Church encoding of linear types

Posted on 2019-06-26 by Oleg Grenrus linear

It's common knowledge that we can encode ADTs using so called Church-encoding. It's more correctly called Böhm-Berarducci encoding, as they showed how to systematically make a translation in a typed setting in 1985.1

Either a b ≡ forall r. (a -> r) -> (b -> r) -> r

or

    (a, b) ≡ forall r. (a -> b -> r)

or

        () ≡ forall r. r -> r

Sometimes these alternative variants are useful: in some mainstream languages there are no sum types, but higher order functions can be at least simulated. Thus: visitor pattern.

However, this blog post is not about visitor pattern, but about linear lambda calculus. The corresponding BB-encodings are easily derived for "normal" types formed with × (pair), + (Either) and recursion. Let's see how similar encodings can be derived for types in linear lambda calculus.

#Short review of Böhm-Berarducci encoding

Based on Beyond Church encoding: Boehm-Berarducci isomorphism of algebraic data types and polymorphic lambda-terms.

Given a type

data D = C1 Int
       | C2 D
       | C3 D D

we can define a corresponding type

data DDict r = DDict { c1 :: Int -> r
                     ; c2 :: r -> r
                     ; c3 :: r -> r -> r
                     }

A version Böhm-Berarducci encoding could be

type D' = forall r. DDict r -> r

We can curry the DDict record, ending up only term and type applications and abstractions:

type D'' = forall r. (Int -> r) -> (r -> r) -> (r -> r -> r) -> r

D'' is the Böhm-Berarducci encoding of D.

More over, we can also define an initial algebra of D:

data DF a = C1F Int
          | C2F a
          | C3F a a

and we notice that DDict r and DF r -> r are isomorphic. Therefore we get third way to represent D:

type D''' = forall r. (DF r -> r) -> r

For this post we won't think about recursive types. An algebra types of non-recursive types have a phantom argument r, so for non-recursive type X we'll have

X ≡ forall r. (X -> r) -> r

Then we can convert X -> r to something simpler. In non-linear setting, we can expand both products and sums to use only functions. Let's see what we can do in linear lambda calculus.

#Linear basics

Linear lambda calculus or linear logics come in many flavours. We will be discussing a higher order intuitionistic linear logic, with ⊸, ⊗, ⊕, &, 1, 0, ⊤, and ∀ connectives. One can think that ⊗ is a linear version ×, and ⊕ is a linear version of +, but it's not that simple, which I'll try to show in this blog post.

The linear type-system (or logic) rules are presented with sequents. I'm lazy, and write them in Unicode-ASCII-diagrams (Haskell highlighting works reasonably well!) instead of LaTeX. The simplest rule is following, saying that if we have a thing in a context, we can use it to conclude our proof/program:


------- ID
 AA

We will write the proof terms too, using heavily Haskell-inspired syntax.


--------------- ID
 a : A ⊢ a : A

Another general rule is CUT, but we won't need it: Cut is admissible. The rest of the rules are right and left sequents for particular combinators. In right rules the connective is on the right of turnstile (⊢), in the left the connective is on the left, respectively. (I'd like to use ⟹, to highlight that this is a sequent calculus, not a natural deduction; but long arrows often render funny in mono-space fonts/views).

Next we'll need to go through three connectives: ⊗, ⊸ and ∀, before we can meaningfully talk about BB-like-encoding.

#times, ⊗

The linear times is probably the most simple connective, illustrating the linearity. Its right rule is

 Γ ⊢ a : A    Δ ⊢ b : B
------------------------ ⊗R
 Γ;Δ ⊢ Pair a b : AB

To construct a Pair: we split the context into two disjoint parts Γ and Δ, and use Γ to construct the first half of a pair, and Δ to construct the second half of a pair. Thus we cannot write a term for A ⊢ A ⊗ A in linear logic.

The left rule is trickier:

 Γ; a : A; b : B ⊢ c : C
------------------------------------------------ ⊗L
 Γ; ab : ABcase ab of Pair a b -> c : C

We case on a Pair to get its halves a and b into the scope, and then continue. We cannot have fst and snd, as those would "use" the whole pair, forgetting other half, violating linearity.

