From 62dd700be785c42f9e5ac9fbfeb1f35785289249 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Tom=C3=A1s=20Ruiz-L=C3=B3pez?= <truizlop@gmail.com>
Date: Tue, 4 Dec 2018 11:17:33 +0100
Subject: [PATCH] Translate chapter 3.10 into Swift

---
 3.10-ends-and-coends.md | 307 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 307 insertions(+)
 create mode 100644 3.10-ends-and-coends.md

diff --git a/3.10-ends-and-coends.md b/3.10-ends-and-coends.md
new file mode 100644
index 0000000..820b3d3
--- /dev/null
+++ b/3.10-ends-and-coends.md
@@ -0,0 +1,307 @@
+```Haskell
+class Profunctor p where
+    dimap :: (c -> a) -> (b -> d) -> p a b -> p c d
+```
+```swift
+protocol Profunctor {
+  associatedtype P
+
+  func dimap<A, B, C, D>(_ pab : Kind2<P, A, B>, _ f : @escaping (C) -> A, _ g : @escaping (B) -> D) -> Kind2<P, C, D>
+}
+```
+................
+```Haskell
+dimap f id pbb :: p a b
+```
+```swift
+dimap(pbb, f, id) : Kind2<P, A, B>
+```
+................
+```Haskell
+dimap id f paa :: p a b
+```
+```swift
+dimap(paa, id, f) : Kind2<P, A, B>
+```
+................
+```Haskell
+dimap id f . alpha = dimap f id . alpha
+```
+```swift
+compose({ pab in dimap(pab, id, f)}, alpha) ==
+  { pab in dimap(pab, f, compose(id, alpha)) }
+```
+................
+```Haskell
+forall a. p a a
+```
+```swift
+// No Rank-N types in Swift
+// We have to introduce polymorphic function
+protocol PolyFunction1 {
+  associatedtype P
+
+  func apply<A>() -> Kind2<P, A, A>
+}
+```
+................
+```Haskell
+dimap f id . pi = dimap id f . pi
+```
+```swift
+compose({ pab in dimap(pab, f, id) }, pi.apply) ==
+  { pab in dimap(pab, id, compose(f, pi.apply))
+```
+................
+```Haskell
+dimap f idb . pib = dimap ida f . pia
+```
+```swift
+compose({ pab in dimap(pab, f, id) }, pi_b.apply) ==
+  compose({ pab in dimap(pab, id, )}, pi_a.apply)
+```
+................
+```Haskell
+Profunctor p => (forall c. p c c) -> p a b
+```
+```swift
+func side<P, A, B, ProfunctorP, PolyFP>(_ f : PolyFP, _ profunctor : ProfunctorP) -> Kind<P, A, B> where
+  PolyFP : PolyFunction1, PolyFP.P == P,
+  ProfunctorP : Profunctor, ProfunctorP.P == P
+```
+................
+```Haskell
+pi :: Profunctor p => forall c. (forall a. p a a) -> p c c
+pi e = e
+```
+```Swift
+protocol PolyFunction2 {
+  associatedtype P
+
+  func apply<C, PolyFP>(_ in : PolyFP) -> Kind2<P, C, C> where PolyFP : PolyFunction1, PolyFP.P == P
+}
+
+class BasePolyFunction2<P> : PolyFunction2 {
+  func apply<C, PolyFP>(_ in : PolyFP) -> Kind2<P, C, C> where PolyFP : PolyFunction1, PolyFP.P == P {
+    return in.apply()
+  }
+}
+
+func pi<P, ProfunctorP>(_ profunctor : ProfunctorP) -> BasePolyFunction2 {
+  return BasePolyFunction2<P>()
+}
+```
+................
+```Haskell
+lambda :: Profunctor p => p a a -> (a -> b) -> p a b
+lambda paa f = dimap id f paa
+
+rho :: Profunctor p => p b b -> (a -> b) -> p a b
+rho pbb f = dimap f id pbb
+```
+```swift
+func lambda<P, A, B, ProfunctorP>(_ paa : Kind2<P, A, A>, _ f : @escaping (A) -> B, _ profunctor : ProfunctorP) -> Kind2<P, A, B> where
+ProfunctorP : Profunctor, ProfunctorP.P == P {
+  return profunctor.dimap(paa, id, f)
+}
+
+func lambda<P, A, B, ProfunctorP>(_ pbb : Kind2<P, B, B>, _ f : @escaping (A) -> B, _ profunctor : ProfunctorP) -> Kind2<P, A, B> where
+ProfunctorP : Profunctor, ProfunctorP.P == P {
+  return profunctor.dimap(pbb, f, id)
+}
+```
+................
+```Haskell
+type ProdP p = forall a b. (a -> b) -> p a b
+```
+```swift
+protocol ProdP {
+  associatedtype P
+
+  func apply<A, B>(_ f : @escaping (A) -> B) -> Kind2<P, A, B>
+}
+```
+................
+```Haskell
+newtype DiaProd p = DiaProd (forall a. p a a)
+```
+```swift
+class DiaProd<PolyFP, P> where PolyFP : PolyFunction1, PolyFP.P == P {
+  let paa : PolyFP
+}
+```
+................
