Hom-set morphism generator for bicartesian closed categories

This is a little Scala-EDSL that enables the user to write down chains of equalities that describe (iso)morphisms between hom-sets in BCCC (bicartesian closed categories), from which it then can reconstruct the complete (iso)morphisms between those hom-sets.

It can also automatically apply the Yoneda lemma to extract morphisms between objects given morphisms between their Yoneda embeddings. Therefore, it is essentially a semi-automated implementation of the general strategy described for example in Awodey's "Category Theory" section 8.4 "Applications of the Yoneda Lemma".

The basic idea is that if we can construct an isomorphism between Hom(A, X) and Hom(B, X) that is natural in X, then the Yoneda lemma gives us a way to construct an explicit isomorphism between A and B. For example, in Proposition 8.6, Awodey wants to show that

A x (B U C) ~ (A x B) U (A x C)

holds (here, A, B, C are some objects of a BCCC, and x, U, ~ are ugly ascii-art representations that stand for product, coproduct, isomorphism respectively). In order to show this, he writes down the following chain of isomorphic hom-sets:

Hom(A x (B U C), X) ~ 
Hom(B U C, X^A) ~
(Hom(B, X^A) x Hom(C, X^A)) ~
(Hom(A x B, X) x Hom(A x C, X)) ~
Hom((A x B) U (A x C), X)

(again, up to differences in type-setting), and then applies the Yoneda lemma corollary to obtain the isomorphism between A x (B U C) and (A x B) U (A x C).

The present EDSL allows us to interpret the above chain of Hom-sets literally as executable Scala code. It then fills out the gaps, replacing the somewhat vague ~-signs by explicit isomorphisms. Finally, it uses the Yoneda lemma to convert those isomorphisms to isomorphisms between underlying objects.

The interesting observation is that it is much easier to write down chains of "types", and then let the "code" be generated automatically. This is exactly the opposite of what happens with type inference where we write "code" and then infer "types".

A wall of examples follows.

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Distributivity of product over coproduct (from right)

Scala code:

Hom((A U B) x C, Y) ~
Hom(A U B, Y^C) ~
(Hom(A, Y^C) x Hom(B, Y^C)) ~
(Hom(A x C, Y) x Hom(B x C, Y)) ~
Hom((A x C) U (B x C), Y)
  

Inferred canonical natural isomorphism between hom-sets:

Isomorphisms extracted using Yoneda lemma between objects Prod[Coprod[A,B],C] and Coprod[Prod[A,C],Prod[B,C]]:

 \varepsilon^{ C, ({  ({ A}\times{ C}) }\amalg{  ({ B}\times{ C}) }) }\circ \langle  [\lambda( \iota_1^{  ({ A}\times{ C}) ,  ({ B}\times{ C}) } ),\lambda( \iota_2^{  ({ A}\times{ C}) ,  ({ B}\times{ C}) } )]\circ \pi_1^{  ({ A}\amalg{ B}) , C} ,\pi_2^{  ({ A}\amalg{ B}) , C}\rangle 

[ \varepsilon^{ C, ({  ({ A}\amalg{ B}) }\times{ C}) }\circ \langle   \lambda( \textrm{Id}_{  ({  ({ A}\amalg{ B}) }\times{ C}) } )\circ \iota_1^{ A, B} \circ \pi_1^{ A, C} ,\pi_2^{ A, C}\rangle , \varepsilon^{ C, ({  ({ A}\amalg{ B}) }\times{ C}) }\circ \langle   \lambda( \textrm{Id}_{  ({  ({ A}\amalg{ B}) }\times{ C}) } )\circ \iota_2^{ A, B} \circ \pi_1^{ B, C} ,\pi_2^{ B, C}\rangle ]

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Distributivity of product over coproduct (from left)

Scala code:

