Resources: Some of the slides of this lecture were derived from [Järvi], with permission of the original author, by copy x = 1 & paste or by selection, annotation, or rewording. [Järvi] is let x = 1 in ... in turn based on [Pierce] as the underlying textbook. x(1). !x(1) x.set(1) Programming Paradigms and Formal Semantics Implementing Lambda Calculi in Prolog Ralf Lämmel <1h [Järvi] Slides by J. Järvi: “Programming Languages”, CPSC 604 @ TAMU (2009) [Pierce] B.C. Pierce: Types and Programming Languages, MIT Press, 2002 Table of contents • Untyped NB Objective: compare grammar- and rule-based formalization • Typed NB with the corresponding Prolog encodings / implementation. • Untyped lambda calculus • Applied, untyped lambda calculus • Applied, typed lambda calculus © Ralf Lämmel, 2009-2011 unless noted otherwise 192 Untyped NB © Ralf Lämmel, 2009-2011 unless noted otherwise 193 This slide is derived from Jaakko Järvi’s slides for his course ”Programming Languages”, CPSC 604 @ TAMU. SyntaxMore than one syntactic of category NB Language NB t ::= terms: v value if t1 then t2 else t3 conditional succ t successor pred t predecessor iszero t test for zero v ::= values: true constant true false constant false nv numeric value nv ::= numeric values: 0 zero value succ nv successor value © Ralf Lämmel, 2009-2011 unless noted otherwise 194 54 / 58 Prolog Syntax of NB term(V) :- value(V). term(if(T1,T2,T3)) :- term(T1), term(T2), term(T3). term(succ(T)) :- term(T). term(pred(T)) :- term(T). term(iszero(T)) :- term(T). We are faithful to the distinction of the value(true). syntactical categories. value(false). value(NV) :- nvalue(NV). nvalue(zero). nvalue(succ(NV)) :- nvalue(NV). © Ralf Lämmel, 2009-2011 unless noted otherwise 195 Prolog NB: sample terms supposed to evaluate to 0 if(iszero(pred(succ(zero))),zero,succ(succ(zero))). supposed to evaluate to false if(iszero(zero),iszero(succ(zero)),true). © Ralf Lämmel, 2009-2011 unless noted otherwise 196 This slide is derived from Jaakko Järvi’s slides for his course ”Programming Languages”, CPSC 604 @ TAMU. Typ es for EvaluationNB rules of NB Reminder evaluation rules NB (SOS) E-Iszero E-IszeroZero t t → iszero 0 true iszero t iszero t → → E-Succ E-Pred E-IszeroSucc t t t t iszero (succ nv) false → → → succ t succ t pred t pred t → → E-PredZero E-PredSucc E-IfTrue pred 0 0 pred (succ nv) nv if true then t else t t → → 2 3 → 2 E-IfFalse Exercise: what if false then t2 else t3 t3 → happens to our type system E-If t t when we omit 1 → 1 if t then t else t if t then t else t the rule? 1 2 3 → 1 2 3 14 / 28 © Ralf Lämmel, 2009-2011 unless noted otherwise 197 Prolog Evaluation rules of NB (SOS) % eval(pred(zero),zero). Disfavored eval(pred(succ(NV)),NV) :- nvalue(NV). semantics eval(succ(T1),succ(T2)) :- eval(T1,T2). eval(pred(T1),pred(T2)) :- eval(T1,T2). Note: appearances of eval(iszero(zero),true). metavariables in SOS eval(iszero(succ(NV)),false) :- nvalue(NV). translate into tests for values. eval(iszero(T1),iszero(T2)) :- eval(T1,T2). eval(if(true,T2,_),T2). eval(if(false,_,T3),T3). eval(if(T1,T2,T3),if(T4,T2,T3)) :- eval(T1,T4). © Ralf Lämmel, 2009-2011 unless noted otherwise 198 Prolog Reflexive, transitive closure manysteps(V,V) :- value(V). manysteps(T1,V) :- eval(T1,T2), manysteps(T2,V). This is like ➝* in the formal setup, and the predicate works for any language with a binary reduction relation eval/2. © Ralf Lämmel, 2009-2011 unless noted otherwise 199 Prolog Composing everything :- [...]. % Import syntax and semantics :- [...]. % Import main predicate :- current_prolog_flag(argv,Argv), ( append(_,['--',Input],Argv), main(Input), halt; true ). Hence, we can invoke the language processor from the command-line prompt. © Ralf Lämmel, 2009-2011 unless noted otherwise 200 Prolog main(Input) :- see(Input), read(Term), seen, format('Input term: ~w~n',[Term]), manysteps(Term,X), show(X,Y), format('Value of term: ~w~n',[Y]). show(zero,0) :- !. show(succ(X),Z) :- !, show(X,Y), Z is Y + 1. show(X,X). © Ralf Lämmel, 2009-2011 unless noted otherwise 201 Running the NB interpreter $ swipl -q -f main.pro -- ../samples/sample1.nb Input term: if(iszero(pred(succ(zero))),zero,succ(succ(zero))) Value of term: 0 $ © Ralf Lämmel, 2009-2011 unless noted otherwise 202 Typed NB © Ralf Lämmel, 2009-2011 unless noted otherwise 203 This slide is derived from Jaakko Järvi’s slides for his course ”Programming Languages”, CPSC 604 @ TAMU. Typ es for NB typing rules NB Types in NB First, we need