Terminal successors, the FOLLOW function, and LL1 conditions for non -free grammars
9.2.2 Terminal successors, the FOLLOW function, and LL1 conditions for non -free grammars
We have already commented that -productions might cause difficulties in parsing. Indeed, Rule 1 is not strong enough to detect another source of trouble, which may arise if such productions are used. Consider the grammar G = { N , T , S , P } N = { A , B } T = { x , y } S = A P = A Bx 1 B xy 2 B 3 In terms of the discussion above, Rule 1 is satisfied. Of the alternatives for the non-terminal B, we see that FIRSTB 1 = FIRSTxy = x FIRSTB 2 = FIRST = Ø which are disjoint. However, if we try to parse the string x we may come unstuck Sentential form S = A Input string x Sentential form Bx Input string x As we are working from left to right and have a non-terminal on the left we substitute for B, to get, perhaps Sentential form xyx Input string x which is clearly wrong. We should have used 3, not 2, but we had no way of telling this on the basis of looking at only the next terminal in the input. This situation is called the null string problem, and it arises only for productions which can generate the null string. One might try to rewrite the grammar so as to avoid -productions, but in fact that is not always necessary, and, as we have commented, it is sometimes highly inconvenient. With a little insight we should be able to see that if a non-terminal is nullable, we need to examine the terminals that might legitimately follow it, before deciding that the -production is to be applied. With this in mind it is convenient to define the terminal successors of a non-terminal A as the set of all terminals that can follow A in any sentential form, that is a FOLLOWA if S Aa A, S N ; a T ; , N T To handle this situation, we impose the further restriction Rule 2 When the productions for a non-terminal A admit alternatives A 1 | 2 | . . . n and in particular where k for some k, the sets of initial terminal symbols of all sentences that can be generated from each of the j for j k must be disjoint from the set FOLLOWA of symbols that may follow any sequence generated from A, that is FIRST j FOLLOWA = Ø, j k or, rather more loosely, FIRSTA FOLLOWA = Ø where, as might be expected FIRSTA = FIRST 1 FIRST 2 . . . FIRST n In practical terms, the set FOLLOWA is computed by considering every production P k of the form P k k A k and forming the sets FIRST k , when FOLLOWA = FIRST 1 FIRST 2 . . . FIRST n with the addition that if any k is also capable of generating the null string, then the set FOLLOWP k has to be included in the set FOLLOWA as well. In the example given earlier, Rule 2 is clearly violated, because FIRSTB 1 = FIRSTxy = { x } = FOLLOWB9.2.3 Further observations
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