Mike Gerwitz

Activist for User Freedom

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authorMike Gerwitz <mike.gerwitz@ryansg.com>2022-07-14 15:12:57 -0400
committerMike Gerwitz <mike.gerwitz@ryansg.com>2022-07-14 15:12:57 -0400
commitcf2cd882ca13d90a6953f49dcbec77a56deea08e (patch)
treeab284be1b562ed84443c8cf766d180ad2421161c /rater/build-aux
parent1fdfc0aa4debd74683b61bc278f1c65d6c45a974 (diff)
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tamer: xir::parse::ele: Introduce sum nonterminalsmain
This introduces `Nt := (A | ... | Z);`, where `Nt` is the name of the nonterminal and `A ... Z` are the inner nonterminals---it produces a parser that provides a choice between a set of nonterminals. This is implemented efficiently by understanding the QName that is accepted by each of the inner nonterminals and delegating that token immediately to the appropriate parser. This is a benefit of using a parser generator macro over parser combinators---we do not need to implement backtracking by letting inner parsers fail, because we know ahead of time exactly what parser we need. This _does not_ verify that each of the inner parsers accept a unique QName; maybe at a later time I can figure out something for that. However, because this compiles into a `match`, there is no ambiguity---like a PEG parser, there is precedence in the face of an ambiguous token, and the first one wins. Consequently, tests would surely fail, since the latter wouldn't be able to be parsed. This also demonstrates how we can have good error suggestions for this parsing framework: because the inner nonterminals and their QNames are known at compile time, error messages simply generate a list of QNames that are expected. The error recovery strategy is the same as previously noted, and subject to the same concerns, though it may be more appropriate here: it is desirable for the inner parser to fail rather than retrying, so that the sum parser is able to fail and, once the Kleene operator is introduced, retry on another potential element. But again, that recovery strategy may happen to work in some cases, but'll fail miserably in others (e.g. placing an unknown element at the head of a block that expects a sequence of elements would potentially fail the entire block rather than just the invalid one). But more to come on that later; it's not critical at this point. I need to get parsing completed for TAME's input language. DEV-7145
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