nimforum mirror - pegs eventParser example not gc-safe

squattingmonk (orginal) [2025-01-29T19:22:18+01:00] view original

Hi, all. I'm trying to learn to use the eventParser template from the std/pegs module. However, the example code doesn't compile under >= nim 2.0.0; it fails with the following error:

/home/squattingmonk/Code/nim/playground/peg_t.nim(34, 19) Hint: 'val`gensym502' is declared but not used [XDeclaredButNotUsed]
/home/squattingmonk/Code/nim/playground/peg_t.nim(17, 26) template/generic instantiation of `eventParser` from here
/home/squattingmonk/.choosenim/toolchains/nim-2.0.0/lib/pure/pegs.nim(1062, 18) Error: 'parseIt' is not GC-safe as it accesses 'opStack' which is a global using GC'ed memory

Because this is a template invocation and not an actual proc definition, I'm not sure how to go about getting around this. The solution I'm using so far is to wrap everything into a proc:


import std/[strutils, pegs]

let
  pegAst = """
Expr    <- Sum
Sum     <- Product (('+' / '-')Product)*
Product <- Value (('*' / '/')Value)*
Value   <- [0-9]+ / '(' Expr ')'
  """.peg
  txt = "(5+3)/2-7*22"

proc parseArithExpr(s: string): int =
  var
    pStack: seq[string] = @[]
    valStack: seq[float] = @[]
    opStack = ""
  let
    p = pegAst.eventParser:
      pkNonTerminal:
        enter:
          pStack.add p.nt.name
        leave:
          pStack.setLen pStack.high
          if length > 0:
            let matchStr = s.substr(start, start+length-1)
            case p.nt.name
            of "Value":
              try:
                valStack.add matchStr.parseFloat
                echo valStack
              except ValueError:
                discard
            of "Sum", "Product":
              try:
                let val = matchStr.parseFloat
              except ValueError:
                if valStack.len > 1 and opStack.len > 0:
                  valStack[^2] = case opStack[^1]
                  of '+': valStack[^2] + valStack[^1]
                  of '-': valStack[^2] - valStack[^1]
                  of '*': valStack[^2] * valStack[^1]
                  else: valStack[^2] / valStack[^1]
                  valStack.setLen valStack.high
                  echo valStack
                  opStack.setLen opStack.high
                  echo opStack
      pkChar:
        leave:
          if length == 1 and "Value" != pStack[^1]:
            let matchChar = s[start]
            opStack.add matchChar
            echo opStack
  s.p

let pLen = parseArithExpr(txt)

Is there a better way to do this?

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12668 :: pegs eventParser example not gc-safe