Mirror of forum.nim-lang.org

3235 :: Error: cannot instantiate: 'T'

• sflennik (orginal) [2017-10-09T02:17:36+02:00] view original

  I'm trying to write a generic function, here is a simplified version. It generates the errors shown below. What am I doing wrong?

  import strutils
  
  proc test16(): uint16 =
    var buffer: array[sizeof(uint16), uint8]
    for ix in 0..sizeof(uint16)-1:
      buffer[ix] = cast [uint8](ix)
    result = cast[uint16](buffer)
  
  proc test[T](): T =
    var buffer: array[sizeof(T), uint8]
    for ix in 0..sizeof(T)-1:
      buffer[ix] = cast [uint8](ix)
    result = cast[T](buffer)
  
  var value16: uint16 = test16()
  echo "value16 = ", toHex(value16)
  
  var valueT: uint16 = test()
  echo "valueT = ", toHex(valueT)
  Run

  Here are the errors:

  
  # t2.nim(18, 26) template/generic instantiation from here
  # t2.nim(9, 11) Error: cannot instantiate: 'T'
  Run

---

• jlp765 (orginal) [2017-10-09T04:16:34+02:00] view original

  Try either (on line 18)

  
  var valueT = test[uint16]()
  Run

  or

  
  var valueT: uint16 = test[uint16]()
  Run

  The issue was that test() needed to be "told" what the generic type T was.

---

• LeuGim (orginal) [2017-10-09T07:32:13+02:00] view original

  I.e. there's no return type overloading, test cannot automatically get, by what type it is parameterised.

---

• sflennik (orginal) [2017-10-10T04:51:35+02:00] view original

  Thank you very much.

---