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Editorial Team
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Asked: 2026-06-10T22:00:11+00:00

Real World Haskell has this example: class BasicEq3 a where isEqual3 :: a ->

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Real World Haskell has this example:

class BasicEq3 a where
    isEqual3 :: a -> a -> Bool
    isEqual3 x y = not (isNotEqual3 x y)

    isNotEqual3 :: a -> a -> Bool
    isNotEqual3 x y = not (isEqual3 x y) 

instance BasicEq3 Bool

And when I run it in GHCI:

#> isEqual3 False False
out of memory

So, you have to implement at least one of the 2 methods or it will loop. And you get the flexibility of choosing which one, which is neat.

The question I have is, is there a way to get a warning or something if didn’t override enough of the defaults and the defaults form a loop? It seems strange to me that the compiler that is so crazy smart is fine with this example.

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1 Answer

  1. Editorial Team

    I think it’s perfectly fine for GHC to issue a warning in case of an “unbroken” cyclic dependency. There’s even a ticket along those lines: http://hackage.haskell.org/trac/ghc/ticket/6028

    Just because something is “undecidable” doesn’t mean no instance of the problem can be solved effectively. GHC (or any other Haskell compiler) already has quite a bit of the information it needs, and it’d be perfectly possible for it to issue a warning if the user is instantiating a class without “breaking” the cyclic dependency. And if the compiler gets it wrong in the rare cases as exemplified in previous posts, then the user can have a -nowarnundefinedcyclicmethods or a similar mechanism to tell GHC to be quiet. In nearly every other case, the warning will be most welcome and would add to programmer productivity; avoiding what’s almost always a silly bug.

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