dots,n},\quad \forall a_i:\texttt{A}_i,\quad \texttt{Inj}_i(a_i) = (i, a_i)$$
Note that:
Either_n[A_1, ... , A_n] is by using one of the constructors
$$\forall v:\texttt{Either}_n[\texttt{A}_1,\dots,\texttt{A}_n],\quad \exists! i\in{1,\dots,n},\quad \texttt{Inj}_i(a_i) = v$$For example with n = 2, the coproduct of two types A and B is defined as
sealed abstract class Either2[A,B] {
def fold[R](inj1: A => R, inj2: B => R): R =
this match {
case Inj1(a) => inj1(a)
case Inj2(b) => inj2(b)
}
}
final case class Inj1[A,B](value: A) extends Either2[A,B]
final case class Inj2[A,B](value: B) extends Either2[A,B]
Saying the pattern matching above is exhaustive is exactly equivalent to say a value of type Either2[A,B] has to be either Inj1(a) for some a:A or Inj2(b) for some b:B. Requiring that Inj1 and Inj2 produce different values is also mandatory as Eirher2[Error, Result] is often used to model computation that may fail. A failed computation would produce a Inj1(e) with e being the error that occurred while a successful computation would produce a Inj2(r) with r the result of the computation. We want to be able to discriminate these two cases, even when the types Error and Result are the same as it is usually the case when the computation need to compute a string or fail with an error message. Likewise, the injectivity of the constructors is mandatory to be able to the result produced r or the error that occurred e.
Once again any *coproductù can be equivalently expressed as a data type Either_n_Ind[A_1, ..., A_n] and as a function type Either_n_Fun[A_1, ..., A_n] with inverse functions ind2fun and fun2ind converting back and forth types Either_n_Ind[A_1, ..., A_n] and Either_n_Fun[A_1, ..., A_n]. For example with n = 2:
sealed abstract class Either2Ind[A,B]
final case class Inj1_Ind[A,B](value: A) extends Either2Ind[A,B]
final case class Inj2_Ind[A,B](value: B) extends Either2Ind[A,B]
trait Either2Fun[A,B] {
def fold[R](inj1: A => R, inj2: B => R): R
}
def inj1_fun[A,B]: A => Either2Fun[A,B] =
(a: A) =>
new Either2Fun[A,B] {
def fold[R](inj1: A => R, inj2: B => R): R = inj1(a)
}
def inj2_fun[A,B]: B => Either2Fun[A,B] =
(b: B) =>
new Either2Fun[A,B] {
def fold[R](inj1: A => R, inj2: B => R): R = inj2(b)
}
def ind2fun[A,B]: Either2Ind[A,B] => Either2Fun[A,B] =
(i: Either2Ind[A,B]) =>
i match {
case Inj1_Ind(a) => inj1_fun(a)
case Inj2_Ind(b) => inj2_fun(b)
}
def fun2ind[A,B]: Either2Fun[A,B] => Either2Ind[A,B] =
(f: Either2Fun[A,B]) =>
f.fold[Either2Ind[A,B]](Inj1_Ind[A,B] _, Inj2_Ind[A,B] _)
data Either2Ind a b where
Inj1_Ind :: a -> Either2Ind a b
Inj2_Ind :: b -> Either2Ind a b
type Either2Fun a b = forall c. (a -> c) -> (b -> c) -> c
inj1_fun :: a -> Either2Fun a b
inj1_fun a f _ = f a
inj2_fun :: b -> Either2Fun a b
inj2_fun b _ g = g b
ind2fun :: Either2Ind a b -> Either2Fun a b
ind2fun (Inj1_Ind a) = inj1_fun a
ind2fun (Inj2_Ind a) = inj2_fun a
fun2ind :: Either2Fun a b -> Either2Ind a b
fun2ind f = f Inj1_Ind Inj2_Ind
The definition of Either2 above can be adapted to Either_n[A_1, ..., A_n] for any value of n (i.e. 0, 1, 2, 3, 4, …). With n = 0, which means a coproduct of zero types, there is no constructor, which means it is impossible to build a value of this type. Thus Either_0 is equivalent to False (i.e. Either_0 ≅ False). With n = 1, Either_n[A] ≅ A.
Further more for any types A, B and C we have the following properties
Either2[A,B] is equivalent to Either2[B,A]Either2[Either2[A,B], C] and Either2[A, Either2[B,C]] and Either3[A,B,C] are all equivalentEithr2[False,A] and Either2[A,False] and A are all equivalentEither_1[A] is equivalent to A(C, Either2[A,B]) is equivalent to Either2[(C,A), (C,B)]Exercise: write conversion functions from and tofor each of these equivalence, such that from and to are inverse functions.
Remember that in Type Theory, False is written 0, Unit is written 1, the product (A_1, ..., A_n) is written A_1 × ... A_n and the coproduct Either_n[A_1, ..., A_n] is written A_1 + ... + A_n. If we express the above properties using Type Theory notation, we get
A + B ≅ B + A(A + B) + C ≅ A + (B + C) ≅ A + B + C0 + A ≅ A + 0 ≅ AC × (A + B) ≅ (C × A) + (C × B)1 × A ≅ A0 × A ≅ A × 0 ≅ 0Is arithmetic familiar to you? If so such equations should be familiar too. But these times there are not between numbers but between equivalent types. If you ever wonder why Algebraic Data Types are called this way, this should give you some hints.
In the next episode, we will see Recursive Data Types.