[Haskell-cafe] Cons of -XUndecidableInstances

[oleg at okmij.org]

It seems that UndercidableInstances keep getting a bad rap. There are
legitimate and decidable applications of UndercidableInstances. These
applications have nothing to do with OOP, or HList for that matter.

This topic is discussed at length in the article
	http://okmij.org/ftp/Haskell/types.html#undecidable-inst-defense

Here are the conclusions:

    Thus there are patently decidable type programs that require
    UndecidableInstances. That extension should not be categorically
    stigmatized.

    In conclusion, UndecidableInstances is not a dangerous flag. It will never
    cause the type-checker to accept a program that `goes wrong.' The only bad
    consequence of using the flag is type checker's might be telling us that it
    cannot decide if our program is well-typed, given the context-stack--depth
    limit. We may ask the type-checker to try a bit harder (with a larger depth
    limit), or look through our program and find the problem.

    UndecidableInstances are quite like the primitive recursion criterion:
    all primitive recursion functions surely terminate; non-primitive recursion
    functions generally don't. Still there are many classes of non-primitive
    recursive functions that are total. To see their totality, one has to use
    more complex criteria.