The parser is based on the excellent Parboiled2 scala library. This is a parsing expression grammar (PEG) parser library. PEGs are great because they overcome the ambiguity problem by greedy choice.

Key Classes

  • Parser, the class that implements the Flix grammar.
  • SourcePosition, a class that represents a specific point in the source code, i.e. a line and column number.
  • SourceLocation, a class that represents a specific range in the source code, i.e. a beginning line and column number and an ending line and column number. In every later stage of the compiler, every AST node is associated with a SourceLocation.


The grammar is pretty straightforward, but there are a couple of things to be aware of:

  • Compilation of the parser class is slow due to the use of macros in Parboiled.
  • The error messages produced by Parboiled, if there is an error in the grammar, are almost unusable.
  • The PEG grammar is greedy. Make sure you understand what this means.
  • The AST is constructed during parsing. However, you should avoid any costly work during AST construction since an AST fragment may be constructed only to be discarded later because of a parse mismatch.
  • Whitespace must be handled carefully:
    • The production rule WS requires some whitespace, whereas
    • The production rule optWS allows whitespace.
  • Optional semicolons are matched by the optSC rule.
  • Source locations are tracked by pushing a SourcePosition on the stack when a syntactic element begins and again when the element ends. For example, the rule SP ~ "Foo" ~ SP can be used to track when the element "Foo" begins and ends. SourcePositions are later translated into proper SourceLocations.

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