On Tue Jun 6 19:22:45 UTC 2017, Richard Eisenberg wrote:

I still don't see why TypeError is problematic once we have instance guards.

Ah, "once we have instance guards", TypeError is never needed. With instance guards, you might sometimes want to put a TypeError instance, to give a more helpful message. (HList nowadays exploits TypeError that way.) OTOH, sometimes currently you want to say: "I don't want to provide an instance for that pattern here, but by all means somebody else can elsewhere." Example code to illustrate the above:

module A class C a b ... instance C Int b ...

module B instance C a Bool ...

Those instances overlap irresolvably. The only way you can catch (not really improve) is:

instance (TypeError <help message>) => C Int Bool ...

But a) which module do you put that in? b) what if somebody wants a `instance C Int Bool`? with guards

module A class {-# INSTANCEGUARDS #-} C a b ... instance C Int b | b /~ Bool ...

module B instance C a Bool | a /~ Int ...

module C -- if 'somebody else' wants instance C Int Bool ...

AntC