context clash
<grammar> When a parser cannot tell which alternative production of a
syntax applies by looking at the next input token ("lexeme").
E.g. given syntax
C > A  b c
A > d  b e
If you're parsing nonterminal C and the next token is 'b', you don't know
whether it's the first or second alternative of C
since they both can start with b.
To discover whether a grammar has a context clash:
For each nonterminal, N, with multiple alternatives, look at the first symbol
of each alternative's righthand side, call it s. If s is the empty string, then
find the set FOLLOWER(N) otherwise find the set FIRST*(s). If any of the sets
for N's alternatives intersect then there will be a context clash when parsing
N. If the next input symbol is one of those in the intersection of two sets then
you won't know which of the alternatives applies.
FIRST(s) is the set of symbols with which s can start, including s itself. If s
is a nonterminal then FIRST(s) also includes the first symbol of each
alternative righthand side of s. The '*' in FIRST*(s) means the "transitive
closure" of FIRST which means keep applying FIRST to each element of the result
until the result doesn't change. I.e. start with just the set R = s, then for
each nonterminal x in R, add FIRST(x) to R. Keep doing this until nothing new
is added. (We are really only interested in the terminals in FIRST*(s) but some
definitions include the nonterminals).
FOLLOWER(N) is the set of symbols which can come after N in a sentence. Find
each occurrence of N on the righthand side of a rule, e.g.
M > ...  ... N ...  ...
If there is a symbol s immediately following N then add FIRST*(s) to the
result (again, we're only interested in the terminal
symbols in FIRST*(s)) if there is no symbol after N
in the alternative then add FOLLOWER(M) to the
result (i.e. if N can be the last symbol in an M
then anything that can follow M can also follow N).
If a grammar can generate the same sentence in multiple different ways (with
different parse tress) then it is ambiguous. An ambiguity must start with a
context clash (but not all context clashes imply ambiguity). The context clash
occurs when trying to parse the first token of the phrase with multiple parses 
you will not be able to tell which alternative to take. To see if a context
clash is also a case of ambiguity you would need to follow the alternatives
involved in each context clash to see if they can generate the same complete
sequence of tokens.
(19950405)
Nearby terms:
contentfree « contention slot « context «
context clash
» contextfree » contextsensitive menu » context
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