a few new syntactic forms T ::= types: Bool the Boolean type Nat the type of numeric values And typing rules: T-If T-True T-False t1 : Bool t2 : Tt3 : T true : Bool false : Bool if t1 then t2 else t3 : T T-Succ T-Pred T-Iszero © Ralf Lämmel, T-Zero2009-2011 unless noted otherwise 204 t : Nat t : Nat t : Nat 0 : Nat succ t : Nat pred t : Nat iszero t : Bool 15 / 28 Typ es for NB typing rules NB This slide is derived from Jaakko Järvi’s slides for his course ”Programming Languages”, CPSC 604 @ TAMU. First, we need a few new syntactic forms T ::= types: Bool typingthe Boolean typerules NatNBthe type of numeric values And typing rules: T-If T-True T-False t1 : Bool t2 : Tt3 : T true : Bool false : Bool if t1 then t2 else t3 : T T-Succ T-Pred T-Iszero T-Zero t : Nat t : Nat t : Nat 0 : Nat succ t : Nat pred t : Nat iszero t : Bool 15 / 28 © Ralf Lämmel, 2009-2011 unless noted otherwise 205 Prolog NB typing rules welltyped(true,bool). 1:1 mapping welltyped(false,bool). welltyped(zero,nat). welltyped(succ(T),nat) :- welltyped(T,nat). welltyped(pred(T),nat) :- welltyped(T,nat). welltyped(iszero(T),bool) :- welltyped(T,nat). welltyped(if(T1,T2,T3),T) :- welltyped(T1,bool), welltyped(T2,T), welltyped(T3,T). © Ralf Lämmel, 2009-2011 unless noted otherwise 206 Prolog Conditional evaluation for typed NB main(Input) :- see(Input), read(Term), seen, format('Input term: ~w~n',[Term]), welltyped(Term, Type), format('Type of term: ~w~n',[Type]), manysteps(Term,X), show(X,Y), format('Value of term: ~w~n',[Y]). © Ralf Lämmel, 2009-2011 unless noted otherwise 207 The Untyped Lambda Calculus © Ralf Lämmel, 2009-2011 unless noted otherwise 208 Summarizing Summarizing the system This slide is derived from Jaakko Järvi’s slides for his course ”Programming Languages”, CPSC 604 @ TAMU. The grammar and (small-step) operational semantics of lambda-calculus Formalizationlambda-calculus of the lambda calculus Evaluation rules (call-by-name) Syntax Summarizing Summarizing the system Syntax t ::=x • t ::=x t t t t t1 → t1 t → t λx.t Terms → → t1 t2 t1 t2 vt vt 1 2 → 1 2 → The grammar and (small-step) operational semantics of tt (λx.t) v [v/x]t lambda-calculus → v ::=λx.t Values (normal forms) Evaluation rules (call-by-name)v ::=λx.t Syntax is the smallest binary relation on • Evaluation → terms satisfying the rules t ::=x t t t t 1 → 1 → λx.t 34 / 36 t1 t2 t1 t2 vt vt tt → → (λx.t) v [v/x]t → v ::=λx.t © Ralf Lämmel, 2009-2011 unless noted otherwise 209 is the smallest binary relation on → terms satisfying the rules 34 / 36 Prolog Syntax of the untyped lambda calculus term(var(X)) :- variable(X). term(app(T1,T2)) :- term(T1), term(T2). term(lam(X,T)) :- variable(X), term(T). value(lam(X,T)) :- variable(X), term(T). variable(X) :- atom(X). Variables are Prolog atoms. © Ralf Lämmel, 2009-2011 unless noted otherwise 210 Prolog λ: sample term app(app(app( % TEST (if-then-else) lam(l,lam(m,lam(n,app(app(var(l),var(m)),var(n))))), % Church Boolean True lam(t,lam(f,var(t)))), We illustrate Church Booleans % Church Numeral 0 and numerals. That is, we use a conditional (TEST) to select lam(s,lam(z,var(z)))), either C0 or C1. % Church Numeral 1 lam(s,lam(z,app(var(s),var(z))))). © Ralf Lämmel, 2009-2011 unless noted otherwise 211 Prolog Evaluation rules of the untyped lambda calculus eval(app(T1,T2),app(T3,T2)) :- eval(T1,T3). eval(app(V,T1),app(V,T2)) :- Substitution (as needed for beta reduction) is the value(V), interesting part--both in the eval(T1,T2). formal setting. and in Prolog. eval(app(lam(X,T1),V),T2) :- value(V), substitute(V,X,T1,T2). © Ralf Lämmel, 2009-2011 unless noted otherwise 212 This slide is derived from Jaakko Järvi’s slides for his course ”ProgrammingIntroduction Languages”, CPSC 604 @ TAMU. InductiveSubstitution definition of substitution in λ calculus [N/x]x = N [N/x]y = y, y any variable different from x [N/x](M1 M2)=([N/x]M1)([N/x]M2) [N/x](λx.M)=λx.MIntroduction Variable[N/x binding](λy.M)= —λ precisey.([N/x definition]M), y not free in N Alpha renaming can be applied freely, variables drawn from the infinite FV(M) defines the set of free variables in the term M pool of variable names FV(x)=Examples:x { } FV(MN)=FV(M) FV(N) [z/x]x ∪ FV(λx.M)=FV(M) x [z/x](λx.xx) \{ } © Ralf Lämmel, 2009-2011[z/x unless](λ ynoted.yx otherwise) 213 [Spotz/x]( freeλz. andxz) bound occurrences here: 12 / 36 (λx.y)(λy.y) λx.(λy.xy)y Combinator —atermwithnofreevariables(alsoclosed term) 9 / 36 Substitution 1/3 Prolog substitute(N,X,var(X),N).