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4239 :: Confusion about macros.

• solo989 (orginal) [2018-09-26T06:31:42+02:00] view original

  This works.

  import macros
  import typetraits
  
  macro t1() : untyped =
    return nnkStmtList.newTree(
      nnkDiscardStmt.newTree(
      newEmptyNode()
    )
    )
  
  t1()
  
  Run

  But if I add:

  macro t2(): untyped =
    result = t1()
  
  #t2()
  Run

  It breaks with the error message:
  Error: expression 'discard' has no type (or is ambiguous).
  Why can't I pass the result of one macro to another?
  I have a feeling I'm missing something fundamental here about how macros work.
  
  Even stranger if I comment out t1 and add discard.

  macro t1() : untyped =
    discard
    #return nnkStmtList.newTree(
    #  nnkDiscardStmt.newTree(
    # newEmptyNode()
   # )
    #)
  Run

  It gives the error not unused.

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• gemath (orginal) [2018-09-26T09:26:04+02:00] view original

  This works:

  macro t2(): untyped =
    result = getAst(t1())
  
  #t2()
  Run

  A macro produces an abstract syntax tree (AST) of nodes, that's what is assigned to its result variable. A macro call like t1() is replaced with the code representation of this AST, not with the AST itself. If you need the AST, and you do in result = t1() because t2 is supposed to return an AST, wrap it with getAST. Alternatively, t1 could become a proc.

  Even stranger if I comment out t1 and add discard...

  The compiler just warns about t2 not being used because the call to it is commented out, not sure what you mean.

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• solo989 (orginal) [2018-09-26T10:17:16+02:00] view original

  Thanks for the help. I didn't quite get what the not unused error was because it was so short and I was hung up on the other error.

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