    Hom(C x (A U B), Y) ~
    Hom((A U B) x C, Y) ~
    Hom(A U B, Y^C) ~
    (Hom(A, Y^C) x Hom(B, Y^C)) ~
    (Hom(A x C, Y) x Hom(B x C, Y)) ~
    Hom((A x C) U (B x C), Y) ~
    Hom((C x A) U (C x B), Y)
  

Constructed canonical natural isomorphism:

Isomorphisms extracted using Yoneda lemma between objects Prod[C,Coprod[A,B]] and Coprod[Prod[C,A],Prod[C,B]]:

  \varepsilon^{ C, ({  ({ C}\times{ A}) }\amalg{  ({ C}\times{ B}) }) }\circ \langle  [\lambda(  [ \iota_1^{  ({ C}\times{ A}) ,  ({ C}\times{ B}) }\circ \langle \pi_2^{ A, C},\pi_1^{ A, C}\rangle , \iota_2^{  ({ C}\times{ A}) ,  ({ C}\times{ B}) }\circ \langle \pi_2^{ B, C},\pi_1^{ B, C}\rangle ]\circ \iota_1^{  ({ A}\times{ C}) ,  ({ B}\times{ C}) }  ),\lambda(  [ \iota_1^{  ({ C}\times{ A}) ,  ({ C}\times{ B}) }\circ \langle \pi_2^{ A, C},\pi_1^{ A, C}\rangle , \iota_2^{  ({ C}\times{ A}) ,  ({ C}\times{ B}) }\circ \langle \pi_2^{ B, C},\pi_1^{ B, C}\rangle ]\circ \iota_2^{  ({ A}\times{ C}) ,  ({ B}\times{ C}) }  )]\circ \pi_1^{  ({ A}\amalg{ B}) , C} ,\pi_2^{  ({ A}\amalg{ B}) , C}\rangle \circ \langle \pi_2^{ C,  ({ A}\amalg{ B}) },\pi_1^{ C,  ({ A}\amalg{ B}) }\rangle 

[  \varepsilon^{ C, ({ C}\times{  ({ A}\amalg{ B}) }) }\circ \langle   \lambda( \langle \pi_2^{  ({ A}\amalg{ B}) , C},\pi_1^{  ({ A}\amalg{ B}) , C}\rangle )\circ \iota_1^{ A, B} \circ \pi_1^{ A, C} ,\pi_2^{ A, C}\rangle \circ \langle \pi_2^{ C, A},\pi_1^{ C, A}\rangle ,  \varepsilon^{ C, ({ C}\times{  ({ A}\amalg{ B}) }) }\circ \langle   \lambda( \langle \pi_2^{  ({ A}\amalg{ B}) , C},\pi_1^{  ({ A}\amalg{ B}) , C}\rangle )\circ \iota_2^{ A, B} \circ \pi_1^{ B, C} ,\pi_2^{ B, C}\rangle \circ \langle \pi_2^{ C, B},\pi_1^{ C, B}\rangle ]

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Iterated exponentiation

Scala code:

Hom(D, Exp(C, A x B)) ~
Hom(D x (A x B), C) ~
Hom(D x (B x A), C) ~
Hom((D x B) x A, C) ~
Hom(D x B, Exp(C, A)) ~
Hom(D, Exp(Exp(C, A), B))
  

Constructed canonical natural isomorphism:

Isomorphisms extracted using Yoneda lemma between objects Exp[C,Prod[A,B]] and Exp[Exp[C,A],B]:

