Debug Example

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

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

The macro call expands to:

write(stdout, "a[0]")
write(stdout, ": ")
writeLine(stdout, a[0])
write(stdout, "a[1]")
write(stdout, ": ")
writeLine(stdout, a[1])
write(stdout, "x")
write(stdout, ": ")
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.