vec: Vec: length-indexed (sized) list

[ bsd3, data, dependent-types, library ] [ Propose Tags ]

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.

For lens or optics support see vec-lens and vec-optics packages respectively.

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.


Manual Flags


Depend on adjunctions to provide its instances


Depend on distributive to provide its instances. Turning on, disables adjunctions too.


Depend on semigroupoids to provide its instances, and traverse1.


Use -f <flag> to enable a flag, or -f -<flag> to disable that flag. More info


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Versions [RSS] 0, 0.1, 0.1.1,, 0.2, 0.3, 0.4, 0.4.1, 0.5, 0.5.1
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Dependencies adjunctions (>=4.4.2 && <4.5), base (>= && <4.21), boring (>=0.2.2 && <0.3), deepseq (>= && <1.6), distributive (>=0.6.2 && <0.7), fin (>=0.3.1 && <0.4), hashable (>= && <1.5), indexed-traversable (>=0.1.4 && <0.2), QuickCheck (>=2.14.2 && <2.15 || >= && <2.16), semigroupoids (>=6.0.1 && <6.1), transformers (>= && <0.7) [details]
License BSD-3-Clause
Copyright (c) 2017-2021 Oleg Grenrus
Author Oleg Grenrus <>
Maintainer Oleg.Grenrus <>
Category Data, Dependent Types
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Source repo head: git clone
Uploaded by phadej at 2024-06-08T14:36:15Z
Distributions Arch:0.5, LTSHaskell:0.5, NixOS:0.5, Stackage:0.5.1
Reverse Dependencies 5 direct, 7784 indirect [details]
Downloads 5758 total (65 in the last 30 days)
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