The Curry-Howard Correspondence

As we observed long ago, OCaml is a language in the ML family, and ML was originally designed as the meta language for a theorem prover—that is, a computer program designed to help prove and check the proofs of logical formulas. When constructing proofs, it's desirable to make sure that you can only prove true formulas, to make sure that you don't make incorrect arguments, etc.

The dream would be to have a computer program that can determine the truth or falsity of any logical formula. For some formulas, that is possible. But, one of the groundbreaking results in the early 20th century was that it is not possible, in general, for a computer program to do this. Alonzo Church and Alan Turing independently showed this in 1936. Church used the lambda calculus as a model for computers; Turing used what we now call Turing machines. The Church-Turing thesis is a hypothesis that says the lambda calculus and Turing machines both formalize what "computation" informally means.

Instead of focusing on that impossible task, we're going to focus on the relationship between proofs and programs. It turns out the two are deeply connected in a surprising way.

Computing with Evidence

We're accustomed to OCaml programs that manipulate data, such as integers and variants and functions. Those data values are always typed: at compile time, OCaml infers (or the programmer annotates) the types of expressions. For example, 3110 : int, and [] : 'a list. We long ago learned to read those as "3110 has type int", and "[] has type 'a list".

Let's try a different reading now. Instead of "has type", let's read "is evidence for". So, 3110 is evidence for int. What does that mean? Think of a type as a set of values. So, 3110 is evidence that type is not empty. Likewise, [] is evidence that the type 'a list is not empty. We say that the type is inhabited if it is not empty.

Are there empty types? There actually is one in OCaml, though we've never had reason to mention it before. It's possible to define a variant type that has no constructors:

type empty = |

We could have called that type anything we wanted instead of empty; the special syntax there is just writing | instead of actual constructors. (Note, that syntax might give some editors some trouble. You might need to put double-semicolon after it to get the formatting right.) It is impossible to construct a value of type empty, exactly because it has no constructors. So, empty is not inhabited.

Under our new reading based on evidence, we could think about functions as ways to manipulate and transform evidence—just as we are already accustomed to thinking about functions as ways to manipulate and transform data. For example, the following functions construct and destruct pairs:

let pair x y = (x, y)
let fst (x, y) = x
let snd (x, y) = y

OCaml infers the following types for those functions:

val pair : 'a -> 'b -> 'a * 'b
val fst : 'a * 'b -> 'a
val snd : 'a * 'b -> 'b

We could think of pair as a function that takes in evidence for 'a and evidence for 'b, and gives us back evidence for 'a * 'b. That latter piece of evidence is the the pair (x, y) containing the individual pieces of evidence, x and y. Similarly, fst and snd extract the individual pieces of evidence from the pair. Thus,

  • If you have evidence for 'a and evidence for 'b, you can produce evidence for 'a and 'b.
  • If you have evidence for 'a and 'b, then you can produce evidence for 'a.
  • If you have evidence for 'a and 'b, then you can produce evidence for 'b.

In learning to do proofs (say, in a discrete mathematics class), you will have learned that in order to prove two statements hold, you individually have to prove that each holds. That is, to prove the conjunction of A and B, you must prove A as well as prove B. Likewise, if you have a proof of the conjunction of A and B, then you can conclude A holds, and you can conclude B holds. We can write those patterns of reasoning as logical formulas, using /\ to denote conjunction and -> to denote implication:

A -> B -> A /\ B
A /\ B -> A
A /\ B -> B

Proofs are a form of evidence: they are logical arguments about the truth of a statement. So another reading of those formulas would be:

  • If you have evidence for A and evidence for B, you can produce evidence for A and B.
  • If you have evidence for A and B, then you can produce evidence for A.
  • If you have evidence for A and B, then you can produce evidence for B.

Notice how we now have given the same reading for programs and for proofs. They are both ways of manipulating and transforming evidence. In fact, take a close look at the types for pair, fst, and snd compared to the logical formulas that describe valid patterns of reasoning:

val pair : 'a -> 'b -> 'a * 'b         A -> B -> A /\ B
val fst : 'a * 'b -> 'a                A /\ B -> A
val snd : 'a * 'b -> 'b                A /\ B -> B

If you replace 'a with A, and 'b with B, and * with /\, the types of the programs are identical to the formulas!

