shapeless: Generic programming for Scala | Latest stable release 2.3.3 | Code of conduct https://www.scala-lang.org/conduct/
def check[A, C <: Coproduct, K <: HList](data: Seq[A])(
implicit
gen: LabelledGeneric.Aux[A,C]
keys: ops.union.Keys.Aux[C,K]
toList: ops.hlist.ToTraversable.Aux[K,List,Symbol]
): List[(String,Boolean)] = {
val classes: List[Symbol] = toList(keys())
classes.map(sym => (sym.name, data.exists(_.getClass.getSimpleName == sym.name)))
}
Hey guys, I'm trying to use shapeless's HMap class, and I'm running into trouble. The docs say that it supports "an arbitrary relation between the key type and the corresponding value type", but it looks like you cannot reuse the key type. I.e., you cannot define both a
Int -> Double and Int -> String. Is this correct? Or am I using the library incorrectly?
if you have both
Rel[Int, Double] and Rel[Int, String], and you do get(1), how will the typesystem know whether the value is of type Double or String?
you might also want to look at the
vault library for a similar abstraction with different constraints
in that case the keys are represented by a value of type
Key[A], and then you can get the corresponding value
it's not quite a Map though, more like a store
and just not use Int in the HMap
although it does seem like you might be able to do this if you specify the return type as well (not sure if that would work with HMap, but some sort of Poly wrapper around a Map should do it)
I can use
implicit ev: SelectAll[H,I] to ensure all members of I are in H.
And I can use
implicit ev: IsHCons[I] to ensure that I is not empty.
But if I combine the two like so...
def foo[H <: Hlist, I <: HList](xs: H, ys: I)(implicit ev: SelectAll[H,I], ev2: IsHCons[I]): Unit = {
println(xs.select[ev2.H])
}
It says that there's no proof that
ev2.H is in H.
SelectAll does not guarantee order, so SelectAll[H, I] does not provide proof that IsHCons[I] is equal to IsHCons[H].
So you cannot do
ev2.head(xs) because while they have the same types, it’s not given that the head of H is the same type as head of I. It could be a type not existing in I, or one of the types in the tail of I. And if you wanna do xs.select, you need to provide a Selector instance for it.
But it looks way better than what I finally arrived at last night.
or rather, there are two different approaches to this problem: HMap and Vault
if you have
A -> B and A -> C then by definition it's not a mapping
what does
A ->map to, at the type level?
B or C?
they will all have different types
and it will work brilliantly
you can then convert strings to objects at the edge (or even build some machinery with witness similarly to how record selection works)
again, if you have
A -> B and A -> C, how is that a mapping?
it sounds like you want to be able to map the value "a" to the type
Boolean and the value "b" to the type Int
this is not directly possible without full dependent types: a value is a runtime entity, whilst a type is a compile time one
the way you simulate it generally is by the pattern of creating singleton types
you create types with exactly one value, provide a type to type mapping, and then need to handle the lifting of values to their singleton types with runtime checks (if they aren't statically known) or macros (if they are)
the pattern I mention above with objects as key + HMap does exactly that
and the same pattern is used by shapeless labelled generic, without going through objects but directly exposing the singleton types of strings through macro magic
If I have a recursively-resolved typeclass like this:
trait Foo[A] {
type Repr
def to: Repr
}With at most one instance for a given A. Is there a way to write an instance summoner that looks something like this:
def instance[A](implicit foo: Foo.Aux[A,R]): R = ???? And if so, will the compiler correctly infer the concrete type rather than some Foo.Aux… type?