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§23.12. Declaring files
Like figures and sounds, files must be declared before they can be used. For instance:
The File of Glaciers is called "ice".
This creates a new named constant "File of Glaciers" to refer to the file, throughout the source text. We use this name for it whether or not the actual disc file exists yet: it might be one that will only be created if something unusual happens in play, for instance.
Quoted filenames should contain only letters and digits, should be 23 characters or fewer, and should begin with a letter. (In particular they can contain no slashes or dots - no subfolders or extensions can be indicated.) The actual filename this translates to will vary from platform to platform, but "ice.glkdata" is typical, stored in some sensible folder.
Every file has an owner - not a person, but the project which normally writes to it. Inform assumes that the current project will be owning any file which it declares - the File of Glaciers, for instance. But we can optionally specify that it is owned by somebody else:
The file of Boundaries (owned by another project) is called "milnor".
The file of Spectral Sequences (owned by project "4122DDA8-A153-46BC-8F57-42220F9D8795") is called "adams".
Inform uses ownership to make sure that we do not accidentally read in a file which has nothing to do with us, but merely happens to use the same name. Thus it is an error to read a file whose ownership does not agree with our declaration. Saying that a file is owned by "another project" allows us to read it whatever the owner is (so this can be used for files shared between multiple projects); specifying exactly where it needs to come from allows us to pass information from one project to another. Note that we identify projects using the IFID number - this can be found in the Contents index for a project, or by typing VERSION during play; see the chapter on Publishing for more about IFIDs.
Files are indexed in the Contents index, alongside figures and sound effects.
Two technicalities. First, constants such as "File of Glaciers" are of a kind of value called "external file" (compare "figure name" and "sound name"). Second, Inform's file-handling is provided for the Glulx machine, which in turn uses the Glk interface. This allows for either text or binary files. Inform's higher-level phrases to do with files, described in this chapter, all use text files, and all declared files are text files by default. But we can optionally add the keyword "binary" to declare a binary file, if needed:
The binary File of Glaciation Data is called "icedata".