Implementing a Vala Language Server

There is a Language Server Specification (LSP) published by Microsoft. Is a set of interfaces to manage information about a set of source files in an specific programming language, like symbols, classes, properties, variables, enumerations, and so on.

LSP uses JSON to communicate between a Server and a Client, so all interfaces must be translated to its counterpart in JSON before to transmit over. The communication is bidirectional, so when a request is sent from one side to another, a response is sent back with the result.

An LSP Client can used by Source Code Editors, like GNOME Builder or elementary’s code, or Anjuta, in order to provide specific services to the coder, like completion, goto symbol definitions, diagnostics, code formatting and others. A Client request all above services from a Server. This way, a source code editor can support multiple programming languages, just implementing an LSP Client and connect to an LSP Server.

Client and Server should cooperate in order to provide power full tools to the user. Some times one or other, provides limitations or the cooperation requires to changes in the source code editor design.

Anjuta for example, has implemented a database to track symbols definitions, this is no necessary as an LSP Client, because the Server should track them for you. Client just need to request if a symbol is found and get back its type and children (a very common concept on Object Oriented Programming Languages like Vala).

A Server requires lot of information from the Client, like the root directory and compilation flags used, so it can provide reliable diagnostics and, like in Vala, the --pkg switches in use so it can find the namespaces in use for both, completion and diagnostics.

Vala has its own Language Compiler and as many others, creates a tree of structured information about the code in order to translate it to C and then use another compiler to transform the resulted code to machine code. The Vala’s tree lot of the information we need to implement an LSP.

Recently GVls was accepted by GNOME Builder, as The Vala Language Server for source editing. It provides completion, goto definition and diagnostics, with more services coming.

There are improvements and new implementations in GVls to work on and hope a Client for Microsoft Visual Studio Code will be implemented soon.

Vala will be impacted by GVls for sure, in order to improve diagnostics messages, symbol resolution, completion and so on, just need to push it to its limits and find new opportunities for improvements.

GNOME Builder, has gained some new LSP implementations while integrating GVls and hope both projects found a way to provide more powerful features for Object Oriented Programmers. One area to work on, is on Vala Source Code Highlighting, in order to help programmers to know if an used symbol is correctly spelled by seeking for in the Server’s symbols database. Diagnostics is other one, where we need a way to introspect the Build System, in order to find Vala’s compiler’s switches in use, so we can improve error or warning messages.

JSON to/from GObject

In order to implement LSP in GVls, a new GVls.VariantObject interface to be implemented by any GObject class. It provides any GObject the capability to be converted to/from its GLib.Variant representation. In GVls we use JSON-GLIB, so a GLib.Variant is converted to JSON and back, so now you can use any JSON based service to serialize and parse information using, for example, JSON-RPC which in turn is the one used to implement a LSP Client and Server communication.

Author: despinosa

Linux and GNOME user, full time, since 2001. Actual maintainer of GXml and contributor to other projects mainly on GObject Introspection support.

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