Re: Solution for including a file only once

[Hans Aberg]

On 7 Feb 2010, at 04:51, Michael J. O'Donnell wrote:

> I *think* that I have the essential solution to include a file exactly
> once, no matter how many times it is mentioned. That is, with the
> following definition,
>
>    \includeIfAbsent "MyFile.ly"
>
> should copy in MyFile.ly, just as though you had written
>
>    \include "MyFile.ly"
>
> unless an inclusion (either through \include or through
> \includeIfAbsent) has already started (and usually finished). If there
> is a circular path of \includes, LilyPond appears to enter an infinite
> loop, but \includeAbsent will break a cycle. That's why it's important
> that I said "unless an inclusion ... has already *started*," rather  
> than
> "already happened."
>
> The sweat here involved avoiding any preparatory code to enable the
> mechanism. You need nothing more than the definition of  
> includeIfAbsent.
>
> I'm not sure how either \include or \includeIfAbsent behave inside a
> context. It's probably not a good idea to use either one except at the
> outermost level---inner stuff should usually be done with variables.
> But, it would be cool if someone tested the possibility.

The Haskell functional language <http://haskell.org> has a module  
system. The command that ensures that a module is loaded only once is  
called "import".

One can only use "import" at the beginning of a file. So it is not  
possible to appear inside code.

There are some other things attached to the module system:

It defines a name-space, like namespace in C++. So
  module Foo

  ...
  f = ...
causes that actual name of f to be Foo.f. If one imports a name f form  
a module Bar, then one has both Foo.f and Bar.f present. If one wants  
to use the name for one of them, one needs to have system for  
disambiguation.

In addition, a module can decide which names to export: all or just  
some of them, creating a user interface. Though the syntax in Haskell  
is just (...) after the module name, here write
  export f
Then f can be use, not not other names not in the list.

  Hans