Daily Challenge #286 - Reverse It Quickly!

ES6

a=>[...a].map(a.pop,a)

Haskell solution in exactly 28 characters:

r=flip(:)`foldl`[]::[a]->[a]

Longer version of the same, but more readable:

myReverse :: [a] -> [a]
myReverse = flip (:) `foldl` []

Couldn't skip type annotations as that raises ambiguous type error.

Explanation:

1. Function foldl applies a given function to all container elements going from inside out. When applied to list, it effectively traverses list in a reverse order.
2. Binary operator foldl is applied to flip (:) and [].
3. foldl has type signature Foldable t => (b -> a -> b) -> b -> t a -> b, which can be rewritten as ([a] -> a -> [a]) -> [a] -> [a] -> [a].
4. Binary operator (:) appends a value (first argument) to a list (second argument). It's type signature is a -> [a] -> [a]. It almost looks like the required signature from the 3. point.
5. Use the flip function in order to switch around (:) first and second argument. This results in a function flip (:) with type signature of [a] -> a -> [a].
6. Now when this flip (:) is partially applied to foldl, it gives a function foldl (flip (:)) with a type signature [a] -> [a] -> [a]. Giving foldl a initial value of [], gives the final form foldl (flip (:)) [] with a type signature [a] -> [a].

Python 🐍

from typing import List, Any

def reverse_(values: List[Any]) -> List[Any]:
  return values[::-1]

Tried it in C# (64 chars)

var r=new ArrayList();foreach(var e in l)r.Insert(0,e);return r;

Well.. 11 characters?

a = a[::-1]

Python

Seems to work:

def reverse(arr):
    rtnArr = []

    for i in arr[::-1]:
        rtnArr.append(i)

    return rtnArr

Bit more elegant that mine.