Geoffrey Romer writes:

Suppose I'm tying to build a nested lambda expression, like so (typename...::type omitted for simplicity):

lambda >, _2> >

- Is this legal?

Yes, although the semantics is probably not what you are trying to achieve: for the purpose of inline metafunction composition, 'lambda' is just a metafunction without any special status, and is treated as such. The 'apply' test (http://cvs.sourceforge.net/viewcvs.py/boost/boost/libs/mpl/test/apply.cpp?rev=1.5&view=markup) actually illustrates this and similar cases well.

- Does the _1 refer to the first argument of the inner lambda, or of the outer lambda?

The outer one.

- Can the above be overridden? I.e. what if I want to refer to the arguments of both lambdas within the inner lambda?

It's not currently supported, but it's on the TODO list -- see http://thread.gmane.org/gmane.comp.lib.boost.devel/115924. -- Aleksey Gurtovoy MetaCommunications Engineering