MPL-2.0 licensed by Veronika Romashkina
Maintained by Kowainik
This version can be pinned in stack with:slist-,3513

Module documentation for

Depends on 2 packages(full list with versions):


GitHub CI Hackage MPL-2.0 license

⚠️ Caution: this is a very opinionated library. There is no intention to replace the standard list data type. We are aware of every design decision we made for this package, and we are taking responsibility for that design. If you find it inappropriate, please, consider to use another library instead, that would fulfil your requirements.

This package introduces sized list data type — Slist. The data type has the following shape:

data Slist a = Slist
    { sList :: [a]
    , sSize :: Size

As you can see that along with the familiar list, it contains Size field that represents the size of the structure. Slists can be finite or infinite, and this is expressed with Size.

data Size
    = Size Int
    | Infinity

⚠️ Caution: Int is used for the size by design. We had to make some trade-offs to provide the better (as we think) interface for users. For more details on the choice, see the Haddock documentation for the Size data type.

This representation of the list gives some additional advantages. Getting the length of the list is the “free” operation (runs in O(1)). This property helps to improve the performance for a bunch of functions like take, drop, at, etc. But also it doesn’t actually add any overhead on the existing functions.

Also, this allows to write a number of safe functions like safeReverse, safeHead, safeLast, safeIsSuffixOf, etc.


Check out the comparison table between lists and slists performance.

Function list (finite) list (infinite) Slist (finite) Slist (infinite)
length O(n) <hangs> O(1) O(1)
safeLast O(n) <hangs> O(n) O(1)
init O(n) <works infinitely> O(n) O(1)
take O(min i n) O(i) 0 < i < n: O(i); otherwise: O(1) O(i)
at O(min i n) (run-time exception) O(i) (run-time exception) 0 < i < n: O(i); otherwise: O(1) O(i)
safeStripPrefix O(m) O(m) (can hang) O(m) O(m)

Potential usage cases

  • When you ask the size of the list too frequently.
  • When you need to convert to data structures that require to know the list size in advance for allocating an array of the elements. Example: Vector data structure.
  • When you need to serialise lists.
  • When you need to control the behaviour depending on the finiteness of the list.
  • When you need a more efficient or safe implementation of some functions.



slist uses PVP Versioning. The changelog is available on GitHub. – Nov 3, 2022

  • #56: Support GHC-9.4. – Oct 7, 2022

  • #53: Support GHC-9.2.
  • Upgrade hedgehog dependency. — Mar 18, 2021

  • #45: Support GHC-9.0. Update older GHC’s bounds.

  • #36: Add strict functions: append', concat' and concatMap'.

  • #30: Add cons and cons' functions.

  • #35: Add integration with the containers library: mapToKeys, mapToVals, mapToPairs, setToSlist.

    Add ordNub.

  • #34: Add partitionWith and listPartitionWith.

  • #29: Add Slist.Maybe module with maybeToSlist, slistToMaybe, catMaybes, mapMaybe, slistWith functions.

  • #31: Add sortWith.

  • #24: Add chunksOf for Slist and listChunksOf for ordinary lists.

  • Move the Slist data type into the separate Slist.Type module. — Apr 18, 2020

  • Fix mconcat for Slist Monoid instance.
  • #25: Support GHC-8.10.
  • Update to GHC-8.8.3 from GHC-8.8.1.

  • #13: Support GHC-8.8.1.
  • #16: Use DerivingStrategies.
  • #9: Implement fromRange function. (by @zfnmxt)
  • #6: Add generic function over the size and indices. (by @waynee95)
  • Make dropWhile work better on infinite lists. (by @chshersh)
  • Support GHC-8.6.5 instead of GHC-8.6.3.
  • #6: Build with Stack.


  • Initially created.