Access Objects in Nim by strings for fields (wrapper automatisation)

strAccessObj.nim

import macros


type Foo = object
  a: int
  b: int
  c: float

#[
dumpTree:
  proc `[]=`(obj: var Foo, field: string, value: int) =
    case field
    of "a": obj.a = value
    of "b": obj.b = value
    else: raise newException(FieldError, "no such field: " & field)

dumpTree:
  proc `[]`(obj: Foo, field: string): int =
    case field
    of "a": return obj.a
    of "b": return obj.b
    else: raise newException(FieldError, "no such field: " & field)
]#


macro makeStringAccessObj(obj: stmt, fld: stmt): stmt =
  var objType = obj.getType

  result = newStmtList()

#[
  proc newProc(name = newEmptyNode(); params: openArray[NimNode] = [newEmptyNode()];
            body: NimNode = newStmtList(); procType = nnkProcDef): NimNode {.
    compileTime, raises: [], tags: [].}
]#

  let procName1 = newNimNode(nnkAccQuoted).add(newIdentNode("[]="))
  var body1 = newStmtList()

  var params1: seq[NimNode] = @[newEmptyNode()] # no return type
  params1.add(newIdentDefs(ident("obj"), newNimNode(nnkVarTy).add(ident(obj.repr))))
  params1.add(newIdentDefs(ident("field"), ident("string")))
  params1.add(newIdentDefs(ident("value"), ident(fld.repr)))

  var casof1 = newNimNode(nnkCaseStmt);
  casof1.add(ident("field"))

  var ty = objType

  while ty[1].kind == nnkSym:
    ty = getType(ty[1])

  if ty.kind != nnkObjectTy or ty[1].kind != nnkRecList:
    error "can't create named accessors"

  for child in ty[1].children:
    if fld.repr == repr(child.getType()):
      var branch = newNimNode(nnkOfBranch)
      branch.add(newStrLitNode($child))
      let ass = newAssignment(newDotExpr(ident("obj"), ident($child)), ident("value"))
      branch.add(newStmtList(ass))
      casof1.add(branch)

  let call1 = newCall(ident("newException"), ident("FieldError"),
    infix(newStrLitNode("No such field: "), "&", ident("field")));

  let els = newNimNode(nnkElse).add(newStmtList().add(newNimNode(nnkRaiseStmt).add(call1)))

  casof1.add(els)
  body1.add(casof1)
  result.add(newProc(procName1, params1, body1))

  let procName2 = newNimNode(nnkAccQuoted).add(newIdentNode("[]"))
  var body2 = newStmtList()

  var params2: seq[NimNode] = @[ident(fld.repr)] # returns int
  params2.add(newIdentDefs(ident("obj"), newNimNode(nnkVarTy).add(ident(obj.repr))))
  params2.add(newIdentDefs(ident("field"), ident("string")))

  var casof2 = newNimNode(nnkCaseStmt);
  casof2.add(ident("field"))

  for child in ty[1].children:
    if fld.repr == repr(child.getType()):
      var branch = newNimNode(nnkOfBranch)
      branch.add(newStrLitNode($child))
      let ass = newNimNode(nnkReturnStmt).add(newDotExpr(ident("obj"), ident($child)))
      branch.add(newStmtList(ass))
      casof2.add(branch)

  casof2.add(copyNimTree(els)) # same as in proc1
  body2.add(casof2)
  result.add(newProc(procName2, params2, body2))

  #echo result.treeRepr()

makeStringAccessObj(Foo, int)

var o = Foo( a: 1, b: 2 )

echo o["b"], ' ', o.b

o["a"] = 3
o.b = 4
echo o.a, ' ', o["b"]

let x = "a"
let y = "b"

o[x]= o[x] + o[y] * 2
echo o.a, ' ', o.b

echo o.c, ' ', o["c"] # runtime error