The Correspondence

What we have just discovered is that computing with evidence corresponds to constructing valid logical proofs. This correspondence is not just an accident that occurs with these three specific programs. Rather, it is a deep phenomenon that links the fields of programming and logic. Aspects of it have been discovered by many people working in many areas. So, it goes by many names. One common name is the Curry-Howard correspondence, named for logicians Haskell Curry (for whom the functional programming language Haskell is named) and William Howard. This correspondence links ideas from programming to ideas from logic:

  • Types correspond to logical formulas (aka propositions).
  • Programs correspond to logical proofs.
  • Evaluation corresponds to simplification of proofs.

We've already seen the first two of those correspondences. The types of our three little programs corresponded to formulas, and the programs themselves corresponded to the reasoning done in proofs involving conjunctions. We haven't seen the third yet; we will later.

Let's dig into each of the correspondences to appreciate them more fully.

Types Correspond to Propositions

In propositional logic, formulas are created with atomic propositions, negation, conjunction, disjunction, and implication. The following BNF describes propositional logic formulas:

p ::= atom
    | ~ p      (* negation *)
    | p /\ p   (* conjunction *)
    | p \/ p   (* disjunction *)
    | p -> p   (* implication *)

atom ::= <identifiers>

For example, raining /\ snowing /\ cold is a proposition stating that it is simultaneously raining and snowing and cold (a weather condition known as Ithacating). An atomic proposition might hold of the world, or not. There are two distinguished atomic propositions, written true and false, which are always hold and never hold, respectively.

All these connectives (so-called because they connect formulas together) have correspondences in the types of functional programs.

Conjunction. We have already seen that the /\ connective corresponds to the * type constructor. Proposition A /\ B asserts the truth of both A and B. An OCaml value of type a * b contains values both of type a and b. Both /\ and * thus correspond to the idea of pairing or products.

Implication. The implication connective -> corresponds to the function type constructor ->. Proposition A -> B asserts that if you can show that A holds, then you can show that B holds. In other words, by assuming A, you can conclude B. In a sense, that means you can transform A into B. An OCaml value of type a -> b expresses that idea even more clearly. Such a value is a function that transforms a value of type a into a value of type b. Thus, if you can show that a is inhabited (by exhibiting a value of that type), you can show that b is inhabited (by applying the function of type a -> b to it). So, -> corresponds to the idea of transformation.

Disjunction. The disjunction connective \/ corresponds to something a little more difficult to express concisely in OCaml. Proposition A \/ B asserts that either you can show A holds or B holds. Let's strengthen that to further assert that in addition to showing one of them holds, you have to specify which one you are showing. Why would that matter?

Suppose we were working on a proof of the twin prime conjecture, an unsolved problem that states there are infinitely many twin primes (primes of the form n and n+2, such as 3 and 5, or 5 and 7). Let the atomic proposition TP denote that there are infinitely many twin primes. Then the proposition TP \/ ~ TP seems reasonable: either there are infinitely many twin primes, or there aren't. We wouldn't even have to figure out how to prove the conjecture! But if we strengthen the meaning of \/ to be that we have to state which one of the sides, left or right, holds, then we would either have to give a proof or disproof of the conjecture. No one knows how to do that currently. So we could not prove TP \/ ~ TP.

Henceforth we will use \/ in that stronger sense of having to identify whether we are giving a proof of the left or the right side proposition. Thus, we can't necessarily conclude p \/ ~ p for any proposition p: it will matter whether we can prove p or ~ p on their own. Technically, this makes our propositional logic constructive rather than classical. In constructive logic we must construct the proof of the individual propositions. Classical logic (the traditional way \/ is understood) does not require that.

Returning to the correspondence between disjunction and variants, consider this variant type:

type ('a, 'b) disj = Left of 'a | Right of 'b

A value v of that type is either Left a, where a : 'a; or Right b, where b : 'b. That is, v identifies (i) whether it is tagged with the left constructor or the right constructor, and (ii) carries within it exactly one sub-value of type either 'a or 'b—not two subvalues of both types, which is what 'a * 'b would be.

Thus, the (constructive) disjunction connective \/ corresponds to the disj type constructor. Proposition A \/ B asserts that either A or B holds as well as which one, left or right, it is. An OCaml value of type ('a, 'b) disj similarly contains a value of type either 'a or 'b as well as identifying (with the Left or Right constructor) which one it is. Both \/ and disj therefore correspond to the idea of unions.

