X-Git-Url: http://lambda.jimpryor.net/git/gitweb.cgi?p=lambda.git;a=blobdiff_plain;f=week6.mdwn;h=57158c6b92d1ac900cdb3a738a35a70c9eb29f64;hp=221e02077378f650111c0facbaea9cc9d649eb23;hb=8e8edc03022be0da1621d6f1b418919c2637856e;hpb=96a8c8c9b81fc914ac7ec368fab0ffa4bcf4177a diff --git a/week6.mdwn b/week6.mdwn index 221e0207..57158c6b 100644 --- a/week6.mdwn +++ b/week6.mdwn @@ -171,3 +171,145 @@ We can use functions that take arguments of type unit to control execution. In Scheme parlance, functions on the unit type are called *thunks* (which I've always assumed was a blend of "think" and "chunk"). +Curry-Howard, take 1 +-------------------- + +We will return to the Curry-Howard correspondence a number of times +during this course. It expresses a deep connection between logic, +types, and computation. Today we'll discuss how the simply-typed +lambda calculus corresponds to intuitionistic logic. This naturally +give rise to the question of what sort of computation classical logic +corresponds to---as we'll see later, the answer involves continuations. + +So at this point we have the simply-typed lambda calculus: a set of +ground types, a set of functional types, and some typing rules, given +roughly as follows: + +If a variable `x` has type σ and term `M` has type τ, then +the abstract `\xM` has type σ `-->` τ. + +If a term `M` has type σ `-->` τ, and a term `N` has type +σ, then the application `MN` has type τ. + +These rules are clearly obverses of one another: the functional types +that abstract builds up are taken apart by application. + +The next step in making sense out of the Curry-Howard corresponence is +to present a logic. It will be a part of intuitionistic logic. We'll +start with the implicational fragment (that is, the part of +intuitionistic logic that only involves axioms and implications): + +
+Axiom: --------- + A |- A + +Structural Rules: + + Γ, A, B, Δ |- C +Exchange: --------------------------- + Γ, B, A, Δ |- C + + Γ, A, A |- B +Contraction: ------------------- + Γ, A |- B + + Γ |- B +Weakening: ----------------- + Γ, A |- B + +Logical Rules: + + Γ, A |- B +--> I: ------------------- + Γ |- A --> B + + Γ |- A --> B Γ |- A +--> E: ----------------------------------- + Γ |- B ++ +`A`, `B`, etc. are variables over formulas. +Γ, Δ, etc. are variables over (possibly empty) sequences +of formulas. Γ `|- A` is a sequent, and is interpreted as +claiming that if each of the formulas in Γ is true, then `A` +must also be true. + +This logic allows derivations of theorems like the following: + +
+------- Id +A |- A +---------- Weak +A, B |- A +------------- --> I +A |- B --> A +----------------- --> I +|- A --> B --> A ++ +Should remind you of simple types. (What was `A --> B --> A` the type +of again?) + +The easy way to grasp the Curry-Howard correspondence is to *label* +the proofs. Since we wish to establish a correspondence between this +logic and the lambda calculus, the labels will all be terms from the +simply-typed lambda calculus. Here are the labeling rules: + +
+Axiom: ----------- + x:A |- x:A + +Structural Rules: + +Exchange: Γ, x:A, y:B, Δ |- R:C + ------------------------------- + Γ, y:B, x:A, Δ |- R:C + +Contraction: Γ, x:A, x:A |- R:B + -------------------------- + Γ, x:A |- R:B + +Weakening: Γ |- R:B + --------------------- + Γ, x:A |- R:B [x chosen fresh] + +Logical Rules: + +--> I: Γ, x:A |- R:B + ------------------------- + Γ |- \xM:A --> B + +--> E: Γ |- f:(A --> B) Γ |- x:A + ------------------------------------- + Γ |- (fx):B ++ +In these labeling rules, if a sequence Γ in a premise contains +labeled formulas, those labels remain unchanged in the conclusion. + +What is means for a variable `x` to be chosen *fresh* is that +`x` must be distinct from any other variable in any of the labels +used in the proof. + +Using these labeling rules, we can label the proof +just given: + +
+------------ Id +x:A |- x:A +---------------- Weak +x:A, y:B |- x:A +------------------------- --> I +x:A |- (\y.x):(B --> A) +---------------------------- --> I +|- (\x y. x):A --> B --> A ++ +We have derived the *K* combinator, and typed it at the same time! + +Need a proof that involves application, and a proof with cut that will +show beta reduction, so "normal" proof. + +[To do: add pairs and destructors; unit and negation...] + +Excercise: construct a proof whose labeling is the combinator S.