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milessabin / shapeless

Generic programming for Scala
Apache License 2.0
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Improve composability of Zipper operations. #143

Open milessabin opened 10 years ago

milessabin commented 10 years ago

The problem is visible when you try and use the zipper in a polymorphic function. If you've used shapeless you'd expect the function to need a bunch of implicit zipper operation witnesses before you can perform the corresponding operation within the method body. Unfortunately it's extremely difficult to write the signatures for those witnesses. For instance in,

import shapeless._, ops.zipper._
def getRight[T, L <: HList](t: T)
  (implicit gen: Generic.Aux[T, L], right: Right[..]) = Zipper(t).right

what should we write as the type argument for Right in the implicit parameter block above?

milessabin commented 10 years ago

It should be possible to write a method that takes a Zipper argument and yields the result of .right.get.

mandubian commented 10 years ago

Not sure to understand what you mean by composability... Do you have the signature of the function you imagine and a short description?

milessabin commented 10 years ago

@mandubian I've updated the description to make it clearer what the issue actually is. I've also removed the "Low hanging fruit" label because I think it might be quite fiddly to solve in a satisfying way.

mandubian commented 10 years ago

It compiles & works with: 

def getRight[T, L <: HList](t: T)
  (implicit gen:   Generic.Aux[T, L],
               right:  Right[Zipper[T, HNil, L, None.type]]
  ) = Zipper(t).right

But we can't write:

def getRight[T, RH, RL <: HList](t: T)
  (implicit gen:   Generic.Aux[T, RH :: RL],
               right:  Right[Zipper[T, HNil, RH :: RL, None.type]]
  ) = Zipper(t).right

The Generic.Aux[T, RH :: RL] can't be inferred by the compiler (the whitebox macro mustn't help it).

If we could write this, it would be easy to infer the Zipper from the Generic.Aux and finally to infer the Right from the Zipper with something like:

implicit def hasRight[C, L <: HList, RH, RT <: HList, P](zipper: Zipper[C, L, RH::RT, P]): Right[Zipper[C, L, RH::RT, P]] = Right.right[C, L, RH, RT, P]

In this case, we wouldn't need the implicit Right[Zipper[...]] in the def getRight but it's not possible as is...