Re: Closure conditions for languages? (Torben AEgidius Mogensen)
Wed, 23 Mar 1994 23:57:45 GMT

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Newsgroups: comp.compilers
From: (Torben AEgidius Mogensen)
Keywords: theory, parse, LL(1)
Organization: Compilers Central
References: 94-03-080
Date: Wed, 23 Mar 1994 23:57:45 GMT (Gary Merrill) writes:

>I see that it is known that the LL languages are not closed under union
>(or any number of other things as well). This has lead me to the
>following questions which I am hoping someone will have ready answers to:

>1. What is the set of conditions C such that the following is
> a theorem:

> If C, then if L is an LL(1) language and L' is an LL(1)
> language, the union of L and L' is an LL(1) language.

Well, if FIRST(L) and FIRST(L') are disjoint, then the union of L and
L' will obviously also be LL(1). I don't know of any stronger results.

>2. Are there "interesting" languages L and L' which are both
> LL(1) languages and whose union is not?

That depends on what you mean by 'interesting'. An example is

L = {a^m b^m c^n}
L' = {a^m b^n c^n}

represented by the LL(1) grammars

L -> A' C
A' -> _ | a A' b /* _ is the "empty" symbol */
C -> _ | c C

L' -> A B'
A -> _ | a A
B' -> _ | b B' c

The union of L and L' is a language that can't be represented by an
unambigous grammar, and hence not by any LL(k) grammar.

>3. Do similar results hold for LR languages?

The same example can be used for LR languages.

Torben Mogensen (

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