Remove duplicate Symbols caused by a Label #33
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**The Problem:** In `NormGrammarReader.processSymbol`, a cache is maintained for symbols that are already processed. However, this cache did not take into account that Symbols with and without a Label should be treated as equal (e.g. `tail:Stm` and `Stm`, see org.spoofax.jsglr2.integration/src/main/resources/grammars/layout-sensitive.sdf3). These labels are only used to reference the symbols from the layout constraints, but do not actually contribute to the meaning of a production. **Why is this a problem?** For SLR parse table generation (see metaborg#27), all grammar symbols are assumed to be unique. However, because Symbols with or without a Label were not treated as equal, the "graph" used to calculate the First- and Follow-sets became disconnected. This in turn caused some symbols to have empty First-/Follow-sets. The productions belonging to these symbols would not get any reduce actions in the parse table, because if the Follow-set is empty, no lookahead is allowed. **Current solution:** When processing symbols in the `NormGrammarReader`, any Label constructor is removed from the symbol term. **Discussion:** I have tried looking around where these labels are used (I am assuming layout constraints, but I couldn't find how they are processed in NormGrammarReader). Depending on their usage, it might be a good idea to apply this filter in Stratego already, e.g. in the strategy `module-to-normal-form`. @udesou any feedback is appreciated, as always 🙂
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The Problem: In
NormGrammarReader.processSymbol, a cache is maintained for symbols that are already processed. However, this cache did not take into account that Symbols with and without a Label should be treated as equal (e.g.tail:StmandStm, see org.spoofax.jsglr2.integration/src/main/resources/grammars/layout-sensitive.sdf3). These labels are only used to reference the symbols from the layout constraints, but do not actually contribute to the meaning of a production.Why is this a problem? For SLR parse table generation (see #27), all grammar symbols are assumed to be unique. However, because Symbols with or without a Label were not treated as equal, the "graph" used to calculate the First- and Follow-sets became disconnected. This in turn caused some symbols to have empty First-/Follow-sets. The productions belonging to these symbols would not get any reduce actions in the parse table, because if the Follow-set is empty, no lookahead is allowed.
Current solution: When processing symbols in the
NormGrammarReader, any Label constructor is removed from the symbol term.Discussion: I have tried looking around where these labels are used (I am assuming layout constraints, but I couldn't find how they are processed in NormGrammarReader).
Depending on their usage, it might be a good idea to apply this filter in Stratego already, e.g. in the strategy
module-to-normal-form. In that case, this PR can be closed without merging.@udesou any feedback is appreciated, as always 🙂