Debug Example

The following example implements a powerful debug command that accepts a variable number of arguments:

  1. # to work with Nim syntax trees, we need an API that is defined in the
  2. # ``macros`` module:
  3. import macros
  5. macro debug(args: varargs[untyped]): untyped =
  6.   # `args` is a collection of `NimNode` values that each contain the
  7.   # AST for an argument of the macro. A macro always has to
  8.   # return a `NimNode`. A node of kind `nnkStmtList` is suitable for
  9.   # this use case.
  10.   result = nnkStmtList.newTree()
  11.   # iterate over any argument that is passed to this macro:
  12.   for n in args:
  13.     # add a call to the statement list that writes the expression;
  14.     # `toStrLit` converts an AST to its string representation:
  15.     result.add newCall("write", newIdentNode("stdout"), newLit(n.repr))
  16.     # add a call to the statement list that writes ": "
  17.     result.add newCall("write", newIdentNode("stdout"), newLit(": "))
  18.     # add a call to the statement list that writes the expressions value:
  19.     result.add newCall("writeLine", newIdentNode("stdout"), n)
  21. var
  22.   a: array[0..10, int]
  23.   x = "some string"
  24. a[0] = 42
  25. a[1] = 45
  27. debug(a[0], a[1], x)

The macro call expands to:

  1. write(stdout, "a[0]")
  2. write(stdout, ": ")
  3. writeLine(stdout, a[0])
  5. write(stdout, "a[1]")
  6. write(stdout, ": ")
  7. writeLine(stdout, a[1])
  9. write(stdout, "x")
  10. write(stdout, ": ")
  11. writeLine(stdout, x)

Arguments that are passed to a varargs parameter are wrapped in an array constructor expression. This is why debug iterates over all of n’s children.