kowainik / slist

♾️ Sized list
https://kowainik.github.io/projects/slist
Mozilla Public License 2.0
46 stars 6 forks source link
data-structure hacktoberfest haskell list sized

slist

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.

Comparison

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