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8298 :: Why generic converter doesn't work?

• alexeypetrushin (orginal) [2021-08-05T01:25:07+02:00] view original

  Why generic convertor doesn't work?

  play

   Nim
  import strutils
  
  type Direction = enum top, right, bottom, left
  
  converter to_enum[T: enum]*(s: string): T = parse_enum[T](s)
  # converter to_enum*(s: string): Direction = parse_enum[Direction](s)
  
  let d: Direction = "top"
  echo d
  Run

  Error:

  
  Error: converter needs a return type
  Run

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• xigoi (orginal) [2021-08-05T01:55:30+02:00] view original

  There is no way for this converter to know what type you want to convert to.

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• demotomohiro (orginal) [2021-08-05T03:21:11+02:00] view original

  Nim can inferences generic type from arguments, but cannot inference return type.

  import strutils
  
  type Direction = enum top, right, bottom, left
  
  proc to_enum[T: enum](s: string): T = parse_enum[T](s)
  
  # Compile Error: cannot instantiate: 'T'
  # let d: Direction = to_enum("top")
  
  let d: Direction = to_enum[Direction]("top")
  
  echo d
  Run

  Generic converter like this work:

  import strutils
  
  type
    StringT[T] = string
  
  type Direction = enum top, right, bottom, left
  
  converter to_enum*[T: enum](s: StringT[T]): T = parse_enum[T](s)
  
  let d: Direction = StringT[Direction]("top")
  echo d
  Run

  By the way, export marker * must be placed right after an identifier like to_enum*[T], not like to_enum[T]*.

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