+```Haskell
+lambdaP :: Profunctor p => DiaProd p -> ProdP p
+lambdaP (DiaProd paa) = lambda paa
+
+rhoP :: Profunctor p => DiaProd p -> ProdP p
+rhoP (DiaProd paa) = rho paa
+```
+```swift
+class LambdaProdP<P, ProfunctorP, PolyFP> : ProdP where
+  ProfunctorP : Profunctor, ProfunctorP.P == P,
+  PolyFP : PolyFunction1, PolyFP.P == P {
+
+  let profunctor : ProfunctorP
+  let dia : DiaProd<PolyFP, P>
+
+  init(_ profunctor : ProfunctorP, _ dia : DiaProd<PolyFP, P>) {
+    self.profunctor = profunctor
+    self.dia = dia
+  }
+
+  func apply<A, B>(_ f : @escaping (A) -> B) -> Kind2<P, A, B> {
+    return lambda(dia.paa, f, profunctor)
+  }
+}
+
+func lambdaP<P, ProfunctorP, PolyFP>(_ profunctor : ProfunctorP) -> (DiaProd<PolyFP, P>) -> LambdaProdP<P> {
+  return { dia in LambdaProdP(profunctor, dia)}
+}
+
+class RhoProdP<P, ProfunctorP, PolyFP> : ProdP where
+  ProfunctorP : Profunctor, ProfunctorP.P == P,
+  PolyFP : PolyFunction1, PolyFP.P == P {
+
+  let profunctor : ProfunctorP
+  let dia : DiaProd<PolyFP, P>
+
+  init(_ profunctor : ProfunctorP, _ dia : DiaProd<PolyFP, P>) {
+    self.profunctor = profunctor
+    self.dia = dia
+  }
+
+  func apply<A, B>(_ f : @escaping (A) -> B) -> Kind2<P, A, B> {
+    return rho(dia.paa, f, profunctor)
+  }
+}
+```
+................
+```Haskell
+forall a. f a -> g a
+```
+```swift
+protocol FunctionK {
+  associatedtype F
+  associatedtype G
+
+  func apply<A>(_ f : Kind<F, A>) -> Kind<G, A>
+}
+```
+................
+```Haskell
+exists a. p a a
+```
+```swift
+Kind2<P, A, A>
+```
+................
+```Haskell
+data Coend p = forall a. Coend (p a a)
+```
+```swift
+protocol Coend {
+  associatedtype P
+
+  func paa<A>() -> Kind2<P, A, A>
+}
+```
+................
+```Haskell
+data SumP p = forall a b. SumP (b -> a) (p a b)
+```
+```swift
+protocol SumP {
+  associatedtype P
+
+  func f<A, B>(_ b : B) -> A
+  func pab<A, B>() -> Kind2<P, A, B>
+}
+```
+................
+```Haskell
+data DiagSum p = forall a. DiagSum (p a a)
+```
+```swift
+protocol DiagSum {
+  associatedtype P
+
+  func paa<A>() -> Kind2<P, A, A>
+}
+```
+................
+```Haskell
+lambda, rho :: Profunctor p => SumP p -> DiagSum p
+lambda (SumP f pab) = DiagSum (dimap f id pab)
+rho    (SumP f pab) = DiagSum (dimap id f pab)
+```
+```swift
+class LambdaDiagSum<P, ProfunctorP, ASumP> : DiagSum where
+  ProfunctorP : Profunctor, ProfunctorP.P == P,
+  ASumP : SumP, ASumP.P == P {
+
+  let profunctor : ProfunctorP
+  let sump : ASumP
+
+  init(_ profunctor : ProfunctorP, _ sump : ASumP) {
+    self.profunctor = profunctor
+    self.sump = sump
+  }
+
+  func paa<A>() -> Kind2<P, A, A> {
+    return profunctor.dimap(sump.pab, sump.f, id)
+  }
+}
+
+func lambda<P, ProfunctorF, ASumP>(_ sum : ASumP, _ profunctor : ProfunctorP) -> LambdaDiagSum<P> {
+  return LambdaDiagSum(profunctor, sum)
+}
+
+class RhoDiagSum<P, ProfunctorP, ASumP> : DiagSum where
+  ProfunctorP : Profunctor, ProfunctorP.P == P,
+  ASumP : SumP, ASumP.P == P {
+
+  let profunctor : ProfunctorP
+  let sump : ASumP
+
+  init(_ profunctor : ProfunctorP, _ sump : ASumP) {
+    self.profunctor = profunctor
+    self.sump = sump
+  }
+
+  func paa<A>() -> Kind2<P, A, A> {
+    return profunctor.dimap(sump.pab, id, sump.f)
+  }
+}
+
+func rho<P, ProfunctorF, ASumP>(_ sum : ASumP, _ profunctor : ProfunctorP) -> RhoDiagSum<P> {
+  return RhoDiagSum(profunctor, sum)
+}
+```
+................
+```Haskell
+(exists x. p x x) -> c ≅ forall x. p x x -> c
+```
+```swift
+(Kind2<P, A, A>) -> C ≅ (BasePolyFunction1<P>) -> C
+```
+................
+```Haskell
+data Procompose q p a b where
+  Procompose :: q a c -> p c b -> Procompose q p a b
+```
+```swift
+class Procompose<Q, P, A, B> {}
+
+func apply<Q, P, A, B, C>(_ qac : Kind2<Q, A, C>, _ pcb : Kind2<Q, C, B>) -> Procompose<Q, P, A, B> {
+  ...
+}
+```
+................
+```Haskell
+exists c. (q a c, p c b)
+```
+```swift
+(Kind2<Q, A, C>, Kind2<P, C, B>)
+```