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While simple programs can be written in a single component, in many cases you will want to develop multi-component projects in which your project is broken down into separate concerns. Developing more complex projects using a set of well-separated components will make your code easier to understand and maintain.

Dana uses a specific convention to organise source code for multi-component projects, which enables its compiler and runtime to locate all of your code. The way in which is organisation works is designed to strongly separate type definitions from implementations.

Programs that you build in Dana are a mixture of your own components and components from Dana's central source tree, which is our standard library. All components in the standard library are open-source so you can browse them yourself. Your own projects and the central source tree use an identical organisation convention, so we can easily migrate code into the central source tree or override components.

Developing a multi-component project

All multi-component projects will have a directory called resources. The resources directory contains all type definitions used in the project, and can have sub-directories representing packages. Components which implement interfaces defined in the resources directory must appear in a corresponding directory with the same relative path from the project root but without resources at the start; this allows the Dana runtime to locate implementations of interfaces.

Let's imagine we're creating a command-line calculator project, in a directory called projectx. We decide to use one main component, providing the App' interface, and two utility components: a parser, to parse command-line input into a set of arithmetic tokens, and an arithmetic processor, to calculate the result of a set of arithmetic tokens.

Our resulting project directory would be organised as shown below:

The lines in this diagram indicate “in-directory” relationships; the projectx directory therefore contains four items (three directories and one file) while the resources directory contains two directories.

The resources directory, and its sub-directories, is used to contain source files that describe interface and data types. In general, a source file that contains a type definition (such as an interface) should have the same file name as the type name that it defines.

The project root directory, and each of its sub-directories (except for the resources sub-directory), contains source files of components that implement interfaces. The general convention for these source files is to name them with the same name as the main interface type that the component provides.

Using this example, the file Calculator.dn might have the following code:

component provides App requires parsing.CalcParser parser,
                                arithmetic.CalcEngine engine {
    	
    int App:main(AppParam params[])
        {
        ArToken tokens[] = parser.parse(params)
		
        dec result = engine.calculate(tokens)
        
        return 0
        }
    }

When we try to compile this component, the compiler will need to search for each of the interface types used by the component. This is done by converting any dots to slashes and adding ".dn" to the end, then searching for the corresponding path within the resources directory. The required interface parsing.CalcParser therefore becomes resources/parsing/CalcParser.dn.

We can compile every component in a project (recursively) from a command prompt in its root directory, using the command:

dnc .

We then run the project with the example command:

dana Calculator 4 * 5

When we do this, the Dana runtime tries to automatically locate default implementing components of each required interface that has been declared by this component. The runtime does this in a very similar way to the approach used by the compiler; for each required interface, a search path is derived by converting any dots to slashes and then adding ".o" to the end. The required interface parsing.CalcParser therefore becomes parsing/CalcParser.o.

Search priority

When searching for a file containing an interface definition, the resources sub-directory of the local directory takes priority over the resources directory of the standard library.

When searching for a default implementing component of a required interface, the local directory is again given higher priority that the standard library (even if both locations contain a file of the same name). For component searches, the Dana runtime also uses manifest files to allow additional control over the search.

For component search manifest files take precedence over simple file name equivalence, and can be used both in a local directory and in the standard library.

Putting this all together, when searching for a default implementing component for io.File, the Dana runtime first converts dots to slashes and adds .o to the end, resulting in the path io/File.o. The runtime then does this, in order:

This search procedure therefore allows the local directory to override the standard library, and allows manifest files to override name equivalence when searching for a default implementing component of a given interface. The structure of manifest files is described next.

Using manifest files

A manifest file has the name .manifest. If present, it describes how to map an interface type onto a default implementing component in the current directory. Manifest files use a simple JSON format as follows:

{
"defaultLinks":	[
    {"interface": "FileSystem", "component": "File"},
    {"interface": "Carem", "component": "Orion"}
    ]
}

If the above file is present in a given directory, an interface type named FileSystem that has been resolved against that directory (using the search procedure described above) will by default be matched to the default implementing component File.o in that same directory.

Manifest files are often not required when simple name equivalence is good enough (i.e. name equivalence between the interface's type name and the name of the source file containing the component that implements that interface).

Manifest files are useful however when the programmer either wishes to override name equivalence, for example to test an alternative implementation, or when it is not possible for name equivalence to exist, for example when the same component is the default implementing component for more than one interface type.