RE: [Caml-list] Lazy modules

["David Allsopp" <dra-news@metastack.com>]

Alain Frisch wrote:
> 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.

I've muddled this with something else! I seem to have muddled a lot in my post... :$

> 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.

Live 'n learn - thanks for the correction!


David