Vec: length-indexed (sized) list

Version on this page:
LTS Haskell 22.29:0.5@rev:1
Stackage Nightly 2024-07-13:0.5.1
Latest on Hackage:0.5.1

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BSD-3-Clause licensed by Oleg Grenrus
Maintained by Oleg.Grenrus
This version can be pinned in stack with:vec-,4691

Module documentation for

This package provides length-indexed (sized) lists, also known as vectors.

data Vec n a where
    VNil  :: Vec 'Nat.Z a
    (:::) :: a -> Vec n a -> Vec ('Nat.S n) a

The functions are implemented in four flavours:

  • naive: with explicit recursion. It's simple, constraint-less, yet slow.

  • pull: using Fin n -> a representation, which fuses well, but makes some programs hard to write. And

  • data-family: which allows lazy pattern matching

  • inline: which exploits how GHC dictionary inlining works, unrolling recursion if the size of Vec is known statically.

As best approach depends on the application, vec doesn't do any magic transformation. Benchmark your code.

This package uses fin, i.e. not GHC.TypeLits, for indexes.

See Hasochism: the pleasure and pain of dependently typed haskell programming by Sam Lindley and Conor McBride for answers to how and why. Read APLicative Programming with Naperian Functors by Jeremy Gibbons for (not so) different ones.

Similar packages

  • linear has V type, which uses Vector from vector package as backing store. Vec is a real GADT, but tries to provide as many useful instances (upto lens).

  • vector-sized Great package using GHC.TypeLits. Current version ( uses finite-typelits and Int indexes.

  • sized-vector depends on singletons package. vec isn't light on dependencies either, but try to provide wide GHC support.

  • fixed-vector

  • sized also depends on a singletons package. The Sized f n a type is generalisation of linear's V for any ListLike.

  • clash-prelude is a kitchen sink package, which has CLaSH.Sized.Vector module. Also depends on singletons.


Revision history for vec

  • Use fin-0.1


  • Add Data.Vec.DataFamily.SpineStrict module
  • Add Data.Vec.DataFamily.SpineStrict.Pigeonhole module: this let us define Representable in a handy way.


  • Reverse dependencies with boring.
  • GHC-8.4.1 support


  • First version. Released on an unsuspecting world.