Trailing commas in "command" syntax?

[shirleyquirk]

i'll bite: 
    
    
    import macros,strutils,tables
    
    macro `~`(op:typed,body:untyped):untyped =
      echo op.treeRepr
      #can't use colons inside, so for a : b in tables, named tuples, object 
constructors instead use a = b
      var mappings:Table[NimNodeKind,NimNodeKind] = 
{nnkAsgn:nnkExprColonExpr}.toTable
      case op.kind
      of nnkCurly:#Table or set
        if body[0].kind == nnkAsgn:
          result = nnkTableConstr.newNimNode
        else:#set
          result = op.copyNimTree()
      of nnkCall:#assumes varargs
          result = op.copyNimTree
          result.del(result.len - 1)#remove empty varargs.
      of nnkSym:#function call
        result = newCall(ident(op.strval))
      else:
        result = op.copyNimTree()
      
      var dest = result
      
      #special case seq constructor
      if op.kind == nnkPrefix and op[0].strval == "@":
        result = nnkPrefix.newTree(ident"@",nnkBracket.newNimNode)
        dest = result[1]
      
      for stmt in body:
        dest.add if stmt.kind in mappings:
          let res = mappings[stmt.kind].newNimNode
          stmt.copyChildrenTo(res)
          res
        else:
          stmt
    
    
    
    
    type Foo = object
      a: int
      b: set[char]
    proc `$`(f:Foo):string = "Foo" & system.`$`(f)
    
    proc weighted_avg(weight:float,data:varargs[float]):float =
      for d in data:
        result += d
      result = result * weight / data.len.float
    
    proc foo(x,y,z:int):int = x + y * z
    
    let nested = toTable: ~{}:
        "thing" = ~():
          "foo"
          ~[]:
            ~Foo():
              a = int: ~weighted_avg(0.5):
                11.0
                13.5
                19.3
              b = {'x'}
          ~ @[]:
            3.0
            3.1
            3.2
        "wrong" = ~():
          "bar"
          ~[]:
            ~Foo():
              a = ~foo:
                3
                4
                2
              b = ~{}:
                'y'
                'z'
          @[]
    
    echo nested
    
    
    Run

<https://play.nim-lang.org/#ix=3BzL>