Haskell insert an element on nth position -


hi started learn haskell can't figure out 1 topic.

let's have list: [1,2,3] , i'm trying write function insert element on nth position. clues how do this?

you can build insertat function this:

insertat :: -> int -> [a] -> [a] insertat newelement 0 = newelement:as insertat newelement (a:as) = : insertat newelement (i - 1)

one strategy such problems write code edge cases , use recursion until edge case found.

the mentioned lens package simplifies handling data structures in sense code may become shorter , nicer write, @ expense of additional library learn.

both examples highlight there several ways achieve solution problem. recommend having @ data.list module gain further insight typical list operations. source drop function start you. provided splitat function fitting building block problem.

as shersh mentions correctly above implementation of insertat flawed: doesn't check negative positions , in case of 1 given continues recursing. can particularly bad in case of infinite list.

we can improve implementation using guards:

insertat :: -> int -> [a] -> [a] insertat newelement _ [] = [newelement] insertat newelement (a:as)   | <= 0 = newelement:a:as   | otherwise = : insertat newelement (i - 1)

this implementation tries right thing inserting newelement when in doubt. possible write version of insertat instead throws errors our face:

import data.monoid ((<>)) import qualified data.list list  insertat :: -> int -> [a] -> [a] insertat newelement   | null && != 0 = error "cannot insert empty list other position 0."   | null && == 0 = [newelement]   | >= 0 = let (prefix, suffix) = list.splitat              in prefix <> [i] <> suffix

this version makes use of list.splitat brevity.


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