Re: data-default WAS: [containers] Proposal: Change to the Data.Map Monoid

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João Cristóvão writes:

The case is that for very long and complex structures, filled with trivial values, and with an obvious default value, writing that default value is a task best left for the compiler, with the help of generics.

Ah, I see your point. Perhaps a compromise solution is possible: A given type can have a generically defined Trivial value if all its members are either also Trivial, OR they have Monoid instances with an mempty value. This way, I know that any Trivial value tree is, at its leaves, composed of trivial or mempty values, giving a principled meaning of 'emptiness', even though the Trivial instance may not itself form a Monoid. The Trivial class would further not allow explicit instances. The core types like Int would have hard-coded trivial values in 'base'; but if you want to define triviality for an ADT in some specific way, it would require making a Monoid instance to demonstrate what emptiness means relative to an operation. John