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3435 :: How do genericHead and stripGenericParams actually work?

• Udiknedormin (orginal) [2017-12-27T22:15:12+01:00] view original

  I followed the manual as for how to use higher-kinded concepts. I was quite surprised when the code containing genericHead actually compiled, but returned something I don't really get...

  import future, typetraits, options
  
  type Functor[A] = concept f
    f.get is A
  
  echo Option[int] is Functor  # prints true
  Run

  import future, typetraits, options
  
  type Functor[A] = concept f
    f.get is A
    
    type MatchedGenericType = genericHead(f.type)
    
    type T = auto
    map(f, A -> T) is MatchedGenericType[T]
  
  echo Option[int] is Functor  # prints false
  Run

  So I checked...

  import options
  type MyOption = genericHead(Option[int])  # type expected
  Run

  Ok, so I'm supposed to instantiate the higher-order type. No problem.

  import options
  type StrOption = genericHead(Option[int])[string]  # no generic parameters allowed for T
  Run

  Ok... So maybe I'll use stripGenericParams?

  import options
  type StrOption = stripGenericParams(Option[int])[string]
  echo StrOption()  # prints: None[string]
  Run

  Perfect. Now make it generic and add to the concept:

  import future, typetraits
  
  type Functor[A] = concept f
    f.get is A
    type MatchedGenericType[T] = stripGenericParams(f.type)[T]
  
  echo Option[int] is Functor  # prints true
  Run

  Nice. Now add the map...

  import future, typetraits
  
  type Functor[A] = concept f
    f.get is A
    type MatchedGenericType[T] = stripGenericParams(f.type)[T]
    
    type T = auto
    map(f, A -> T) is MatchedGenericType[T]
  
  echo Option[int] is Functor  # prints false
  Run

  Ok, no I have no idea what happened. {.explain.} doesn't seem to work either...

  Does anyone have any idea what's going on?

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