\lambda( \lambda(    \varepsilon^{  ({ A}\times{ B}) ,C}\circ \langle \pi_1^{  ({ C}^{  ({ A}\times{ B}) }) ,  ({ A}\times{ B}) },\pi_2^{  ({ C}^{  ({ A}\times{ B}) }) ,  ({ A}\times{ B}) }\rangle \circ \langle \pi_1^{  ({ C}^{  ({ A}\times{ B}) }) ,  ({ B}\times{ A}) }, \langle \pi_2^{ B, A},\pi_1^{ B, A}\rangle\circ \pi_2^{  ({ C}^{  ({ A}\times{ B}) }) ,  ({ B}\times{ A}) } \rangle \circ \langle  \pi_1^{  ({ C}^{  ({ A}\times{ B}) }) , B}\circ \pi_1^{  ({  ({ C}^{  ({ A}\times{ B}) }) }\times{ B}) , A} ,\langle  \pi_2^{  ({ C}^{  ({ A}\times{ B}) }) , B}\circ \pi_1^{  ({  ({ C}^{  ({ A}\times{ B}) }) }\times{ B}) , A} ,\pi_2^{  ({  ({ C}^{  ({ A}\times{ B}) }) }\times{ B}) , A}\rangle\rangle  ) )

\lambda(    \varepsilon^{ A,C}\circ \langle   \varepsilon^{ B, ({ C}^{ A}) }\circ \langle \pi_1^{  ({  ({ C}^{ A}) }^{ B}) , B},\pi_2^{  ({  ({ C}^{ A}) }^{ B}) , B}\rangle \circ \pi_1^{  ({  ({  ({ C}^{ A}) }^{ B}) }\times{ B}) , A} ,\pi_2^{  ({  ({  ({ C}^{ A}) }^{ B}) }\times{ B}) , A}\rangle \circ \langle \langle \pi_1^{  ({  ({ C}^{ A}) }^{ B}) ,  ({ B}\times{ A}) }, \pi_1^{ B, A}\circ \pi_2^{  ({  ({ C}^{ A}) }^{ B}) ,  ({ B}\times{ A}) } \rangle, \pi_2^{ B, A}\circ \pi_2^{  ({  ({ C}^{ A}) }^{ B}) ,  ({ B}\times{ A}) } \rangle \circ \langle \pi_1^{  ({  ({ C}^{ A}) }^{ B}) ,  ({ A}\times{ B}) }, \langle \pi_2^{ A, B},\pi_1^{ A, B}\rangle\circ \pi_2^{  ({  ({ C}^{ A}) }^{ B}) ,  ({ A}\times{ B}) } \rangle  )

---

Product of exponents

Scala code:

Hom(Y, (C^A) x (C^B)) ~
(Hom(Y, C^A) x Hom(Y, C^B)) ~
(Hom(Y x A, C) x Hom(Y x B, C)) ~
(Hom(A x Y, C) x Hom(B x Y, C)) ~
(Hom(A, C^Y) x Hom(B, C^Y)) ~
Hom(A U B, C^Y) ~
Hom((A U B) x Y, C) ~
Hom(Y x (A U B), C) ~
Hom(Y, C^(A U B))
  

Constructed canonical natural isomorphism:

Isomorphisms extracted using Yoneda lemma between objects Prod[Exp[C,A],Exp[C,B]] and Exp[C,Coprod[A,B]]:

\lambda(   \varepsilon^{  ({  ({ C}^{ A}) }\times{  ({ C}^{ B}) }) ,C}\circ \langle  [\lambda(   \varepsilon^{ A,C}\circ \langle  \pi_1^{  ({ C}^{ A}) ,  ({ C}^{ B}) }\circ \pi_1^{  ({  ({ C}^{ A}) }\times{  ({ C}^{ B}) }) , A} ,\pi_2^{  ({  ({ C}^{ A}) }\times{  ({ C}^{ B}) }) , A}\rangle \circ \langle \pi_2^{ A,  ({  ({ C}^{ A}) }\times{  ({ C}^{ B}) }) },\pi_1^{ A,  ({  ({ C}^{ A}) }\times{  ({ C}^{ B}) }) }\rangle  ),\lambda(   \varepsilon^{ B,C}\circ \langle  \pi_2^{  ({ C}^{ A}) ,  ({ C}^{ B}) }\circ \pi_1^{  ({  ({ C}^{ A}) }\times{  ({ C}^{ B}) }) , B} ,\pi_2^{  ({  ({ C}^{ A}) }\times{  ({ C}^{ B}) }) , B}\rangle \circ \langle \pi_2^{ B,  ({  ({ C}^{ A}) }\times{  ({ C}^{ B}) }) },\pi_1^{ B,  ({  ({ C}^{ A}) }\times{  ({ C}^{ B}) }) }\rangle  )]\circ \pi_1^{  ({ A}\amalg{ B}) ,  ({  ({ C}^{ A}) }\times{  ({ C}^{ B}) }) } ,\pi_2^{  ({ A}\amalg{ B}) ,  ({  ({ C}^{ A}) }\times{  ({ C}^{ B}) }) }\rangle \circ \langle \pi_2^{  ({  ({ C}^{ A}) }\times{  ({ C}^{ B}) }) ,  ({ A}\amalg{ B}) },\pi_1^{  ({  ({ C}^{ A}) }\times{  ({ C}^{ B}) }) ,  ({ A}\amalg{ B}) }\rangle  )

