COMP6463: λ-calculus

Transcription

1 COMP6463: λ-calculus 1. Basics Michael Norrish Canberra Research Lab., NICTA Semester 2, 2015

2 Outline Introduction Lambda Calculus Terms Alpha Equivalence Substitution Dynamics Beta Reduction Eta Reduction Normal Forms Evaluation Strategies Conclusion

3 The Lambda Calculus The λ-calculus is a fundamental computational model. It also has a number of connections to logic. Alonzo Church ( ) The next five lectures (including this one) will concentrate on the untyped calculus. The typed calculus has the most elegant connections to logic, so we will focus on computation. But there is also an interesting equational logic to consider. Introduction 3/30

4 Course Plan By me: 1. Basics (today) 2. Equational Logic connection to β examples inconsistent assumptions soundness & completeness 3. Soundness via Church-Rosser 4. Standardisation 5. Computation Church Numerals Encoding Other Types Encoding the Recursive Functions And then, five lectures on the typed calculus by Jeremy Dawson. Introduction 4/30

5 Some Sources H. P. Barendregt, The Lambda Calculus: Its Syntax and Semantics. Elsevier, ISBN Chris Hankin, Lambda Calculi: A Guide for Computer Scientists. Oxford University Press, ISBN Introduction 5/30

6 What are Lambda Terms? Lambda terms make up the world s simplest programming language. A lambda term is either a variable v; or the application of term M to term N, written M N; or the abstraction of variable v in term M, written (λv. M). Examples: f x f, a function, applied to an argument x (f x) y a function applied to two arguments f (g x) two function calls (λv. v) the identity function (λu. (λv. u z)) x abstraction and application Lambda Calculus Terms 6/30

7 Term Shorthands Applications are left-associative. So, you can write M N P, and don t have to write (M N) P (You do have to write M (N P) with the parentheses.) You can chain binders. Instead of (λu. (λv. M)), just write (λu v. M) Thus: instead of S = (λa b c. a c (b c)) S = (λa. (λb. (λc. (a c) (b c)))) Think of abstract syntax trees if necessary. Lambda Calculus Terms 7/30

8 Abstractions are (Anonymous) Functions In secondary school mathematics, you learn to write things like f(x) = x 2 + 2x + 1 meaning that f is a function that takes a parameter x and returns a value derived from that x. When you learnt to program, you might have learnt to write int f (int x) { return x*x + 2*x + 1; } instead. Lambda Calculus Terms 8/30

9 Abstractions are (Anonymous) Functions In λ-calculus (with arithmetic added), you might write f = (λx. x 2 + 2x + 1) In other words, (λx. x 2 + 2x + 1) is a function that takes a value x as a parameter, and calculates a return value. This λ-term is a function without a name. We can decide to give it the name f (or g, or nothing) later. Lambda Calculus Terms 9/30

10 Free and Bound Names Consider: (λv. u v) The variable v is bound. It s the name of a parameter. The variable u is free. It s not the name of an enclosing parameter. The same notions are apparent in other languages: int f (int v) { return u + v; } Lambda Calculus Terms Alpha Equivalence 10/30

11 Bound Names Can be Renamed These two functions are the same: int f (int v) { return u + v; } int f (int x) { return u + x; } But this one is different: int f (int u) { return u + u; } Clearly, not all bound variable renaming is OK. Lambda Calculus Terms Alpha Equivalence 11/30

12 Alpha Equivalence Two λ-terms M and N are alpha-equivalent (written M N) if they are the same up to renaming of bound variables Proof rules v v M 1 M 2 N 1 N 2 M 1 N 1 M 2 N 2 v not free in M N (uv) M (λu. M) (λv. N) where (uv) M = swap u and v everywhere they appear in M Lambda Calculus Terms Alpha Equivalence 12/30

13 Alpha Equivalence Examples Rule for (λv. M) again: v not free in M N (uv) M (λu. M) (λv. N) where (uv) M = swap u and v everywhere they appear in M So, (λv. v u) x (λw. w u) x (λu. u u) x Lambda Calculus Terms Alpha Equivalence 13/30

