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3253 :: what does macros.quote() do exactly?

• bkerin (orginal) [2017-10-17T23:10:42+02:00] view original

  Docs say:

  Quasi-quoting operator. Accepts an expression or a block and returns the AST that represents it. Within the quoted > AST, you are able to interpolate NimNode expressions from the surrounding scope

  By this, I guess it likely means it pastes the sub-tree in at the given position? Or does it truly interpolate it in then reconstruct the AST from the resulting text?

  I was trying to extend the class OOP programming macro form nim by example to create parametrized classes (e.g. Cage[Animal]. Is this even a sane idea? I notice expandMacros refers to expanding one level of macros, but I'm not sure that generics that come out of macros get handled?

  I notice that

  dumpTree:
    class Cage[Animal] of RootObj:
      var occupant: T
  Run

  shows a BracketExpr node, while

  dumpTree:
    type Cage[Animal] of RootObj:
  Run

  shows a GenericParams node, with no BracketExpr node anywhere. But I guess there's no reason to be scared about the lack of a common factor since BracketExpr is more general and the parser only produces GenericParams in a type context? Just recreate tree to taste?

  This simple quote() code also confuses me:

  import future
  import macros
  
  macro decDumbType(): untyped =
    result =
      macros.quote do:
        type Dumb = ref object of RootObj
          contents: int
        method frobnicate(this: Dumb) {.base.} =
          echo "frobnicating!"
  
  expandMacros:
    decDumbType()
  Run

  by producing this:

  type

  Dumb112019 = ref object of RootObj

  contents: int

  method frobnicate(this112017: Dumb112015) {.base.} = echo ["frobnicating!"]

  generic_obj_mac.nim(31, 1) template/generic instantiation from here generic_obj_mac.nim(32, 14) Error: illformed AST: method frobnicate(this112017:

  Dumb112015) {.base.} =

  echo ["frobnicating!"]

  why is Dumb being rewritten?

---

• Araq (orginal) [2017-10-18T09:45:53+02:00] view original

  expandMacros with its conversion to typed is really fragile with today's compiler. Instead use this to see what's going on:

  import future
  import macros
  
  macro decDumbType(): untyped =
    result =
      macros.quote do:
        type Dumb = ref object of RootObj
          contents: int
        method frobnicate(this: Dumb) {.base.} =
          echo "frobnicating!"
    echo repr result
  
  decDumbType()
  Run

---

• bkerin (orginal) [2017-10-20T19:53:57+02:00] view original

  Seems to give almost the same output:

  type

  Dumb112023 = ref object of RootObj

  contents: int

  method frobnicate(this112025: Dumb112023) {.base.} =

  echo "frobnicating!"

  and I still don't understand why Dumb turns into Dumb112023, maybe I will later

---

• Udiknedormin (orginal) [2017-11-05T01:44:36+01:00] view original

  Sorry to hear it breaks for you, it works on my compiler. :-/

  But give quote + getAst at try, it's recommended over quote-do anyway, if I recall.

  import macros
  
  macro decDumbType(): untyped =
    let sym = genSym(nnkType, "Dumb")
    template decl(ty) =
      type Dumb = ref object of RootObj
        contents: int
      method frobnicate(this: Dumb) {.base.} =
        echo "frobnicating!"
    result = getAst(decl(sym))
    echo repr result
  
  decDumbType()
  Run

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