#linear implication, ⊸

Linear implication uses the premise exactly once. Something you would hope politicians to use more (neither omitting the facts, nor repeating the same things over and over again).

The right rule for linear implication is similar to introduction rule of normal implication

 Γ; a : A ⊢  b : B
---------------------- ⊸R
 Γ ⊢ λ a -> b : AB

The left rule is again tricky. When we have a linear implication in the context, we can split the rest of the context into two halves. Then we use the other one to deduce the argument to function, and another with new assumption f a : B to deduce the final conclusion.

 Γ ⊢ a : A    Δ ; b : B ⊢ c : C
--------------------------------------- ⊸L
 Γ;Δ; f : ABlet b = f a in c : C

We can derive another rule:

 Δ ; b : B ⊢ c : C
--------------------------------------------- ⊸l'
 Δ ; a : A; f : ABlet b = f a in c : C

which follows by taking Γ = a : A, and discharging the Γ ⊢ a : A using ID-rule. The ⊸L' rule cannot be taken as a definition, as using it would often require CUT (which we avoid).

#universal quantification, ∀

For some reason, universal quantification is often omitted from linear logics. Its rules are similar to as in System F, as in this restricted (non-dependent) settings types are not terms.

The rules involve substitution in terms, also Y is assumed to be fresh. They resemble the rules for implication. The right rule says: to construct universally quantified expression, we need to be able to construct it for any variable name (i.e. X is not used in Γ).

 Γ ⊢ b : B [Y/X]
--------------------- ∀R
 Γ ⊢ Λ X -> b : X.B

Left rule is almost exact copy of ⊸L, except we can make up new type expressions as we go, they won't consume anything in the context. T type is a pedantic addition to remind us about that, but we'll omit it for brevity.

 T type    Γ; b' : B [T/X] ⊢ c : C
-------------------------------------- ∀L
 Γ; b : X. Blet b' = b @T in c : C

Now we have rules and syntax for times, implication and universal quantification, so we can BB-encode times-connective next.

#Encoding of times

It might be easy to guess the BB-encoding of times:

ABR. (ABR) ⊸ R

That's a reasonable guess if we can show A ⊗ B ⊸ R ≡ A ⊸ B ⊸ R isomorphism.

Here, the LHS ≡ RHS notation means LHS ⊢ RHS and RHS ⊢ LHS. The ⇒ direction is quite direct (read bottom-up):

------- ID
 RR
--------------- ⊸L'
 B ; BRR
----------------------- ⊸L'
 A ; B ; ABRR
----------------------- ⊗L
 AB ; ABRR
------------------------- ⊸R
 AB ⊢ (ABR) ⊸ R
----------------------------- ∀R
 ABR. (ABR) ⊸ R

Terms were omitted for clarity, but the last line would look like (after in lining lets).

lhs : AB
    ⊢ Λ R -> λ f -> case lhs of Pair a b -> f a b
    : R. (ABR) ⊸ R

The reverse, ⇐ direction is quite direct too (~all proofs in this fragment of linear logic are quite direct, that's the nice thing about whole thing)

 ...
--------------- ⊗R
 A ; BAB
------------------- ⊸R²    --------------- ID
 ⊢ (ABAB)          ABAB
------------------------------------------ ⊸L
 (ABAB) ⊸ ABAB
--------------------------------- ∀L
 R. (ABR) ⊸ RAB

The final line after inlining is:

rhs : R. (ABR) ⊸ R
    ⊢ rhs @(AB) (λ a b -> Pair a b)
    : AB

These are about what you would expect in non-linear calculus for × and →. The derivation trees would be similar. However with ⊕ (and &) things get more interesting.

