Chapter 3 Context-Free Grammars, Context-Free Languages, Parse ...
Chapter 3 Context-Free Grammars, Context-Free Languages, Parse ... Chapter 3 Context-Free Grammars, Context-Free Languages, Parse ...
52 CHAPTER 3. CONTEXT-FREE GRAMMARS AND LANGUAGES Remark: Note that we are not requiring that an ω-continuous function f: A1 → A2 preserve least elements, i.e., it is possible that f(⊥1) =⊥2. We now define the crucial concept of a least fixed-point. Definition 3.7.3 Let 〈A, ≤〉 be a partially ordered set, and let f: A → A be a function. A fixed-point of f is an element a ∈ A such that f(a) =a. The least fixed-point of f is an element a ∈ A such that f(a) =a, and for every b ∈ A such that f(b) =b, thena ≤ b. The following lemma gives sufficient conditions for the existence of least fixed-points. It is one of the key lemmas in denotational semantics. Lemma 3.7.4 Let 〈A, ≤〉 be an ω-chain complete poset with least element ⊥. Every ωcontinuous function f: A → A has a unique least fixed-point x0 given by x0 = f n (⊥). Furthermore, for any b ∈ A such that f(b) ≤ b, thenx0 ≤ b. Proof . First, we prove that the sequence ⊥ ,f(⊥) ,f 2 (⊥), ..., f n (⊥), ... is an ω-chain. This is shown by induction on n. Since⊥ is the least element of A, wehave ⊥≤ f(⊥). Assuming by induction that f n (⊥) ≤ f n+1 (⊥), since f is ω-continuous, it is monotonic, and thus we get f n+1 (⊥) ≤ f n+2 (⊥), as desired. Since A is an ω-chain complete poset, the ω-chain (f n (⊥)) has a least upper bound Since f is ω-continuous, we have and x0 is indeed a fixed-point of f. x0 = f n (⊥). f(x0) =f( f n (⊥)) = f(f n (⊥)) = f n+1 (⊥) =x0, Clearly, if f(b) ≤ b implies that x0 ≤ b, thenf(b) =b implies that x0 ≤ b. Thus, assume that f(b) ≤ b for some b ∈ A. We prove by induction of n that f n (⊥) ≤ b. Indeed, ⊥≤ b, since ⊥ is the least element of A. Assuming by induction that f n (⊥) ≤ b, by monotonicity of f, weget f(f n (⊥)) ≤ f(b), and since f(b) ≤ b, this yields f n+1 (⊥) ≤ b.
3.8. CONTEXT-FREE LANGUAGES AS LEAST FIXED-POINTS 53 Since f n (⊥) ≤ b for all n ≥ 0, we have x0 = f n (⊥) ≤ b. The second part of lemma 3.7.4 is very useful to prove that functions have the same least fixed-point. For example, under the conditions of lemma 3.7.4, if g: A → A is another ω-chain continuous function, letting x0 be the least fixed-point of f and y0 be the least fixed-point of g, iff(y0) ≤ y0 and g(x0) ≤ x0, we can deduce that x0 = y0. Indeed, since f(y0) ≤ y0 and x0 is the least fixed-point of f, wegetx0≤y0, and since g(x0) ≤ x0 and y0 is the least fixed-point of g, wegety0≤x0, and therefore x0 = y0. Lemma 3.7.4 also shows that the least fixed-point x0 of f can be approximated as much as desired, using the sequence (f n (⊥)). We will now apply this fact to context-free grammars. For this, we need to show how a context-free grammar G =(V,Σ,P,S)withmnonterminals induces an ω-continuous map ΦG:2 Σ∗ ×···×2 Σ∗ → 2 m Σ∗ ×···×2 Σ∗ . m 3.8 Context-Free Languages as Least Fixed-Points Given a context-free grammar G =(V,Σ,P,S)withm nonterminals A1,...Am, grouping all the productions having the same left-hand side, the grammar G can be concisely written as A1 → α1,1 + ···+ α1,n1, ···→··· Ai → αi,1 + ···+ αi,ni , ···→··· Am → αm,1 + ···+ αm,nn. Given any set A, letPfin(A) be the set of finite subsets of A. Definition 3.8.1 Let G =(V,Σ,P,S) be a context-free grammar with m nonterminals A1, ..., Am. For any m-tuple Λ = (L1,...,Lm) of languages Li ⊆ Σ∗ , we define the function Φ[Λ]: Pfin(V ∗ ) → 2 Σ∗ inductively as follows: Φ[Λ](∅) =∅, Φ[Λ]({ɛ}) ={ɛ}, Φ[Λ]({a}) ={a}, if a ∈ Σ, Φ[Λ]({Ai}) =Li, if Ai ∈ N, Φ[Λ]({αX}) =Φ[Λ]({α})Φ[Λ]({X}), if α ∈ V + , X ∈ V, Φ[Λ](Q ∪{α}) =Φ[Λ](Q) ∪ Φ[Λ]({α}), if Q ∈Pfin(V ∗ ), Q = ∅, α ∈ V ∗ , α/∈ Q.
