Re: [boost] optional<optional<T>>

Subject: Re: [boost] optional<optional<T>>
From: Andrew Bromage (ajb_at_[hidden])
Date: 2009-09-04 02:54:57

G'day.

Andrei Alexandrescu <andrei <at> metalanguage.com> writes:

> I'm defining an "optional" type for D's standard library modeled 
> similarly to Boost.optional. An interesting question came up - should 
> optional<optional<T>> fold itself into optional<T>, or is "double 
> optional" an interesting concept of its own?

Automatically folding optional<optional<T>> doesn't make any more sense
than automatically folding vector<vector<T>>.

The reason is very simple: It's reasonable for templated code to use
optional<T> (where T is a template argument) for its own internal
purposes.  This code should not misbehave just because a client wants
that template argument to be optional<U>.

Don't break parametricity without an extremely good reason.

Andrew Bromage