14 Alpha Equivalence Examples Rule for (λv. M) again: v not free in M N (uv) M (λu. M) (λv. N) where (uv) M = swap u and v everywhere they appear in M So, (λv. v u) x (λw. w u) x (λu. u u) x (λu. (λv. u v) u) (λv. (λu. v u) v) (λu. (λu. u v) u) Lambda Calculus Terms Alpha Equivalence 13/30

15 Alpha Equivalence Examples Rule for (λv. M) again: v not free in M N (uv) M (λu. M) (λv. N) where (uv) M = swap u and v everywhere they appear in M So, (λv. v u) x (λw. w u) x (λu. u u) x (λu. (λv. u v) u) (λv. (λu. v u) v) (λu. (λu. u v) u) (λx. (λy. f y) x) (λy. (λy. f y) y) (λf. (λy. f y) f) Lambda Calculus Terms Alpha Equivalence 13/30

16 Substitution Substitution is a ternary operation: M[v := N] means replace free occurrences of v in M with N Conditions: (making sure that free variables in N are not captured by binders in M) (alpha-convert M as necessary) Freeness: (λv. M)[v := N] (λv. M) (as v is not free in (λv. M)) Capture-avoiding: (λv. u v)[u := v] (λv. v v) (!!!) Lambda Calculus Terms Substitution 14/30

17 Substitution Substitution is a ternary operation: M[v := N] means replace free occurrences of v in M with N Conditions: (making sure that free variables in N are not captured by binders in M) (alpha-convert M as necessary) Freeness: (λv. M)[v := N] (λv. M) (as v is not free in (λv. M)) Capture-avoiding: (λv. u v)[u := v] (λv. v v) (!!!) (λv. u v)[u := v] (λw. u w)[u := v] (λw. v w) Lambda Calculus Terms Substitution 14/30

18 Substitution: Please Take Care! Substitution is tricky! You have seen it before (in handling f.o.l. quantifiers) Use the Barendregt Variable Convention: rename bound variables so they don t overlap with free variables. Lambda Calculus Terms Substitution 15/30

19 Substitution Drives Computation A General Programming Language Axiom: When you apply a function to an argument, the result is as if you substituted the actual argument for the formal parameter and then performed the specified computation For example, how do we figure out what f(3,5) will return? int f (int x, int y) { return 2*x + y; } Dynamics Beta Reduction 16/30

20 Beta Reduction: λ-terms Doing Things Substitution of actuals for formals is the essence of beta-reduction: (λv. M) N β M[v := N] Reduction can occur anywhere within a term. Examples: (λv. v) (λx. x v) β (λx. x v) Dynamics Beta Reduction 17/30

21 Beta Reduction: λ-terms Doing Things Substitution of actuals for formals is the essence of beta-reduction: (λv. M) N β M[v := N] Reduction can occur anywhere within a term. Examples: (λv. v) (λx. x v) β (λx. x v) (λu. (λv. v z) (u z)) β (λu. (u z) z) Dynamics Beta Reduction 17/30

22 Beta Reduction: λ-terms Doing Things Substitution of actuals for formals is the essence of beta-reduction: (λv. M) N β M[v := N] Reduction can occur anywhere within a term. Examples: (λv. v) (λx. x v) β (λx. x v) (λu. (λv. v z) (u z)) β (λu. (u z) z) (λu. (λv. v u) (u z)) v z β (λw. w v) (v z) z β (v z) v z Dynamics Beta Reduction 17/30

23 Beta Reduction: λ-terms Doing Things Substitution of actuals for formals is the essence of beta-reduction: (λv. M) N β M[v := N] Reduction can occur anywhere within a term. Examples: (λv. v) (λx. x v) β (λx. x v) (λu. (λv. v z) (u z)) β (λu. (u z) z) (λu. (λv. v u) (u z)) v z β (λw. w v) (v z) z β (v z) v z (λx. x x) (λy. y y) β (λy. y y) (λy. y y) (λx. x x) (λy. y y) Dynamics Beta Reduction 17/30

24 Beta Reduction Rules and Notation (λv. M) N β M[v := N] M β M M N β M N N β N M N β M N M β M (λv. M) β (λv. M ) Write M β N to mean M can take zero or more β-reduction steps and evolve to N. Write M + β N to mean M can take one or more β-reduction steps and evolve to N. Dynamics Beta Reduction 18/30

