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Lazy modules
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Date: 2010-03-22 (18:11)
From: Yoann Padioleau <padator@w...>
Subject: Re: [Caml-list] Lazy modules

On Mar 17, 2010, at 11:23 AM, 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.
> At runtime, a structure is represented simply by a block with GC tag 0, exactly as a record or a tuple.

Which makes me wonder why people not use simply record or tuples to solve such problem ...
Many times I've found that passing a record of functions was as good as functors and simply
more flexible because they are in the same "value" world. 

> 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
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