[RFC] Working with types in macro is difficult.

[mratsim]

Context

I find working with types in macro quite difficult and I think Nim would gain a lot if it was easier.
The 2 main issues I've noted are:

• getTypeInst only works in a typed macro, but it doesn't work in a compile-time proc. In an untyped context, this forces us to use a helper macro with getAST/quote do as we can't call a macro from a macro.
• It is very easy to get Cannot instantiate or type mismatch because of forgetting to convert the NimSym given by getTypeInst to NimIdent if they are not used raw.

Example:

import macros

type Conv2DLayer[T] = object
  data: T
type Context[T] = ref object
  data: T

proc replaceNodes*(ast: NimNode): NimNode =
  # Replace NimIdent and NimSym by a fresh ident node
  proc inspect(node: NimNode): NimNode =
    case node.kind:
    of {nnkIdent, nnkSym}:
      return ident($node)
    of nnkEmpty:
      return node
    of nnkLiterals:
      return node
    else:
      var rTree = node.kind.newTree()
      for child in node:
        rTree.add inspect(child)
      return rTree
  result = inspect(ast)

####################################

####################################

macro ctxSubtype(context: Context): untyped =
  ## Extract the subtype from a Context
  echo context.getTypeInst.treerepr
  result = context.getTypeInst[1] # This gives a cryptic error got <Foo> expected <bar>
  # result = replaceNodes(context.getTypeInst[1]) # If we replace the symbol by ident it works!

macro genModelFromCtx(modelType: untyped, context: Context): untyped =

  var records = nnkRecList.newTree

  let subtype = getAST(ctxSubtype(context))

  let field_type = nnkBracketExpr.newTree(
      ident("Conv2DLayer"), subtype
    )

  records.add nnkIdentDefs.newTree(
      newIdentNode("field_name"),
      field_type,
      newEmptyNode()
    )

  result = nnkStmtList.newTree(
    nnkTypeSection.newTree(
      nnkTypeDef.newTree(
        newIdentNode($modelType),
        newEmptyNode(),
        nnkObjectTy.newTree(
          newEmptyNode(),
          newEmptyNode(),
          records
        )
      )
    )
  )

var ctx: Context[seq[int]]

expandMacros:
  genModelFromCtx(FooType2, ctx)

## Output - before replaceNodes:
# got: <seq[int]>
# but expected: <T>

## Output fixed:
# type
#   FooType2 = object
#     field_name: Conv2DLayer[seq[int]]

[mratsim]

Not sure if a bug or expected but the following getAST gives node has no type while quote do works, reported in #7739:

import macros, typetraits

proc getSubType(T: NimNode): NimNode =
  echo getTypeInst(T).treerepr
  result = getTypeInst(T)[1]

macro typed_helper(x: typed): untyped =

  result = getSubType(x)

macro untyped_heavylifting(x: varargs[untyped]): untyped =

  let first = x[0]

  # result = quote do: # Works
  #   typed_helper(`first`)

  result = getAST(typed_helper(first)) # Node has no type

var a: seq[int]
echo untyped_heavylifting(a)

[CodeDoes]

result = getAST(typed_helper(first.type))

[mratsim]

That gives typedesc[NimNode]

[CodeDoes]

getAST(typed_helper(first.type))[0]?

[mratsim]

@CodeDoes first.type is of type NimNode because it's an AST node representing x, it is not x. You need to use one of the getType, getTypeInst or getTypeImpl to access the type of x.

[mratsim]

The linked #8531 is another example of the pitfalls of getTypeInst returning NimSym that has to be converted to NimIdent otherwise we get cryptic errors in certain cases. cc @timotheecour.

[iffy]

I don't completely understand everything in this post, but I do know that trying to use quote to make ASTs in macros is causing problems because it generates nnkSymNode where I want nnkIdentNode.

To get my code working, I used a modified version of replaceNodes from above (adding the branch for nnkOpenSymChoice):

proc replaceNodes*(ast: NimNode): NimNode =
  # Replace NimIdent and NimSym by a fresh ident node
  proc inspect(node: NimNode): NimNode =
    case node.kind:
    of {nnkIdent, nnkSym}:
      return ident($node)
    of nnkEmpty:
      return node
    of nnkLiterals:
      return node
    of nnkOpenSymChoice:
      return inspect(node[0])
    else:
      var rTree = node.kind.newTree()
      for child in node:
        rTree.add inspect(child)
      return rTree
  result = inspect(ast)

For reference, here's my original problem: https://forum.nim-lang.org/t/4300#26770