On Sun, Feb 16, 2020 at 03:39:24PM +0000, Juan Casanova wrote:
I simply do not agree with this, and I don't think (?) that GHCi implementors and designers agree with you either, as exemplified by the fact that indeed overlapping instances appear when using instances and not when defining them. While it doesn't clearly agree with what I said, the general phrasing and approach followed here: https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_exts... makes me think indeed the approach is that potentially overlapping instances are allowed so long as each of their uses are unambiguous.
And yet the particular overlapping instances leave room for unexpected results. For example, if the code is specialized a bit to impose "Eq a" and "Typeable b, Eq b" constaints on "Bar", and one implements a heterogenous comparison function for "Bar", not quite the right thing happens: {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE ExistentialQuantification #-} module Main (main) where import Data.Typeable (Typeable, cast) class Class1 a b where foo :: a -> b instance {-# INCOHERENT #-} Monoid a => Class1 a (Either b a) where foo x = Right (x <> x) instance {-# INCOHERENT #-} Monoid a => Class1 a (Either a b) where foo x = Left x data Bar a = Dir a | forall b. (Typeable b, Eq b, Class1 b a) => FromB b instance Eq a => Eq (Bar a) where (Dir x) == (Dir y) = x == y (FromB x) == (FromB y) = case cast x of Just x' -> x' == y _ -> False _ == _ = False getA :: Bar a -> a getA (Dir a) = a getA (FromB b) = foo b createBar :: Eq t => Bar (Either String t) createBar = FromB "abc" createBar2 :: Eq t => Bar (Either t String) createBar2 = FromB "abc" main :: IO () main = do let x = createBar :: Bar (Either String String) y = createBar2 :: Bar (Either String String) print $ map getA [x, y] print $ x == y If your run the above, the output you get is: [Left "abc",Right "abcabc"] True Which shows some of the trouble one can run into with incoherent definitions. Structurally equal terms are no longer equivalent. -- Viktor.