Which dictionary does GHC choose when more than one is in scope?

Question

Consider the following example:

import Data.Constraint

class Bar a where
  bar :: a -> a

foo :: (Bar a) => Dict (Bar a) -> a -> a
foo Dict = ba         


        

Answer 1

It just picks one. This isn't the correct choice; it's a pretty well-known wart. You can cause crashes this way, so it's a pretty bad state of affairs. Here is a short example using nothing but GADTs that demonstrates that it is possible to have two different instances in scope at once:

-- file Class.hs
{-# LANGUAGE GADTs #-}
module Class where

data Dict a where
  Dict :: C a => Dict a

class C a where
  test :: a -> Bool

-- file A.hs
module A where

import Class

instance C Int where
  test _ = True

v :: Dict Int
v = Dict

-- file B.hs
module B where

import Class

instance C Int where
  test _ = False

f :: Dict Int -> Bool
f Dict = test (0 :: Int)

-- file Main.hs
import TestA
import TestB

main = print (f v)

You will find that Main.hs compiles just fine, and even runs. It prints True on my machine with GHC 7.10.1, but that's not a stable outcome. Turning this into a crash is left to the reader.