25 Beta Reduction Does It All! Lambda Terms + Beta Reduction = All Computation Any computation can be encoded in what we have just seen. We will look at computational aspects of the λ-calculus in detail in later lectures. In particular, it is easy to encode Numbers Recursion And that s all you need. Dynamics Beta Reduction 19/30

26 Eta Reduction We can also add another computational rule, η-reduction: v not free in M (λv. M v) η M (Can perform η-reduction anywhere within a term, as with β.) We will later see how η ties into extensionality. Eta-reduction and eta-expansion (!) also play a role in the typed λ-calculus. Dynamics Eta Reduction 20/30

27 Can Combine Beta and Eta For example: βη βη (λf. f x) (λy. g y) (λf. f x) g g x Dynamics Eta Reduction 21/30

28 Can Combine Beta and Eta For example: Or: (λf. f x) (λy. g y) βη (λf. f x) g βη g x (λf. f x) (λy. g y) βη (λy. g y) x βη g x (Eta and Beta coincide here) Dynamics Eta Reduction 21/30

29 Normal Forms A normal form is a term that cannot be further reduced. For example, (λx. g x) is a β-normal form, but not an η-normal form. It is reasonable to think of normal forms as values towards which we want evaluation to proceed. Dynamics Normal Forms 22/30

30 Normal Form Questions It is natural to ask: 1. Does term M have a normal form? 2. Does M have more than one normal form? And, we can ask these questions of all terms too. Dynamics Normal Forms 23/30

31 Lambda Term Evaluation is Non-Deterministic Recall: M β M M N β M N N β N M N β M N Remember also: (λx. x x) (λy. y y) β (λy. y y) (λy. y y) Call (λx. x x) (λy. y y), the self-looping term, Ω. Dynamics Evaluation Strategies 24/30

32 Infinite Loops (Depending on Evaluation Order) Consider what (λx. y) Ω might do. If we keep evaluating the argument (Ω), we never stop: (λx. y) Ω β (λx. y) Ω β (λx. y) Ω β If we apply the top-level function, substituting Ω for x we get: Done! (λx. y) Ω β y This term ((λx. y) Ω) has one normal form (y), but not all evaluation choices reach it. Dynamics Evaluation Strategies 25/30

33 How Do Other Languages Do This? int g l o b a l = 3; unsigned f (void) { unsigned x = 0; while (1) { x++; } return x; } int g(unsigned i ) { return g l o b a l ; } int main(void) { return g( f ()); } What happens in C? Dynamics Evaluation Strategies 26/30

34 Evaluation Strategy #1: Applicative Order Evaluate everything from the bottom up. I.e., in (λv. M) N work will start with M, passing to N and performing the top-level β-reduction last A function s arguments (and the function itself) will be evaluated before the argument is passed to the function. Also known as strict evaluation. (Used in Fortran, C, Pascal, Ada, SML, Java... ) Causes (λx. y) Ω to go into an infinite loop. Dynamics Evaluation Strategies 27/30

35 Evaluation Strategy #2: Normal Order Evaluate top-down, left-to-right. With (λv. M) N, start by performing the β-reduction, producing M[v := N] Find the top-most, left-most β-redex and reduce it. Keep going This strategy is behind the lazy evaluation of languages like Haskell. Normal order evaluation will always terminate with a normal form if a term has one. (Proof in Lecture 4) Dynamics Evaluation Strategies 28/30

36 Strategy Rules Write M n N for M normal order reduces to N. Then: (λv. M) N n M[v := N] M n M (λv. M) n (λv. M ) M n M M not an abstraction M N n M N N n N M not an abstraction M in β-nf M N n M N Rules for applicative order evaluation will be an assignment question. Dynamics Evaluation Strategies 29/30

37 Summary Lambda Terms: Variables, applications, abstractions Bound names, free names, alpha equivalence Substitution Dynamic Behaviour: Beta reduction: computation through substitution Eta reduction Normal forms, and strategies for achieving them Conclusion 30/30