Simple Text Editor

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#include <string>
#include <stack>
#include <iostream>
#include <algorithm>
using namespace std;

int main()
{
    string S, tmp; //S.reserve(1000000);
    int Q, op, K; cin >> Q;
    stack<string> states;
    while (Q--)
    {
        cin >> op;
        switch(op)
        {
        case 1:
            states.push(S);
            cin >> tmp;
            S.append(tmp);
            break;
        case 2:
            states.push(S);
            cin >> K;
            S.erase(S.end()-K, S.end());
            break;
        case 3:
            cin >> K;
            cout << S[K-1] << endl;
            break;
        case 4:
            S = move(states.top());
            states.pop();
            break;
        }
    }
}

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data State = State String [String]

atIndex k lst = lst !! (length lst - k)

edit fn (State o (x:xs)) = State o (fn x:x:xs)
out fn (State o (x:xs)) = State (fn o x) (x:xs)
undo (State o xs) = State o (drop 1 xs)

-- reverse is needed for better performance
append w x = reverse w ++ x
printChar k o x = '\n':atIndex k x:o

cmd ('1':' ':param) = edit . append $ param
cmd ('2':' ':param) = edit . drop . read $ param
cmd ('3':' ':param) = out . printChar . read $ param
cmd "4" = undo

runOne state command = command state

runAll = foldl runOne (State "" [""])

takeOutput (State o _) = reverse o

main = do
    _ <- getLine
    contents <- getContents
    putStr . takeOutput . runAll $ cmd <$> lines contents