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3.8. CONTEXT-FREE LANGUAGES AS LEAST FIXED-POINTS 53<br />
Since f n (⊥) ≤ b for all n ≥ 0, we have<br />
x0 = f n (⊥) ≤ b.<br />
The second part of lemma 3.7.4 is very useful to prove that functions have the same<br />
least fixed-point. For example, under the conditions of lemma 3.7.4, if g: A → A is another<br />
ω-chain continuous function, letting x0 be the least fixed-point of f and y0 be the least<br />
fixed-point of g, iff(y0) ≤ y0 and g(x0) ≤ x0, we can deduce that x0 = y0. Indeed, since<br />
f(y0) ≤ y0 and x0 is the least fixed-point of f, wegetx0≤y0, and since g(x0) ≤ x0 and y0<br />
is the least fixed-point of g, wegety0≤x0, and therefore x0 = y0.<br />
Lemma 3.7.4 also shows that the least fixed-point x0 of f can be approximated as much as<br />
desired, using the sequence (f n (⊥)). We will now apply this fact to context-free grammars.<br />
For this, we need to show how a context-free grammar G =(V,Σ,P,S)withmnonterminals induces an ω-continuous map<br />
ΦG:2 Σ∗<br />
×···×2 Σ∗<br />
<br />
→ 2<br />
m<br />
Σ∗<br />
×···×2 Σ∗<br />
<br />
.<br />
m<br />
3.8 <strong>Context</strong>-<strong>Free</strong> <strong>Languages</strong> as Least Fixed-Points<br />
Given a context-free grammar G =(V,Σ,P,S)withm nonterminals A1,...Am, grouping all<br />
the productions having the same left-hand side, the grammar G can be concisely written as<br />
A1 → α1,1 + ···+ α1,n1,<br />
···→···<br />
Ai → αi,1 + ···+ αi,ni ,<br />
···→···<br />
Am → αm,1 + ···+ αm,nn.<br />
Given any set A, letPfin(A) be the set of finite subsets of A.<br />
Definition 3.8.1 Let G =(V,Σ,P,S) be a context-free grammar with m nonterminals A1,<br />
..., Am. For any m-tuple Λ = (L1,...,Lm) of languages Li ⊆ Σ∗ , we define the function<br />
Φ[Λ]: Pfin(V ∗ ) → 2 Σ∗<br />
inductively as follows:<br />
Φ[Λ](∅) =∅,<br />
Φ[Λ]({ɛ}) ={ɛ},<br />
Φ[Λ]({a}) ={a}, if a ∈ Σ,<br />
Φ[Λ]({Ai}) =Li, if Ai ∈ N,<br />
Φ[Λ]({αX}) =Φ[Λ]({α})Φ[Λ]({X}), if α ∈ V + , X ∈ V,<br />
Φ[Λ](Q ∪{α}) =Φ[Λ](Q) ∪ Φ[Λ]({α}), if Q ∈Pfin(V ∗ ), Q = ∅, α ∈ V ∗ , α/∈ Q.
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