Truth and Falsity The atomic proposition true is the only proposition that is guaranteed to always hold. There are many types in OCaml that are always inhabited, but the simplest of all of them is unit: there is one value () of type unit. So the proposition true (best) corresponds to the type unit.

Likewise, the atomic proposition false is the only proposition that is guaranteed to never hold. That corresponds to the empty type we introduced earlier, which has no constructors. (Other names for that type could include zero or void, but we'll stick with empty.)

There is a subtlety with empty that we should address. The type has no constructors, but it is nonetheless possible to write expressions that have type empty. Here is one way:

# type empty = |;;
type empty = |

# let rec loop x = loop x;;
val loop : 'a -> 'b = <fun>

# let e : empty = loop ();;
(no response from utop)

That expression type checks successfully, then enters an infinite loop. So, there is never any value of type empty that is produced, even though the expression has that type.

Here is another way:

# let e : empty = failwith "";;
Exception: Failure "".

Again, the expression type checks, but it never produces an actual value of type empty. Instead, this time an exception is produced.

So the type empty is not inhabited, even though there are some expressions of that type. But, if we require programs to be total, we can rule out those expressions. That means eliminating programs that raise exceptions or go into an infinite loop. We did in fact make that requirement when we started discussing formal methods, and we will continue to assume it.

Negation. This connective is the trickiest. Let's consider negation to actually be syntactic sugar. In particular, let's say that the propositional formula ~ p actually means this formula instead: p -> false. Why? The formula ~ p should mean that p does not hold. So if p did hold, then it would lead to a contradiction. Thus, given p, we could conclude false. This is the standard way of understanding negation in constructive logic.

Given that syntactic sugar, ~ p therefore corresponds to a function type whose return type is empty. Such a function could never actually return. Given our ongoing assumption that programs are total, that must mean it's impossible to even call that function. So, it must be impossible to construct a value of the function's input type. Negation therefore corresponds to the idea of impossibility, or contradiction.

Propositions as types. We have now created the following correspondence that enables us to read propositions as types:

  • /\ and *
  • -> and ->
  • \/ and disj
  • true and unit
  • false and empty
  • ~ and ... -> false

But that is only the first level of the Curry-Howard correspondence. It goes deeper...

Programs Correspond to Proofs

We have seen that programs and proofs are both ways to manipulate and transform evidence. In fact, every program is a proof that the type of the program is inhabited, since the type checker must verify that the program is well typed.

The details of type checking, though, lead to an even more compelling correspondence between programs and proofs. Let's restrict our attention to programs and proofs involving just conjunction and implication, or equivalently, pairs and functions. (The other propositional connectives could be included as well, but require additional work.)

Type checking rules. For type checking, we gave many rules to define when a program is well typed. Here are rules for variables, functions, and pairs:

{x : t, ...} |- x : t

A variable has whatever type the environment says it has.

env |- fun x -> e : t -> t'
if env[x -> t] |- e : t'

An anonymous function fun x -> e has type t -> t' if e has type t' in a static environment extended to bind x to type t.

env |- e1 e2 : t'
if env |- e1 : t -> t'
and env |- e2 : t

An application e1 e2 has type t' if e1 has type t -> t' and e2 has type t.

env |- (e1, e2) : t1 * t2
if env |- e1 : t1
and env |- e2 : t2

The pair (e1, e2) has type t1 * t2 if e1 has type t1 and e2 has type t2.

env |- fst e : t1
if env |- e : t1 * t2

env |- snd e : t2
if env |- e : t1 * t2

If e has type t1 * t2, then fst e has type t1, and snd e has type t2.

Proof trees. Another way of expressing those rules would be to draw proof trees that show the recursive application of rules, as we saw in recitation. Here are those proof trees:

{x : t, ...} |- x : t

   env[x -> t1] |- e2 : t2
env |- fun x -> e2 : t1 -> t2

env |- fun x -> e2 : t1 -> t2        env |- e1 : t1
          env |- (fun x -> e2) e1 : t2

env |- e1 : t1         env |- e2 : t2
     env |- (e1, e2) : t1 * t2

 env |- e : t1 * t2
 env |- fst e : t1

 env |- e : t1 * t2
 env |- snd e : t2

Proof trees, logically. Let's rewrite each of those proof trees to eliminate the programs, leaving only the types. At the same time, let's use the propositions-as-types correspondence to re-write the types as propositions:

 p |- p

   env, p1 |- p2
  env |- p1 -> p2

env |- p1 -> p2     env |- p1
        env |- p2

env |- p1      env |- p2
    env |- p1 /\ p2

env |- p1 /\ p2
   env |- p1

env |- p1 /\ p2
   env |- p2

Each rule can now be read as a valid form of logical reasoning. Whenever we write env |- t, it means that "from the assumptions in env, we can conclude p holds". A rule, as usual, means that from all the premisses above the line, the conclusion below the line holds.