#plus, ⊕

The plus connective looks very much like + in non-linear logic. I'd say, there shouldn't be anything surprising in the rules

 Γ ⊢ a : A
------------------- ⊕R₁
 Γ ⊢ InL a : AB

 Γ ⊢ b : B
------------------- ⊕R₂
 Γ ⊢ InR b : AB
 Γ ; a : A ⊢ c₁ : C    Γ ; b : B ⊢ c₂ : C
--------------------------------------------------------------- ⊕L
 Γ ; ab : ABcase ab of { InL a -> c₁ ; InR b -> c₂ } : C

At this point, one could think that we could do

ABR. (AR) ⊸ (BR) ⊸ R

analogously to either. But that doesn't work: Try! The right hand side type says that we have to use both: (A ⊸ R) and (B ⊸ R) arguments, yet we'll need only either one. We'll need a &-connective.

#with, &

With, & is an interesting construction. Its rules are mirrored versions of ⊕ rules, on the other hand they look like rules for ×.

Right rule is very similar to ⊗R, but note that both halves are constructed using the same context:

 Γ ⊢ a : A    Γ ⊢ b : B
------------------------ &R
 Γ ⊢ With a b : A & B

The above works as left rules only allow to use either half of the with-pair. We can choose, but cannot use both; in ⊗L-rule we cannot choose, must use both.

 Γ ; a : A ⊢ c : C
------------------------------------------- &L₁
 Γ ; ab : A & Blet a = fst ab in c : C

 Γ ; b : B ⊢ c : C
------------------------------------------- &L₂
 Γ ; ab : A & Blet b = snd ab in c : C

#Encoding of plus

Using with we can BB-encode plus.

ABR. ((AR) & (BR)) ⊸ R

That's using

ABR ≡ (AR) & (BR)

distributivity property.

Instead of derivation tree, I only write the proof terms. The ⇒ direction:

lhs : AB
    ⊢ Λ R -> λ f -> case lhs of
      InL a -> fst f a
      InR b -> snd f b
    : R. ((AR) & (BR)) ⊸ R

Depending on a lhs branch we pick proper half of with argument.

The reverse, ⇐ direction

rhs : R. ((AR) & (BR)) ⊸ R
    ⊢ rhs @(AB) (With (λ a -> InL a) (λ b -> InR b))
    : AB

#Encoding of with

So let's try to write a BB-encoding for A & B. Reasonable guess is

A & BR. ((AR) ⊕ (BR)) ⊸ R

The ⇒ direction works out:

lhs : A & B
    ⊢  Λ R -> λ arbr -> case arbr of
       InL ar -> ar (fst lhs)
       InR br -> br (snd lhs)
    : R. ((AR) ⊕ (BR)) ⊸ R

However the opposite direction doesn't:

rhs : R. ((AR) ⊕ (BR)) ⊸ R
    ⊢ rhs @(A & B) ???
    : A & B

where ⊢ ??? : (A ⊸ A & B) ⊕ (B ⊸ A & B). If we knew the future, we could put the correct InL or InR argument for ???, to match the later happening fst or snd. But we don't know how the result will be used.

We have to give up.

If we lookup in Girard's Linear Logic we'll learn that there are following properties

ABRABR
ABR ≡ (AR) & (BR)

but the third one is only half-distributive, i.e. works only in one direction.

A & BR  (AR) ⊕ (BR)

Unfortunate, but known fact. So, we cannot (at least obviously) simulate A & B using something else. Maybe with is an essential connective, you simply cannot admit. In other words, if you want something like with, you need to add that.

One way to understand linear logic is the resource-interpretation. If we set A = 2€, B = 3$, R = soda, then

  • A & B ⊸ R means "we put both 2€ and 3$ into a machine, it will make a choice (not using other currency) and give use a soda back"
  • (A ⊸ R) ⊕ (B ⊸ R) means "we make a choice, whether to put in 2€ and get a soda, or to put in 3$ and get a soda"

This example is especially relevant when paying with a credit card. You may pay in a local currency or your currency. If the exchange rate is fair, i.e. A ≈ B, then the two approaches are essentially the same. But when A ≉ B, maybe it's good to have a choice.

Note, that ⇒ direction is a machine asking you to make a choice (often they do), but it could make choice for you (charge local currency if you use contactless; or better option for merchant). But if you use cash, making a choice which currency to use, you cannot be charged price in different one.

