parser_generator_ideas.nim

# The 'lexer' macro generates not only the lexing proc but also
# the relevant 'Token' type, including a TokenKind enum that is
# extracted from the actions via the 'token Kind()' syntax.
# There are two special TokenKind values always available:
# Error and Eof.
# '_' refers to the matched string.

lexer:
  "[A-Za-z][A-Za-z0-9]*": 
    # 'keywords' is a special construct. We want to map the pattern
    # to the 'Identifier' token kind, unless it is a keyword. Every
    # keyword is mapped to its own TokenKind enum value.
    keywords("""
      if else for while case of
    """, token Identifier(_))

  "[0-9]+": token Float(_)
  ",": token Comma()
  "+": token Plus()
  "{.": token PragmaLe()
  ".}": token PragmaRi()
  "{": token CurlyLe()
  "}": token CurlyRi()
  ".*": token SomethingElse(_)


# Likewise the 'parser' macro generates a 'Tree' type, including a
# NodeKind enum that is extracted from the actions via 
# the 'node Kind(...)' syntax. 
# There is one special NodeKind value always available: Error. 
# it[i] refers to the i-th parsed subtree. 'it' refers to all parsed
# subtree as a seq, '_' to the current token.

parser:
  expr "expr ParLe expr *^ Comma ParRi":
    # 'a *^ sep' is a shortcut for (a (sep a)*)? 
    node Call(it)

  expr "Identifier":
    node Ident(_)

  expr "expr Plus expr":
    node Add(it)

  stmt "expr": 
    discard "every expression is a statement"
  stmt "If expr Colon expr Else expr":
    node IfStmt(it)
  stmt "PragmaLe stmt (CurlyRi|PragmaRi)":
    # There is no 'or' operator beyond "token or" because it is not 
    # clear what parse tree to attach to an 'or'.
    node Pragma(it[0])