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Sized collections for Scala 2.13 and Dotty

Build status Coverage status Maven Central

What is it?

This library provides a statically-sized wrapper for Scala 2.13's IndexedSeq:

scala> import dev.travisbrown.sized._

scala> val xs = SizedVector(1, 2, 3, 4, 5)
val xs: dev.travisbrown.sized.SizedVector[Int, 5] = SizedVector(1, 2, 3, 4, 5)

scala> xs.sizedReverse.sizedTail.sizedMap(_.toString)
val res0: dev.travisbrown.sized.SizedVector[String, 4] = SizedVector(4, 3, 2, 1)

scala> xs[4]
val res1: Int = 5

scala> xs.sizedConcat(xs.sizedReverse)
val res2: dev.travisbrown.sized.SizedVector[Int, 10] = SizedVector(1, 2, 3, 4, 5, 5, 4, 3, 2, 1)

All ordinary collections methods are available and return the underlying unsized type:

scala> xs.toVector
val res3: Vector[Int] = Vector(1, 2, 3, 4, 5)

scala> xs.take(100)
val res4: Vector[Int] = Vector(1, 2, 3, 4, 5)

scala> * 1000)
val res5: Vector[Int] = Vector(1000, 2000, 3000, 4000, 5000)

Accessing out-of-bounds elements fails at compile time:

scala> xs[-1]
1 |xs[-1]
  |Can't get element at a negative index

scala> xs[10]
1 |xs[10]
  |Index out of bounds

scala> xs.sizedDrop[5].safeHead
1 |xs.sizedDrop[5].safeHead
  |Can't take head of empty collection

This is all done without any macros (or custom type classes) on Dotty. Scala 2.13 requires a (fairly minimal) Pair type class with a macro-powered materializer. Neither version has any non-standard-library dependencies.


I've been wanting an excuse to experiment with implementing a collection type with Scala 2.13's new collection library, and also an excuse to spend some time playing with the new type-level singleton operations in Dotty.

Other questions

What about Scala 2.12?


What about Scala.js?

PRs are open.

What about other collection types?

PRs are open.

Why not just use Shapeless's sized collections?

Sized in Shapeless is great but I want to represent lengths with Int singletons, not Nat.

Why not just use Refined?

Size[Equal[N]] in Refined is also great, but not having support for collections in refineMV means it can feel a little awkward to work with.


This project supports the Scala code of conduct and wants all of its channels (Gitter, GitHub, etc.) to be welcoming environments for everyone.


This project is licensed under the Apache License, Version 2.0 (the "License"); you may not use this software except in compliance with the License.

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.