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1943 :: Expressing monads using concepts

• zielmicha (orginal) [2016-01-15T10:41:29+01:00] view original

  Not that it's useful, but it would be very cool to be able to express monad typeclass in Nim. I'd like to express that monad instance needs to have the flatMap procedure, like this:

  proc flatMap[T, R](m: Monad[T], proc(x: T): Monad[R]): Monad[R]
  Run

  The most obvious thing doesn't work:

  type
    Monad*[T] = concept m
      # flatMap i.e. monadic bind
      flatMap(m, proc(x: T): Monad[R]) is Monad[R]
  
  # somewhere else
  proc flatMap*[T; R](coll: Iterator[T], f: (proc(item: T): Iterator[R])): Iterator[R]
  
  # and
  terator[int] is Monad[int]
  Run

  Error from --reportConceptFailures:on:

  
  base.nim(18, 5) Error: type mismatch: got (Iterator[system.int], typedesc[proc (x: typedesc[int]): Monad[Error Type]{.closure.}])
  but expected one of:
  iterate.flatMap(coll: seq[T], f: proc (item: T): seq[R]{.closure.})
  iterate.flatMap(coll: Iterator[flatMap.T], f: proc (item: T): Iterator[flatMap.R]{.closure.})
  Run

  Do you have any ideas?

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• Araq (orginal) [2016-01-15T12:04:51+01:00] view original

  Monad[Error Type] implies the R in your concept is undeclared. And indeed it is.

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• zielmicha (orginal) [2016-01-15T15:08:52+01:00] view original

  Well, I know its undefined, I've just hoped that undefined types will default to "unique type variable". Is it possible to make something like:

  
  forall R: flatMap(m, proc(x: T): Monad[R]) is Monad[R]
  Run

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