On Sat, Jul 5, 2014 at 6:35 PM, Andre Tampubolon via Digitalmars-d-learn <digitalmars-d-learn@puremagic.com> wrote: > I've been reading the newsgroup for a while, and it seems that one of the > reason folks like D is because it supports module. > > My question is: what does module mean? > A quick google pointed my this page: http://dlang.org/module.html. > Still cannot understand it, though :) > > How does it differ from the old C's #include?
Well, module are 'more first-class' in D than in C. By this, I mean that they have an identity: a name, members, symbols. You can see them as a enormous struct/class. You can use introspection to obtain the symbols defined in a module, query their types, etc. Heck, in some cases, module names can even by used as arguments for templates. As others said, a D module is a unit of encapsulation: inside a module, functions, structs and classes can freely call one other and access their internal fields. Outside the module, only what's not private is visible. Thus, you can create complex, interdependent structures inside your module, but export only some parts for public access. This way, you control a module 'interface', its external frontier. Importing a module in D means the public symbols defined in the module become accessible in the importer. There is no need to import all visible symbols in your current scope: you can restrict your import to only one or two symbols, rename them, etc. That's much more powerful than C when it comes to name clashes. Different 'downstream' modules can import the same 'upstream' module, and choose to import different symbols. No need to do a big 'import everything in the current scope'. symbols names are also defined on a per-module basis: two different modules can have members with a similar name, without clash: just prefix the names with the module name to differentiate them (or rename one while importing). Also, `import foo;` is a statement, you can put it everywhere a statement is legit. That's a recent addition (ie, 1-2 year?), that Andrei likes to call 'Turtles all the way down' and is quickly becoming a D idiom: import a symbol near its use place: inside a function, a method, even inside an if branch: that avoid scope pollution and nicely documents where a symbol is necessary.
