Hello Holger,
Can you define an associative operation
orElse :: CMaybe r a -> CMaybe r a -> CMaybe r a
with identity `mzero` that satisfies the cancellation law?
No, because that function would need to cancel values of type a, but the arguments of type (CMaybe r a) can only compute values of type r.
I'm afraid, I don't understand.
But you can define
orElse :: CMaybe a a -> CMaybe a a -> CMaybe r a CMaybe ca `orElse` CMaybe cb = CMaybe (\k -> (ca return `mplus` cb return) >>= k)
Good point. But with this restricted type `orElse` is less useful. For example, one cannot compute fromCMaybe ((return False `orElse` return True) >>= guard) because there, the arguments of `orElse` are of type `CMaybe () Bool`. Cheers, Sebastian P.S.
fromCMaybe :: CMaybe a a -> Maybe a fromCMaybe (CMaybe ca) = ca Just
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