On 3/17/2010 6:42 PM, David Allsopp wrote:
> AFAIK local modules is a syntax extension not a compiler extension - I
> expect (not looked at it) that the syntax extension simply alpha renames
> all the local module declarations to make them unique and puts them
> globally... a very useful extension but no expressive power added.

This is not true. Local modules are not lifted in any way. This is not 
simply a syntax extension. For instance, if the local module has 
toplevel side-effects (e.g. a structure item like: let () = 
print_endline "Hello"), then the side effect will occur every time the 
local module is evaluated.

At runtime, a structure is represented simply by a block with GC tag 0, 
exactly as a record or a tuple. The block contains dynamic components of 
the structure (values, sub-modules, exceptions, classes) in the order 
given by its signature. Evaluating a structure simply evaluates its 
runtime components a build the block.

A functor is represented as a function.

>The module system at present is a compile time feature (I think that's
> universally true - even with weird things like recursive modules) -
> functors are simply a way of introducing more modules so there is no
> runtime overhead in using a functor.

Modules and functors are much more dynamic than what you believe. The 
introduction of first-class module did not require any change in the way 
modules are compiled.

A local module which is a functor application really applies the functor 
at runtime and evaluates the functor body every time the local module 
expression is evaluated.


Alain