3. Core concepts

Regardless of which language you use in order to write your programs, the Libadalang API comes with several core concepts. Understanding these upfront will make your programming experience easier, so the goal of this section is to make you familiar with them.

3.1. Analysis contexts and units

In order to use Libadalang, the first thing you need to do is to create an analysis context. These are holders that embed all data and computations done during source code analysis. The second step is to create what we call an analysis unit: a holder for computations with respect to a single source file.

Analysis units really map to source files: Libadalang will process one source file that contains several compilation units as one analysis unit; similarly a subunit (“separate”) gets a dedicated unit.

In order to create an analysis unit, you just need to have a context at hand and provide either a source file or a buffer that contains the source text to be analyzed. This will decode the source text (from UTF-8, ISO-8859-1, etc.), run the lexer on it to get a stream of tokens, and then run the parser to get the parsing tree. Note that creating an analysis unit always succeeds: if there are parsing errors or if there is an error opening the file, a analysis unit is returned nevertheless, with error messages stored in a diagnostics vector.

While you can create analysis units explicitly using the analysis context API, semantic analysis also create them on demand. For instance, trying to resolve Ada.Containers.Hash_Type will automatically load an analysis unit for the Ada.Containers runtime unit in order to fetch and return the node corresponding to the Hash_Type type declaration.

3.2. Tokens and trivias

While traditional lexers only give you access to regular tokens and discard information such as white spaces and comments, Libadalang can preserve all formatting information. This option, enabled by default when creating analysis contexts, makes Libadalang suitable for a lot of usages beyond regular compiler-style analysis.

In order to achieve this, Libadalang keeps track of two distinct sequences of “lexemes”: regular tokens and trivias. The sequence of regular tokens (identifiers, keywords, etc.) is the one that the parser will use to create its parsing tree, while the sequence of trivias is just ignored by parsers, but still preserved for later analysis.

Although Libadalang exposes both sequences as a single one (ranges of tokens for the analysis units or for single nodes), getting the index of a regular token will return the position of this token in the list of regular tokens, and conversely for trivias. You can see this in the following example:

procedure Foo;  --  Comment

This source will yield 3 tokens and 3 trivias:

  1. the procedure keyword (token index 1),
  2. the first white space trivia (trivia index 1),
  3. the Foo identifier (token index 2),
  4. the ; token (token index 3),
  5. the second white space trivia (trivia index 2),
  6. the comment trivia (trivia index 3).

Depending on the context, “token” designates either only regular tokens or both regular tokens and trivias.

3.3. Syntax and semantic tree

After a successful parsing step, an analysis unit holds the resulting parsing tree. Each node in this tree has several characteristics:

  • a specific kind (identifier, object declaration, binary expression, ...),
  • a first token and a last one (we derive source location and source text excerpts from these).

Nodes can be grouped by kind in three categories:

Regular nodes
These have a definite number of children. For instance, binary operations always have three children: the left operand, the operator and the right operand. Some children can be null, for example the default expression in a parameter specification node.
List nodes
These can have any number of children (including zero).
Token nodes
They encompass exactly one token and have no children. For instance: string literals.

Sometimes, a node is created to mean the absence of some keywords. For example the Mode_Default node is created to describe the absence of explicit mode for parameter specification. We call these “ghost” nodes.

Children for all nodes can be accessed by index, but only the children of regular nodes get named accessors. For instance, parameter specification nodes have a mode field as their third children; this field is exposed as the F_Mode function in the Ada API and the .f_mode attribute in the Python API.

The syntax tree also serves for semantic analysis. Look for functions whose names match P_* in the Ada API or .p_* node members in the Python API: these are what we call node properties (hence the P prefix). They compute semantic information on top of the syntax tree. The most useful example is the P_Xref property, which computes cross-reference information. On legal Ada code bases, properties are always supposed to return a correct result. However, on incorrect code, they can either return approximate results, or raise Property_Error exceptions (PropertyError in Python).

In interactive tools such as IDEs, the source code can evolve so that it is necessary to recompute information. In Libadalang, the analysis unit creation primitives can reload an analysis unit if the corresponding source file was already analyzed before. As a result, the previous syntax tree is released and replaced with the new one: any attempt to use references to released nodes will raise a Stale_Reference_Error exception (StaleReferenceError in Python).

3.4. Unit providers

Some Ada constructs such as with clauses require name resolution in Libadalang to “jump” from one source file to another. As an example, processing the with Ada.Text_IO; statement requires to load the source files that contain the Ada package specification and the Ada.Text_IO one.

Depending on projects, knowing which source file to use in order to find a compilation unit can be arbitrarily complex. This is why Libadalang comes with the concept of unit provider: given a unit name (Ada.Strings.Unbounded) and a unit kind (specification/body), a unit provider must either return a source file to read, or raise an error if the unit name is not valid.

The analysis context constructor takes an unit provider: name resolution will use this provider whenever it needs to resolve a unit reference to a source file. In the absence of an explicit unit provider, the default one will look for source files in the current directory following the GNAT naming convention: for instance parent-child.ads for the specification of the Parent.Child unit.

Libadalang allows you to create your own unit provider, should the naming convention of your project be totally custom, but it also comes with two very useful providers:

  • one that takes a GPR project file and gives access to all Ada sources referenced by the corresponding project tree (the “project unit provider”),
  • one that looks for all source files that match a given file name pattern in a given list of directory (the “auto unit provider”).

3.4.1. Aggregate projects

Due to the very nature of Libadalang’s operations regarding name resolution, it expects exactly one source file for each unit name/kind. This assumption allows the “get to the definition” operation to return exactly one result. The aggregate project formalism is lax and allows concurrent declarations and implementations to coexist in a single project tree, which violates the assumption described in the previous paragraph.

For instance, consider the following aggregate project:

--  "agg.gpr"

aggregate project Agg is
   for Project_Files use ("arch32/arch.gpr", "arch64/arch.gpr");
end Agg;

The two following projects (two alternative implementations of the same package):

--  "arch32/arch.gpr"
project Arch is
end Arch;

--  "arch32/arch.ads"
package Arch is
   type Target_Address is mod 2 ** 32;
end Arch;

--  "arch64/arch.gpr"
project Arch is
end Arch;

--  "arch64/arch.ads"
package Arch is
   type Target_Address is mod 2 ** 64;
end Arch;

And finally the following project:

--  "main.gpr"
with "arch";
project Main is
   for Main use ("main.adb");
end Main;

--  "main.adb"
with Ada.Text_IO, Arch;
procedure Main is
begin
   Ada.Text_IO.Put_Line
     ("Arch.Target_Address'Size =" & Arch.Target_Address'Size'Image);
end;

Just like the output of the “main” program depends on which Arch package is used, the result of the “get to the definition” query on Arch.Target_Address above depends on it.

Because of this, Libadalang’s project provider has restrictions on the aggregate projects passed to it:

  1. Either the given aggregate project must contain at most one source file per unit name/kind. It is not the case in the example above, because there are two files (arch32/arch.ads and arch64/arch.ads) associated to the spec of the Arch unit.
  2. Either the aggregate project must come with a reference to a specific project file in the whole project tree. The unit provider will consider only the source files that this project file and all its dependencies contain. In the example above, this means that one could give the agg.gpr project file plus a reference to the arch32/arch.gpr project: Libadalang would then only analyze the arch32/arch.ads source file.
  3. Libadalang can also create several project providers, each one having a restricted view on the sub-project it has access to, so that only one source file is available for each unit name/kind.

While the two first options are available in all Libadalang APIs, option 3. is currently available only in its Ada API.

See the Dealing with aggregate projects section for related code examples.