Dependent types: Vector of vectors

Question

I'm new to dependent types (I'm trying both Idris and Coq, despite their big differences).

I'm trying to express the following type: given a type T and a sequence of k nats n1, n2, ... nk, a type consisting of k sequences of T with length n1, n2, ... nk respectively.

That is, a vector of k vectors, whose lengths are given by a parameter. Is this possible?

Answer 1

You can do this with a heterogeneous list, as follows.

Require Vector.
Require Import List.
Import ListNotations.

Inductive hlist {A : Type} (B : A -> Type) : list A -> Type :=
| hnil : hlist B []
| hcons : forall a l, B a -> hlist B l -> hlist B (a :: l).

Definition vector_of_vectors (T : Type) (l : list nat) : Type :=
  hlist (Vector.t T) l.

Then if l is your list of lengths, the type vector_of_vectors T l with will the type you describe.

For example, we can construct an element of vector_of_vectors bool [2; 0; 1]:

Section example.
  Definition ls : list nat := [2; 0; 1].

  Definition v : vector_of_vectors bool ls :=
    hcons [false; true]
          (hcons []
                 (hcons [true] hnil)).
End example.

This example uses some notations for vectors that you can set up like this:

Arguments hnil {_ _}.
Arguments hcons {_ _ _ _} _ _.

Arguments Vector.nil {_}.
Arguments Vector.cons {_} _ {_} _.

Delimit Scope vector with vector.
Bind Scope vector with Vector.t.
Notation "[ ]" := (@Vector.nil _) : vector.
Notation "a :: v" := (@Vector.cons _ a _ v) : vector.
Notation " [ x ] " := (Vector.cons x Vector.nil) : vector.
Notation " [ x ; y ; .. ; z ] " :=  (Vector.cons x (Vector.cons y .. (Vector.cons z Vector.nil) ..)) : vector.

Open Scope vector.

Answer 2

In Idris, besides creating a custom inductive type, we can reuse the standard type of heterogeneous vectors -- HVect:

import Data.HVect

VecVec : Vect k Nat -> Type -> Type
VecVec shape t = HVect $ map (flip Vect t) shape

val : VecVec [3, 2, 1] Bool
val = [[False, False, False], [False, False], [False]] -- the value is found automatically by Idris' proof-search facilities