Shooting your self in the foot

The following list was proposed by members of Haskell-Cafe.

The original thread can be found here.

How to Shoot Your Self in the Foot With Haskell: Putting the unsafe in unsafePerformIO!

• You shoot the gun, but the bullet gets trapped in the IO monad.
• Couldn't match expected type 'Deer' against inferred type 'Foot'.
• While compiling your program the compiler produces a type error long enough to overflow a kernel buffer, overwrite the trigger control register and shoot you in the foot.
• After trying to decipher the type errors from the compiler, your head explodes.
• After you've finally found a way to circumvent the type system and shoot yourself in the foot, Oleg appears out of nothing and shoots you in the foot for coming up with it before him.
• You shoot the gun but nothing happens (Haskell is pure, after all).
• Your foot is fine, until you try to walk on it, at which point it becomes mangled.
• You have a shootFoot function which you've proven correct. QuickCheck validates it for arbitrary you-like values. It will be evaluated only when you end up at the hospital. You hope this doesn't come to pass, as it actually returns a bullet-ridden copy of yourself and you don't want to be garbage-collected.

• foreign import ccall "shootparts.h shootfoot" shoot_foot :: Gun -> Programmer -> IO ()

• shootSelfInFoot = unsafePerformIO . shoot . foot $ self -- Shoot self in foot 0 or more times depending on evaluation order

• No instance for (Target Foot)
  arising from use of `shoot' at SelfInflictedInjury.hs:1:0
  Possible fix: add an instance declaration for (Target Foot)
  In the expression: shoot foot
• You go to shoot yourself in the foot but the bullet is in the ST monad and the gun is in the IO monad, so you can't.
• You ask Haskell to shoot you in the foot but by the rules of lazy evaluation you don't need the result yet so it doesn't happen.
• You decide to shoot yourself in the foot but get distracted devising a ballistics algebra and wondering if you can do the calculations in the type system.
• You want to shoot yourself in the foot but realize there is no Gun datatype so use Arrows instead.
• You shoot in the direction of your foot, but since you are inside the STM monad you can just retry until you figure out what to do.
• You shoot yorself in the foot, but you are perfectly fine as long you just don't evaluate the foot.
• You shoot yourself in the foot, but nothing happens unless you start walking.
• Don't forget about memory consumption! If you don't look, the bullet causes heap overflow. If you look, the bullet causes stack overflow.
• You *appear* to have deliberately shot yourself in the foot, and yet your program actually runs perfectly OK due to lazy evaluation. (So long as you remember to not look at your foot...)
• You aim the gun at your foot, pull the trigger and remove the clip. When you look at your undamaged foot, the hammer clicks on an empty barrel.

This above list is inspired by this list.