Mirror of forum.nim-lang.org

397 :: Hi and some beginners questions

• rvd (orginal) [2014-03-11T19:29:12+01:00] view original

  Hi!

  I stumbled upon nimrod while searching for examples involving perlin noise and found this little benchmark ( https://github.com/nsf/pnoise ) which shows how fast nimrod can be (only surpassed by gcc and rust here). So i dived into nimrod and i think it looks really awesome.

  However i'm having some difficulty getting the right/most efficient "nimrodic way" into my head. I'm now playing with it for a day of 4 and i get the feeling a lot is possible but beyond basic stuff mentioned in the tutorial, not including macros/typeclasses at which point the learning curve makes a significant jump, I hit some sort of wall getting things done, fighting the compiler every 10'th line or so.

  Which is of course a clear sign i'm looking at things the wrong way.

  At this point i'm building a tree of objects that differ in type only in "kind" and their type of value field. It felt a bit overkill to use inheritance for this (as the tutorial suggests), but i want to access this value field in the same way throughout the program (and in the real program this list of kinds is much larger, so using a case-of block every time looks awkward). So the problem here is assigning/returning a value of which the type is only known at runtime.

  I know i'm possibly trying to make nimrod behave more dynamic than it probably is, but then again nimrod has an auto type, gives me templates, "cast" and references, so i guess it can be done.

  Googling the forum resulted in being directed multiple times to typeclasses, but as someone without formal education in type theory, its a bit beyond ready knowledge for me - and they seem to be more concerned with contracts & type safety anyway (correct me if i'm wrong)

  So if someone could point me in the right direction with the following questions i would be very grateful :)

  Is it somehow possible to create an array of typeDesc?

  var ary : array[0..2, typeDesc] = [int32, string, float]
  Run

  This doesn't work, but the error isn't very enlightening either: internal error: GetUniqueType

  Leaving out the last float in the array constructor is obviously wrong but tells me:

  type mismatch: got (Array constructor[0..1, typedesc]) but expected 'array[0..2, typedesc]

  So why does it choke on an array of typedesc's of the right length?

  With the following setup, trying to get ival/sval dynamically:

  type
    TKind = enum
      Kind1, Kind2
    PTyp = ref TTyp
    TTyp = object
      case kind: TKind
      of Kind1: ival: int
      of Kind2: sval: string
    # cannot use same name (val) for ival/sval => Error: redefinition of 'val'
  
  proc newTyp(k: TKind): PTyp =
    result = PTyp(kind: k)
  
  var
    t1 = newTyp(Kind1)
    t2 = newTyp(Kind2)
  
  t1.ival = 1
  t2.sval = "foo"
  Run

  # Try 1

  proc ret_auto(typ: PTyp): auto =
    case typ.kind
    of Kind1: result = typ.ival
    of Kind2: result = typ.sval
  
  var x = ret_auto(t1) # Error: type mismatch: got (string) but expected 'int'
  # So, alright. I cant delay to runtime to determine what type this proc has.
  # Fair enough.
  Run

  # Try 2

  proc dyn_param(typ: PTyp, dyn: var auto) =
    case typ.kind
    of Kind1: dyn = typ.ival
    of Kind2: dyn = typ.sval
  
  var x : auto
  t1.dyn_param(x) # Error: internal error: getTypeDescAux(tyExpr)
  # Why does this fail? I would expect var dyn to become an int/string which propagates to x
  Run

  # Try 3

  template value(name: expr, typ: PTyp): stmt {. immediate .} =
    var name: auto
    case typ.kind
    of Kind1:
      name = typ.ival
    of Kind2:
      name = typ.sval
  
  
  
  value(x, t1) # Error: internal error: getTypeDescAux(tyExpr)
  # Same as 2, but i need an auto var to make "name" visible outside the case-of scope
  Run

  ---

  Edited: can never check long posts enough.

---

• LeuGim (orginal) [2014-03-11T21:15:01+01:00] view original

  This is done easier with generic types (http://nimbuild.nimrod-lang.org/manual.html#generics):

  type
    PTyp[T] = ref TTyp[T]
    TTyp[T] = object
      val: T
  
  proc newTyp[T]: PTyp[T] =
    result = new TTyp[T]
  
  var
    t1 = newTyp[int]()
    t2 = newTyp[string]()
  t1.val=5
  t2.val="test"
  
  
  proc ret[T](obj: PTyp[T]): T =
    result = obj.val
  
  var
    x = ret(t1)
    y = ret(t2)
  
  echo x, y
  Run

  ---

  Your example works for compatible types:

  type
    TKind = enum
      Kind1, Kind2
    PTyp = ref TTyp
    TTyp = object
      case kind: TKind
      of Kind1: ival: int
      of Kind2: sval: int8
    # cannot use same name (val) for ival/sval => Error: redefinition of 'val'
  
  proc newTyp(k: TKind): PTyp =
    result = PTyp(kind: k)
  
  var
    t1 = newTyp(Kind1)
    t2 = newTyp(Kind2)
  
  t1.ival = 1
  t2.sval = 2
  
  # Try 1
  proc ret_auto(typ: PTyp): auto =
    case typ.kind
    of Kind1: result = typ.ival
    of Kind2: result = typ.sval
  
  var x = ret_auto(t1)
  var y = ret_auto(t2)
  
  echo x, y
  Run

  ---

  type mismatch: got (Array constructor[0..1, typedesc]) but expected 'array[0..2, typedesc]'

  You took away one item (float) and didn't change array size, so you got another error.

---

• rvd (orginal) [2014-03-11T21:46:07+01:00] view original

  Thanks for your reply. I will try using generics then. :)

  I was hoping i could eventually instantiate PTyp's in newTyp with some array.

  So using generics :

  proc newTyp(kind: TKind): PTyp[T] =
    var ary: array[Kind1..Kind2, typeDesc] = [int, string]
    result = new TTyp[ ary[kind] ]
  Run

  This could be also done with a case-of block, but what confused me is that building an array of types is possible in the repl but not in normal code.

---

• zahary (orginal) [2014-03-11T22:28:47+01:00] view original

  Arrays of types are possible within compile-time contexts (i.e. they could be assigned to constants, used inside macros and accidentally in the REPL too)

---