Add section on interface identification to IDL design doc

Signed-off-by: Jacob Perron <[email protected]>

Author: jacobperron

articles/142_idl.md

@@ -40,7 +40,7 @@ Both line comments (`//`) as well as block comments (`/* ... */`) are being supp
 
 #### 7.2.3 Identifiers
 
-An identifier must start with an is an ASCII alphabetic characteran followed by any number of ASCII alphabetic, digit and underscore (`_`) characters.
+An identifier must start with an ASCII alphabetic character followed by any number of ASCII alphabetic, digit and underscore (`_`) characters.
 
 #### 7.2.6 Literals
 
@@ -129,7 +129,7 @@ the DDS-XTypes specification 1.2 defines it with 16-bit.
 | ---------------------- | ---------------------------------------------------------- |
 | sequence\<type\_spec>  | sequence of items of the specific type\_spec               |
 |                        | the sequence is unbounded and no maximum size is specified |
-| sequence<type_spec, N> | sequence of of up to N items of the specified type\_spec   |
+| sequence<type_spec, N> | sequence of up to N items of the specified type\_spec      |
 |                        | the sequence is bounded and contain between 0 and N items  |
 
 #### 7.4.1.4.4.3.2 Strings
@@ -159,6 +159,60 @@ An enumerated type consist of an ordered list of enumerators.
 A multidimensional, fixed-size array is defined by the type of each item and the explicit sizes for each dimension.
 For now only a one-dimensional, fixed-size array is supported though.
 
+### Interface Identification
+
+#### Uniform Resource Name (URN)
+
+Following 7.5.1 from the IDL 4.2 specification, every IDL definition can be uniquely identified with a qualified name of the form
+**<scoped\_name>::<identifier>**.
+In addition to the specification, the scope of an IDL definition is prefixed with the package name from which the defintions is derived.
+This is to guarantee uniqueness of definitions across packages (under the assumption that package names are unique).
+Therefore, each interface defintion can be resolved with a URN of the form: **<package\_name>/<scoped\_name>/<identifier>**.
+
+#### Uniform Resource Locator (URL)
+
+For source code to use an interface (e.g. include or import) there needs to be a way to reference the file containing the defintion.
+To reference the location of an IDL defintion we use a URL of the form: **rosidl:<package\_name>/<subfolder>/<file>**.
+Where *<subfolder>* is the relative path from the root of the package where generated defintions can be found.
+
+#### Example
+
+Imagine we have a package `foo_pkg` containing an IDL file `Bar.idl` with the following contents:
+
+```idl
+struct MsgA {
+  boolean bool_member;
+};
+
+module bar {
+  const int8 MY_CONSTANT = 42;
+
+  struct MsgB {
+    int8 int_member;
+  };
+
+  module baz {
+    struct MsgC {
+      string string_member;
+    };
+  };
+};
+```
+
+Also imagine the message generation pipeline places the generated implementation to a subfolder `interface`.
+
+Then, we can reference the interfaces with the following URNs:
+
+- `foo_pkg/MsgA`
+- `foo_pkg/bar/MY_CONSTANT`
+- `foo_pkg/bar/MsgB`
+- `foo_pkg/bar/baz/MsgC`
+
+And all can be located with the URL: `rosidl:foo_pkg/interface/Bar`.
+
+For example, the definitions could be included in C++ with `#include <foo_pkg/interface/Bar.hpp>`
+or imported in Python with `import foo_pkg.interface.Bar`.
+
 ### Annotations
 
 The syntax for arbitrary annotations is supported.