\langle \lambda(   \varepsilon^{  ({ C}^{  ({ A}\amalg{ B}) }) ,C}\circ \langle   \lambda(   \varepsilon^{  ({ A}\amalg{ B}) ,C}\circ \langle \pi_1^{  ({ C}^{  ({ A}\amalg{ B}) }) ,  ({ A}\amalg{ B}) },\pi_2^{  ({ C}^{  ({ A}\amalg{ B}) }) ,  ({ A}\amalg{ B}) }\rangle \circ \langle \pi_2^{  ({ A}\amalg{ B}) ,  ({ C}^{  ({ A}\amalg{ B}) }) },\pi_1^{  ({ A}\amalg{ B}) ,  ({ C}^{  ({ A}\amalg{ B}) }) }\rangle  )\circ \iota_1^{ A, B} \circ \pi_1^{ A,  ({ C}^{  ({ A}\amalg{ B}) }) } ,\pi_2^{ A,  ({ C}^{  ({ A}\amalg{ B}) }) }\rangle \circ \langle \pi_2^{  ({ C}^{  ({ A}\amalg{ B}) }) , A},\pi_1^{  ({ C}^{  ({ A}\amalg{ B}) }) , A}\rangle  ),\lambda(   \varepsilon^{  ({ C}^{  ({ A}\amalg{ B}) }) ,C}\circ \langle   \lambda(   \varepsilon^{  ({ A}\amalg{ B}) ,C}\circ \langle \pi_1^{  ({ C}^{  ({ A}\amalg{ B}) }) ,  ({ A}\amalg{ B}) },\pi_2^{  ({ C}^{  ({ A}\amalg{ B}) }) ,  ({ A}\amalg{ B}) }\rangle \circ \langle \pi_2^{  ({ A}\amalg{ B}) ,  ({ C}^{  ({ A}\amalg{ B}) }) },\pi_1^{  ({ A}\amalg{ B}) ,  ({ C}^{  ({ A}\amalg{ B}) }) }\rangle  )\circ \iota_2^{ A, B} \circ \pi_1^{ B,  ({ C}^{  ({ A}\amalg{ B}) }) } ,\pi_2^{ B,  ({ C}^{  ({ A}\amalg{ B}) }) }\rangle \circ \langle \pi_2^{  ({ C}^{  ({ A}\amalg{ B}) }) , B},\pi_1^{  ({ C}^{  ({ A}\amalg{ B}) }) , B}\rangle  )\rangle

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Exponent of products

Scala code:

Hom(Y, (A x B)^C) ~
Hom(Y x C, A x B) ~
(Hom(Y x C, A) x Hom(Y x C, B)) ~
(Hom(Y, A^C) x Hom(Y, B^C)) ~
Hom(Y, (A^C) x (B^C))
  

Generated canonical natural isomorphism:

Isomorphisms extracted using Yoneda lemma between objects Exp[Prod[A,B],C] and Prod[Exp[A,C],Exp[B,C]]:

