St. - 1 year ago 69

Scala Question

Suppose I have a method without params. How can I determine a length of type parameter?

`def func[T <: HList]: Nat = {`

// some magic

}

Answer Source

You can use `ops.hlist.Length`

operation to calculate the `Nat`

length of an `HList`

.

Also, getting it as an opaque `Nat`

is not very useful, because you lose all the type-level information about the actual number. So you have to get the exact `Nat`

type from the function:

```
import shapeless._
import shapeless.ops.hlist.Length
def func[T <: HList](implicit len: Length[T]): len.Out = len()
```

Usage:

```
scala> natLen[Int :: String :: HNil]
res1: shapeless.Succ[shapeless.Succ[shapeless._0]] = Succ()
```

Getting the length as an `Int`

seems more tricky. It seems you can't use `ops.nat.ToInt`

, because it would require a `N <: Nat`

type parameter and basically make it useless:

```
def uselessIntLen[T <: HList, N <: Nat](implicit
len: Length.Aux[T, N],
toInt: ToInt[N]
): Int = toInt()
```

I've found the following workaround using `HKernel`

(of course, it's also possible to write a new typeclass `IntLength`

manually). Maybe someone can help with a more direct built-in method:

```
import shapeless.ops.hlist.HKernelAux
def intLen[T <: HList](implicit ker: HKernelAux[T]): Int = ker().length
```

Usage:

```
scala> intLen[Int :: String :: HNil]
res2: Int = 2
```