Proofs and programs. Now consider the following proof tree, showing the derivation of the type of a program:

------------------------           ------------------------
{p : a * b} |- p : a * b           {p : a * b} |- p : a * b
------------------------           ------------------------
{p : a * b} |- snd p : b           {p : a * b} |- fst p : a
        {p : a * b} |- (snd p, fst p) : b * a
     {} |- fun p -> (snd p, fst p) : a * b -> b * a

That program shows that you can swap the components of a pair, thus swapping the types involved.

If we erase the program, leaving only the types, and re-write those as propositions, we get this proof tree:

----------------           ----------------
a /\ b |- a /\ b           a /\ b |- a /\ b
----------------          -----------------
  a /\ b |- b                a /\ b |- a
              a /\ b |- b /\ a
           {} |- a /\ b -> b /\ a

And that is a valid proof tree for propositional logic. It shows that you can swap the sides of a conjunction.

What we see from those two proof trees is: a program is a proof. A well-typed program corresponds to the proof of a logical proposition. It shows how to compute with evidence, in this case transforming a proof of a /\ b into a proof of b /\ a.

Programs are proofs. We have now created the following correspondence that enables us to read programs as proofs:

  • A program e : t corresponds to a proof of the logical formula to which t itself corresponds.
  • The proof tree of |- t corresponds to the proof tree of {} |- e : t.
  • The proof rules for typing a program correspond to the rules for proving a proposition.

But that is only the second level of the Curry-Howard correspondence. It goes deeper...

Evaluation Corresponds to Simplification

We will treat this part of the correspondence only briefly. Consider the following program:

fst (a, b)

That program would of course evaluate to a.

Next, consider the typing derivation of that program. The variables a and b must be bound to some types in the static environment for the program to type check.

------------------------         -------------------------
{a : t, b : t'} |- a : t         {a : t, b : t'} |- b : t'
         {a : t, b : t'} |- (a, b) : t * t'
         {a : t, b : t'} |- fst (a, b) : t

Erasing that proof tree to just the propositions, per the proofs-as-programs correspondence, we get this proof tree:

-----------            -----------
t, t' |-  t            t, t' |- t'
         t, t' |- t /\ t'
            t, t' |- t

However, there is a much simpler proof tree with the same conclusion:

  t, t' |- t

In other words, we don't need the detour through proving t /\ t' to prove t, if t is already an assumption. We can instead just directly conclude t.

Likewise, there is a simpler typing derivation corresponding to that same simpler proof:

{a : t, b : t'} |- a : t

Note that typing derivation is for the program a, which is exactly what the bigger program fst (a, b) evaluates to.

Thus evaluation of the program causes the proof tree to simplify, and the simplified proof tree is actually (through the proofs-as programs correspondence) a simpler proof of the same proposition. Evaluation therefore corresponds to proof simplification. And that is the final level of the Curry-Howard correspondence.

What It All Means

Logic is a fundamental aspect of human inquiry. It guides us in reasoning about the world, in drawing valid inferences, in deducing what must be true vs. what must be false. Training in logic and argumentation—in various fields and disciplines—is one of the most important parts of a higher education.

The Curry-Howard correspondence shows that logic and computation are fundamentally linked in a deep and maybe even mysterious way. The basic building blocks of logic (propositions, proofs) turn out to correspond to the basic building blocks of computation (types, functional programs). Computation itself, the evaluation or simplification of expressions, turns out to correspond to simplification of proofs. The very task that computers do therefore is the same task that humans do in trying to present a proof in the best possible way.

Computation is thus intrinsically linked to reasoning. And functional programming is a fundamental part of human inquiry.

Could there be a better reason to take this course?


This section is based on the paper Propositions as Types by Philip Wadler. You can watch an entertaining recorded lecture lecture by Prof. Wadler on this material.

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