\langle \lambda(  \pi_1^{ A, B}\circ  \varepsilon^{ C, ({ A}\times{ B}) }\circ \langle \pi_1^{  ({  ({ A}\times{ B}) }^{ C}) , C},\pi_2^{  ({  ({ A}\times{ B}) }^{ C}) , C}\rangle   ),\lambda(  \pi_2^{ A, B}\circ  \varepsilon^{ C, ({ A}\times{ B}) }\circ \langle \pi_1^{  ({  ({ A}\times{ B}) }^{ C}) , C},\pi_2^{  ({  ({ A}\times{ B}) }^{ C}) , C}\rangle   )\rangle

\lambda( \langle  \varepsilon^{ C,A}\circ \langle  \pi_1^{  ({ A}^{ C}) ,  ({ B}^{ C}) }\circ \pi_1^{  ({  ({ A}^{ C}) }\times{  ({ B}^{ C}) }) , C} ,\pi_2^{  ({  ({ A}^{ C}) }\times{  ({ B}^{ C}) }) , C}\rangle , \varepsilon^{ C,B}\circ \langle  \pi_2^{  ({ A}^{ C}) ,  ({ B}^{ C}) }\circ \pi_1^{  ({  ({ A}^{ C}) }\times{  ({ B}^{ C}) }) , C} ,\pi_2^{  ({  ({ A}^{ C}) }\times{  ({ B}^{ C}) }) , C}\rangle \rangle )

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The rule (A^1 = A)

Scala code:

Hom(Y, A^Terminal) ~
Hom(Y x Terminal, A) ~
Hom(Y, A)
  

Generated canonical natural isomorphism:

Isomorphisms extracted using Yoneda lemma between objects Exp[A,Terminal] and A:

  \varepsilon^{ \mathbb{T},A}\circ \langle \pi_1^{  ({ A}^{ \mathbb{T}}) , \mathbb{T}},\pi_2^{  ({ A}^{ \mathbb{T}}) , \mathbb{T}}\rangle \circ \langle \textrm{Id}_{  ({ A}^{ \mathbb{T}}) },\dagger_{  ({ A}^{ \mathbb{T}}) }\rangle 

\lambda( \pi_1^{ A, \mathbb{T}} )

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The rule (A^0 = 1)

Scala code:

Hom(Terminal, A^Initial) ~
Hom(Terminal x Initial, A) ~
Hom(Initial, A)
  

Here, we cannot (and need not) use the Yoneda-lemma. Instead, we simply apply the inverse of the above natural isomorphism to the morphism !_A, and obtain a morphism from the terminal object to A^{\mathbb{I}}. Hence we obtain that A^{\mathbb{I}} must be isomorphic to the terminal object.

InitMor[A]

Lambda[Terminal,Initial,A]((
InitMor[A] o 
P2[Terminal, Initial]))

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Analogon of the disjunction elimination

Scala code:

(Hom(D, C^A) x Hom(D, C^B) x Hom(D, A U B)) ~
(Hom(D x A, C) x Hom(D x B, C) x Hom(D, A U B)) ~
(Hom(A x D, C) x Hom(B x D, C) x Hom(D, A U B)) ~
(Hom(A, C^D) x Hom(B, C^D) x Hom(D, A U B)) ~
(Hom(A U B, C^D) x Hom(D, A U B)) ~
(Hom((A U B) x D, C) x Hom(D, A U B)) ~
(Hom(D x (A U B), C) x Hom(D, A U B)) ~
(Hom(D, C^(A U B)) x Hom(D, A U B)) ~
Hom(D, (C^(A U B)) x (A U B)) ->
Hom(D, C)
  

Generated morphism:

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Generating this readme

This readme has been generated automatically using

scala bccc_morphism_construction.scala > README.md

To look at it locally, you might want to run

markdown README.md > readme.html && firefox readme.html