machdata.nim

import maker
import streams
  
makeFormat(MachData):
  f(firstField,  int32, default: 4)
  f(secondField, string, length: firstFiend)
  f(thirdField,  string, length: firstField + 6, value: abc)

let s = "\x05\x00\x00\x00abcxxxdefxxx"
let stream = newStringStream(s)
let x: MachData = read(stream)
echo(x)

maker.nim

import macros
import strutils

macro makeFormat(name: expr, description: expr): stmt =
  var typBodyStr = "type $1 = object" % [$name]
  
  for ex in description:
    let s = """  $1: $2""" % [$ex[1], $ex[2]]
    typBodyStr = "$1\n$2" % [typBodyStr, s]

  var readStr = "proc read(s: Stream): $1 =" % [$name]  
  var readCons = newSeq[string]()
  for ex in description:
    let fName = $ex[1]
    let fType = $ex[2]
    
    if fType == "int32":
      let s = "  let $1 = readInt32(s)" % [fName]
      readStr = "$1\n$2" % [readStr, s]
      add(readCons, "$1: $1" % [fName])
    elif fType == "string":
      let s = "  let $1 = readStr(s, 6)" % [fName]
      readStr = "$1\n$2" % [readStr, s]
      add(readCons, "$1: $1" % [fName])

  readStr = "$1\n  result = $2($3)" % [readStr, $name, join(readCons, ", ")]
  
  result = newStmtList()
  add(result, parseStmt(typBodyStr))
  add(result, parseStmt(readStr))