# Curry-Howard-Lambek Correspondence The Curry-Howard-Lambek correspondence is a three way isomorphism between types (in programming languages), propositions (in logic) and objects of a Cartesian closed category. Interestingly, the isomorphism maps programs (functions in Haskell) to (constructive) proofs in logic (and vice versa). ## Life, the Universe and Everything As is well established by now, ~~~ {.literatehaskell} > theAnswer :: Integer > theAnswer = 42 ~~~ The logical interpretation of the program is that the type Integer is inhabited (by the value 42), so the existence of this program proves the proposition Integer (a type without any value is the "bottom" type, a proposition with no proof). A (non-trivial) Haskell function maps a value (of type a, say) to another value (of type b), therefore, given a value of type a (a proof of a), it constructs a value of type b (so the proof is transformed into a proof of b)! So b is inhabited if a is, and a proof of a -> b is established (hence the notation, in case you were wondering). ~~~ {.haskell} representation :: Bool -> Integer representation False = 0 representation True = 1 ~~~ says, for example, if Boolean is inhabited, so is Integer (well, the point here is demonstration, not discovery). ## Connectives Of course, atomic propositions contribute little towards knowledge, and the Haskell type system incorporates the logical connectives $\wedge$ and $\vee$, though heavily disguised. Haskell handles $\vee$ conjuction in the manner described by Intuitionistic Logic. When a program has type $A \vee B$, the value returned itself indicates which one. The algebraic data types in Haskell has a tag on each alternative, the constructor, to indicate the injections: ~~~ {.literatehaskell} > data Message a = OK a | Warning a | Error a > > p2pShare :: Integer -> Message String > p2pShare n | n == 0 = Warning "Share! Freeloading hurts your peers." > | n < 0 = Error "You cannot possibly share a negative number of files!" > | n > 0 = OK ("You are sharing " ++ show n ++ " files." ~~~ {.haskell} So any one of OK String, Warning String or Error String proves the proposition Message String, leaving out any two constructors would not invalidate the program. At the same time, a proof of Message String can be pattern matched against the constructors to see which one it proves. On the other hand, to prove String is inhabited from the proposition Message String, it has to be proven that you can prove String from any of the alternatives... ~~~ {.literatehaskell} > show :: Message String -> String > show (OK s) = s > show (Warning s) = "Warning: " ++ s > show (Error s) = "ERROR! " ++ s ~~~ The $\wedge$ conjuction is handled via an isomorphism in Closed Cartesian Categories in general (Haskell types belong to this category): $\mathrm{Hom}(X\times Y,Z) \cong \mathrm{Hom}(X,Z^Y)$. That is, instead of a function from $X \times Y$ to $Z$, we can have a function that takes an argument of type $X$ and returns another function of type $Y \rightarrow Z$, that is, a function that takes $Y$ to give (finally) a result of type $Z$: this technique is (known as currying) logically means $A \wedge B \rightarrow C \equiv A \rightarrow (B \rightarrow C)$. (insert quasi-funny example here) So in Haskell, currying takes care of the $\wedge$ connective. Logically, a proof of $A \wedge B$ is a pair (a,b) of proofs of the propositions. In Haskell, to have the final $C$ value, values of both $A$ and $B$ have to be supplied (in turn) to the (curried) function. # Theorems for free! Things get interesting when polymorphism comes in. The composition operator in Haskell proves a very simple theorem. ~~~ {.literatehaskell} > (.) :: (a -> b) -> (b -> c) -> (a -> c) >(.) f g x = f (g x) ~~~ The type is, actually, forall a b c. (a -> b) -> (b -> c) -> (a -> c), to be a bit verbose, which says, logically speaking, for all propositions a, b and c, if from a, b can be proven, and if from b, c can be proven, then from a, c can be proven (the program says how to go about proving: just compose the given proofs!) # Negation Of course, there's not much you can do with just truth. forall b. a -> b says that given a, we can infer anything. Therefore we will take forall b. a -> b as meaning not a. Given this, we can prove several more of the axioms of logic. ~~~ {.literatehaskell} > type Not x = (forall a. x -> a) > > doubleNegation :: x -> Not (Not x) > doubleNegation k pr = pr k > > contraPositive :: (a -> b) -> (Not b -> Not a) > contraPositive fun denyb showa = denyb (fun showa) > > deMorganI :: (Not a, Not b) -> Not (Either a b) > deMorganI (na, _) (Left a) = na a > deMorganI (_, nb) (Right b) = nb b > > deMorganII :: Either (Not a) (Not b) -> Not (a,b) > deMorganII (Left na) (a, _) = na a > deMorganII (Right nb) (_, b) = nb b ~~~ # Type classes A type class in Haskell is a proposition about a type. ~~~ {.literatehaskell} > class Eq a where > (==) :: a -> a -> Bool > (/=) :: a -> a -> Bool ~~~ means, logically, there is a type a for which the type a -> a -> Bool is inhabited, or, from a it can be proved that a -> a -> Bool (the class promises two different proofs for this, having names == and /=). This proposition is of existential nature (not to be confused with [existential type]()). A proof for this proposition (that there is a type that conforms to the specification) is (obviously) a set of proofs of the advertised proposition (an implementation), by an instance declaration: ~~~ {.literatehaskell} > instance Eq Bool where > True == True = True > False == False = True > _ == _ = False > > (/=) a b = not (a == b) ~~~ A not-so-efficient sort implementation would be: ~~~ {.literatehaskell} > sort [] = [] > sort (x : xs) = sort lower ++ [x] ++ sort higher > where lower = filter (<= x) xs > higher = filter (> x) xs ~~~ Haskell infers its type to be forall a. (Ord a) => [a] -> [a]. It means, if a type a satisfies the proposition about propositions Ord (that is, has an ordering defined, as is necessary for comparison), then sort is a proof of [a] -> [a]. For this to work, somewhere, it should be proved (that is, the comparison functions defined) that Ord a is true. # Multi-parameter type classes Haskell makes frequent use of multiparameter type classes. Type classes constitute a Prolog-like logic language, and multiparameter type classes define a relation between types. ## Functional dependencies These type level functions are set-theoretic. That is, class TypeClass a b | a -> b defines a relation between types a and b, and requires that there would not be different instances of TypeClass a b and TypeClass a c for different b and c, so that, essentially, b can be inferred as soon as a is known. This is precisely functions as relations as prescribed by set theory. # Indexed types *(please someone complete this, should be quite interesting, I have no idea what it should look like logically)*