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It's possible to provide rich information and flexible error reporting using error combinators, which merge results from failed parser combinators. The error co

It's possible to provide rich information and flexible error reporting using error combinators, which merge results from failed parser combinators. The error combinators are in some sense dual to the parser combinators.

shakna 9y ago
How could an error combinator possibly approach something like a misnamed variable? It would need access to the current scope's environment, or a version of it, to suggest what the spelling error might be. Not-yet-in-scope errors are similar, but need both past and future scopes as well. I don't know of any parser combinator that can handle those very human mistakes, because they require partial evaluation to understand. I might be mistaken, but if I'm not, parser combinators are great for saving time, but if you have the time to do it right, then a hand-rolled parser will give your users a much better experience.
- tomp 9y ago
  Misnamed variables are actually one of the simplest problems to deal with ehile parsing. The solution is simple - you don't! The purpose of the parser is to parse the syntax, so you can simply use it to build a syntax tree and postpone the resolution of names until the next step in compiler's pipeline (also useful if you language supports macros).
  - pornel 9y ago
    C is a well-known exception to this. The grammar depends on typedef name resolution. `foo * bar` and `(foo)-1` parse as different things depending on whether `foo` is a typedef or a variable.
  - thesz 9y ago
    In some areas of the world (VHDL/Ada), you have to maintain symbol table for correct lexing. These languages have constructions like OBJECT_NAME'ATTRIBUTE (object name can be anything with visible name in the scope and attributes depend on its kind) and RECORD_TYPE_NAME'SLICE where we create value of RECORD_TYPE_NAME using slice, which is collection of values of record fields. There are also character literals, which have form like 'a', '1', '#', ''''. Just as you can expect here. The combination of record value construction and lexer forms for character literals makes it hard to parse something like MY_RECORD'('1','0',false, 10). Symbol table and/or dynamic parse construction allows you to parse these texts easily and specify the syntax straightforwardly. The other approach with construction of separate AST complicates things a lot.

shakna 9y ago

How could an error combinator possibly approach something like a misnamed variable? It would need access to the current scope's environment, or a version of it, to suggest what the spelling error might be. Not-yet-in-scope errors are similar, but need both past and future scopes as well. I don't know of any parser combinator that can handle those very human mistakes, because they require partial evaluation to understand. I might be mistaken, but if I'm not, parser combinators are great for saving time, but if you have the time to do it right, then a hand-rolled parser will give your users a much better experience.

tomp 9y ago
Misnamed variables are actually one of the simplest problems to deal with ehile parsing. The solution is simple - you don't! The purpose of the parser is to parse the syntax, so you can simply use it to build a syntax tree and postpone the resolution of names until the next step in compiler's pipeline (also useful if you language supports macros).
- pornel 9y ago
  C is a well-known exception to this. The grammar depends on typedef name resolution. `foo * bar` and `(foo)-1` parse as different things depending on whether `foo` is a typedef or a variable.
- thesz 9y ago
  In some areas of the world (VHDL/Ada), you have to maintain symbol table for correct lexing. These languages have constructions like OBJECT_NAME'ATTRIBUTE (object name can be anything with visible name in the scope and attributes depend on its kind) and RECORD_TYPE_NAME'SLICE where we create value of RECORD_TYPE_NAME using slice, which is collection of values of record fields. There are also character literals, which have form like 'a', '1', '#', ''''. Just as you can expect here. The combination of record value construction and lexer forms for character literals makes it hard to parse something like MY_RECORD'('1','0',false, 10). Symbol table and/or dynamic parse construction allows you to parse these texts easily and specify the syntax straightforwardly. The other approach with construction of separate AST complicates things a lot.