def _bnf_symbol(stream: TokenStream, gram: Grammar) -> RHSItem:
"""A single identifier or literal, or a parenthesized group"""
if stream.peek().kind == TokenCat.LPAREN:
stream.take()
subseq = _bnf_rhs(stream, gram)
require(stream, TokenCat.RPAREN, consume=True)
# log.debug(f"Subsequence group: {subseq}")
return subseq
token = stream.take()
if token.kind == TokenCat.STRING or token.kind == TokenCat.CHAR:
# log.debug("Forming literal")
return gram.literal(token.value[1:-1]) # Clips quotes
elif token.kind == TokenCat.IDENT:
# log.debug("Forming symbol")
return gram.symbol(token.value)
else:
raise InputError(f"Unexpected input token {token.value}")
def _statement(stream: TokenStream, gram: Grammar):
"""
_statement == production | merge
(left-factored for lookahead)
"""
require(stream, TokenCat.IDENT, desc="Statement should begin with symbol")
lhs_ident = stream.take().value
prod_type = stream.take()
if prod_type.kind == TokenCat.BNFPROD:
lhs_sym = gram.symbol(lhs_ident)
rhs = _bnf_rhs(stream, gram)
gram.add_cfg_prod(lhs_sym, rhs)
elif prod_type.kind == TokenCat.BNFMERGE:
merge_list = _merge_symbols(stream)
# Merges are symmetric, so order doesn't matter
merge_list.append(lhs_ident)
gram.merge_symbols(merge_list)
require(stream,
TokenCat.TERMINATOR,
"Statement must end with terminator",
consume=True)