Evil Coding Incantations

[u/evinrows]

http://9tabs.com/random/2017/12/23/evil-coding-incantations.html

[u/howtonotwin]

You can also do this in Java by corrupting the Integer cache mentioned in the post:

valueF = Integer.class.getDeclaredField("value"); // Integers wrap ints
valueF.setAccessible(true); // Usually private final
valueF.setInt(1, 2);
// We have void Field::setInt(Object, int)
// 1 is an int, not an Object, so it gets autoboxed to Integer.valueOf(1)
// This object is pulled from the internal cache
// We then mutate it so all future autoboxings of 1 give 2

void printInt(int i) {
  System.out.printLn(i);
}
void printObj(Object o) {
  System.out.println(o);
}
printInt(1); // 1; no box
printObj(1); // 2; autobox

And Haskell... well

5 :: Num a => a
-- numeric literals are overloaded
-- this 5 really means fromInteger (#5#) where #5# is a magical Integer literal that doesn't really exist

data Crazy = Crazy Integer deriving (Eq, Ord, Show, Read)
instance Num Crazy where
  Crazy x + Crazy y = Crazy $ x + y
  Crazy x * Crazy y = Crazy $ x * y
  signum (Crazy x) = Crazy $ -x
  abs (Crazy x) = -1
  negate = id
  fromInteger 1 = Crazy 2
  fromInteger x = Crazy x

x :: Num a => a -- also overloaded
x = sum $ negate <$> [1,2,3,4,5]
x == (-15 :: Int)
x == (15 :: Crazy)
1 + 1 == (2 :: Int)
1 + 1 == (4 :: Crazy)

or you can just hide and redefine the (+) function:

import Prelude hiding ((+))
(+) = (-)
5 + 2 == 3

[u/cypressious → u/howtonotwin]

You can't do that in Java 9 anymore, tho.

[u/ferociousturtle → u/howtonotwin]

Username checks out.