#one, 1; zero, 0; top, ⊤

Story with units follows the lines of related binary-connectives.

One, 1 is unit of times. One doesn't carry any information, so we can freely introduce and remove it from the context. t has expected rules:

----------- 1R
One : 1
 Γ         ⊢ c : C
------------------- 1L
 Γ ; x : 1 ⊢ c : C

Zero, 0 is unit of plus. It has only a left rule, if there's a value of type 0 in a context, "all is lost", anything is derivable.

------------------------- 0L
 Γ; x : 0 ⊢ absurd x : C

Top, ⊤ is unit of with. It has only a right rule. It's an interesting type which let us dismiss stuff, putting "boring" stuff a side. But we cannot ever get completely rid of of them, neither extract back. A bit like black hole, I guess.

------------------------- ⊤R
 Γ ⊢ boring {x y...} :

Corresponding BB-encodings are

1R. RR

Conversions: an exercise.

0R. R

The ⇒ direction is given by absurd. The ⇐ by instantiating with @0.

⊤, as &, cannot really be admitted. We can squash everything into ⊤, but it's impossible to get rid of it.

#Conclusion

In higher order linear lambda calculus, type and term abstraction and application is not enough. The plus & and top ⊤ stand out.

On the other hand, if your linear language doesn't have plus and top, there are still a lot of you can do. Then the setup is close to normal lambda calculus; yet subtly different, you must be aware that not all tricks will work out.

This post is a not-so-gentle introduction into linear lambda calculus. I plan to write-mumble more, so this post will act as a reference point.

#Appendix: All rules in one place

--------------- ID
 a : A ⊢ a : A
 Γ ⊢  a : A    Δ; a' : A ⊢ b : B
--------------------------------- CUT
 Γ;Δ ⊢ let a' = a in b : B
 Γ; a : A ⊢  b : B
---------------------- ⊸R
 Γ ⊢ λ a -> b : AB
 Γ ⊢ a : A    Δ ; b : B ⊢ c : C
--------------------------------------- ⊸L
 Γ;Δ; f : ABlet b = f a in c : C
 Γ ⊢ a : A    Δ ⊢ b : B
------------------------ ⊗R
 Γ;Δ ⊢ Pair a b : AB
 Γ; a : A; b : B ⊢ c : C
------------------------------------------------ ⊗L
 Γ; ab : ABcase ab of Pair a b -> c : C
 Γ ⊢ b : B [Y/X]
--------------------- ∀R
 Γ ⊢ Λ X -> b : X.B
 T type    Γ; b' : B [T/X] ⊢ c : C
-------------------------------------- ∀L
 Γ; b : X. Blet b' = b @T in c : C
 Γ ⊢ a : A
------------------- ⊕R₁
 Γ ⊢ InL a : AB

 Γ ⊢ b : B
------------------- ⊕R₂
 Γ ⊢ InR b : AB
 Γ ; a : A ⊢ c₁ : C    Γ ; b : B ⊢ c₂ : C
--------------------------------------------------------------- ⊕L
 Γ ; ab : ABcase ab of { InL a -> c₁ ; InR b -> c₂ } : C
 Γ ⊢ a : A    Γ ⊢ b : B
------------------------ &R
 Γ ⊢ With a b : A & B
 Γ ; a : A ⊢ c : C
------------------------------------------- &L₁
 Γ ; ab : A & Blet a = fst ab in c : C

 Γ ; b : B ⊢ c : C
------------------------------------------- &L₂
 Γ ; ab : A & Blet b = snd ab in c : C
----------- 1R
One : 1
 Γ         ⊢ c : C
------------------- 1L
 Γ ; x : 1 ⊢ c : C
------------------------- 0L
 Γ; x : 0 ⊢ absurd x : C
---------------- ⊤R
 Γ ⊢ boring {x y...} :

  1. See Beyond Church encoding: Boehm-Berarducci isomorphism of algebraic data types and polymorphic lambda-terms

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