Support for query fragments about doobie HOT 4 CLOSED

my proposed data structure is something like this:

case class QueryFragment[L <: HList: Composite](query: Cord, params: L) {
  def ++[P <: HList, POut <: HList](other: QueryFragment[P])
    (implicit p: Prepend.Aux[L, P, POut], cout: Composite[POut]): QueryFragment[POut] =
      QueryFragment(query ++ other.query, params ++ other.params)
}

you don't need to do anything special to support HList just make product and emptyProduct from LowerPriorityComposite implicitly available

UPDATE: this will not work as type of HList changes. more successfull attempt would be with coproducts or sigma types (https://gist.github.com/wedens/2c854d03f5c8b40bbb12).

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I like it.

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I've been running into some issues with this idea... specifically, I can't make a semigroup out of this, since what we want is precisely to be able to append arbitrary QueryFragments together...

see:
https://gist.github.com/kryptt/b2f7ef6719a252dd316c

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It can never be a semigroup because the type argument changes, but it should definitely be possible to append fragments (that's the whole point). Sorry I haven't had time to focus on this; I'm working on some less interesting things.

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