It is possible to have marshalling functions (and tracing functions) generated for types defined outside an orpc interface file. This is useful if you want to use the same types in more than one RPC protocol, if you have some existing types which you would like to transmit over RPC without having to move them into the interface file, and if you would like to keep types abstract but still be able to marshal them (however orpc makes no attempt to enforce abstraction across the RPC interface).
To use an external type in an interface, just refer to it by its module-qualified name. To generate the marshalling functions, build with Camlp4 and the orpc-syntax package, and define your types like
type foo = Bar | Baz of int with orpc(aux, trace)
Generators are given as a comma-separated list after with orpc; the available generators are as follows:
| tag | generator |
|---|---|
aux | marshalling for ONC RPC |
js_aux | marshalling for RPC over HTTP |
trace | tracing functions |
See the external example for more details.
You could also use external types to override the marshalling that orpc uses for a particular type. If you want to do this try dumping out the generated code for a type to see how it works; you can just replace the functions instead of calling the generator.
If you want to use the same external type (defined in a single module) over both ONC RPC and RPC over HTTP, it is slightly tricky. The code for ONC RPC can’t be generated when compiling with ocamljs, and the code for RPC over HTTP can’t be generated when compiling with ocamlc/ocamlopt, because the libraries each depends on are not available. But there is only one .cmi file which is shared between the two compilations, so you can’t just omit the unneeded functions. To get around this there is a special argument to Camlp4, --ocamljs, which you add when ccompiling with ocamljs, so that the unneeded functions are stubbed out.