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11038 :: Is it impossible to declare an _empty enum_ under quote in order to populate it later in a macro?

• xealits (orginal) [2024-02-18T23:48:07+01:00] view original

  I would like to generate an enum and a matching array of strings in a macro, as in this example but with quote instead of direct tree nodes like nnkTypeSection.newTree(...) in order to keep the code more readable in simple macros. Is it possible to declare an empty enum under a quote at all?

  
  macro defineSoA_with_enum(enumName, .., definitionsAoS: untyped): untyped =
    var
      enumDef = quote do:
        type
          `enumName` = enum <make it empty>
          # maybe something like:
          #`enumName` = enum discard
          # or something like this:
          #`enumName` = `nnkEnumTy.newTree(newEmptyNode())`
          # but it does not work: enumDef.repr is `enumName = nnkEnumTy` not `enumName = enum`
    
    # the rest of the macro populates enumDef
    for i in countup(0, definitionsAoS.len - 1):
      ...
      enumDef[0][2].add nnkEnumFieldDef.newtree(...)
  Run

  Other objects can be empty, like strings and arrays etc. But it seems an empty enum might not make sense for the language. Or is there a way to do it? Probably, it is possible to add a dummy node under quote and then remove it in the macro?

  On the same topic, is there a quote-base way to define a completely branch-less case statement? Some sort of the same quote do: case ...: discard?

  Although, it may be OK for many situations to define the case statement with the else branch only, then the macro could insert the other branches:

  
  caseSwitch = quote do:
    case parseEnum[`enumName`](`templateArgument`):
      else:
        stack.push parseFloat(`templateArgument`)
  
  # insert the new nodes instead of adding, so that the else branch is the last one, as the case order demands
  # insert at position 1, because 0 is the case parsing expression
  caseSwitch.insert 1, nnkOfBranch.newTree(
    enumInfo[0],
    commandInfo[1])
  Run

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• Araq (orginal) [2024-02-19T08:13:22+01:00] view original

  I think it is not possible and in general there is no design rule like "quote do must be able to do anything" as the declarative way to construct trees in inherently more inflexible than the imperative way.

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• awr1 (orginal) [2024-02-19T10:01:14+01:00] view original

  nim is not lisp, that it has a more complicated grammar beyond sexps means ultimately that there are AST substitutions you cannot do in quote do. this is why when building macros you should always keep astGenRepr close by in case you do have to get dirtier.

  as your concern is with readability, what i would probably go for is populating the enum with a dummy field and then deleting/replacing the fields from there

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• PMunch (orginal) [2024-02-19T10:32:20+01:00] view original

  You've basically stumbled upon the solutions. quote requires actual valid Nim syntax, so the way to solve it is to add dummy nodes which you can then later replace. I do that in Futhark in order to get doc comments into nodes (which you can't do with the AST generators), but the concept is the same. I've also done the case trick you mentioned in protobuf-nim. I think I leave the case statement with the else: discard node in the end, but you can delete it once you have the AST and before you start adding things to the node.

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• mratsim (orginal) [2024-02-19T10:58:18+01:00] view original

  It's not hard to create the enum without quote do:

  https://github.com/mratsim/trace-of-radiance/blob/master/trace_of_radiance/support/emulate_classes_with_ADTs.nim#L94C7-L132

  #
    var enumValues = nnkEnumTy.newTree()
    enumValues.add newEmptyNode()
    
    for subtype in class.subtypes:
      let kind = ident("k" & $subtype)  # kSphere, kLambertian
      
      enumValues.add kind
    
    # Create the type section
    result = nnkStmtList.newTree(
      nnkTypeSection.newTree(
        # type MaterialKind* = enum
        #   kLambertian, kMetal, ...
        nnkTypeDef.newTree(
          exported($className & "Kind"),
          newEmptyNode(),
          enumValues
        ),
  Run

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• xealits (orginal) [2024-02-19T23:07:31+01:00] view original

  It looks like newEnum also requires the enum to have at least one field:

  
  macro ... =
    enumDef = newEnum(`enumName`, [], true, true)
  
  $ nim c -r -d:release basic_macro.nim
  ...
  /.../.local/nim/lib/core/macros.nim(1272, 10) Error: Enum must contain at least one field
  Run

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• xealits (orginal) [2024-02-19T23:10:26+01:00] view original

  Thanks for the links!

  I think it may actually be a good idiom to declare dummy nodes and delete them in the macro. The dummy nodes can serve as a demo for other developers of what you intend to do in the macro.

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