Vala Documentation

Appearance

3.4. Semantic Analyzer ​

Vala.SemanticAnalyzer (vala/valasemanticanalyzer.vala) is a CodeVisitor that looks up the symbols that have been, parsed and checks if the symbols are being used correctly.

This includes:

  • Verifying that for each symbol type, the syntax is being followed
  • The rules of how each symbol can be used is being followed

The key method to look at is Vala.SemanticAnalyzer.analyze ()

3.4.1 How The Built-in Symbol Types Are Looked Up ​

Initially symbol types are looked up from the Vala.CodeContext (vala/valacodecontext.vala) root (Vala.Namespace, vala/valanamespace.vala).

In Vala.Compiler (compiler/valacompiler.vala) for the run() method, for each source filename given to the compiler, the Vala.CodeContext.add_source_file_name () is called (unless the compiler is run with the Fast VAPIs feature enabled).

On each call to the add_source_file_name () method on Vala.CodeContext, a default namespace using directive is added to the Vala.SourceFile (vala/valasourcefile.vala) object representing each source file.

The default namespace that gets added depends on the profile option set when the compiler is executed:

  • gobject (default): using GLib;
  • posix: using Posix;

This provides the implementation of the built-in types of the language.

The implementation of each type are then added to the namespace scope which is then looked up when the Vala.SemanticAnalyzer.analyze() method is called.

3.4.2 How Each Symbol in the Symbol Tree Is Checked ​

After looking up the built-in symbol types, in Vala.SemanticAnalyzer.analyze (), the method then continues on by calling Vala.CodeNode.check () (vala/valacodenode.vala) on the namespaces in the code context root.

After, CodeContext.accept () is called with the semantic instance and the code visitor recursively goes through the entire symbol tree to ensure that all the symbols are being used correctly.

Also, Vala.SemanticAnalyzer contains methods and static functions that are used as shared reusable logic across the symbol tree of ensuring that symbol's are being used correctly. For example Vala.SemanticAnalyzer.check_arguments () checks the arguments of an expression, ensuring that the correct amount of arguments are being used.

3.4.3 Error Handling ​

If while checking the symbols, any errors have been reported, after the semantic analyzer has finishing checking through the entire symbol tree, the compiler program will exit early instead of proceeding with the Flow Analyzer.

3.4.4. Attributes ​

Semantic analysis is where most attribute-driven validation happens, since this is the stage that walks every symbol and calls Vala.CodeNode.check () on it. There's no separate step for this - the same attribute accessors described in 3.2.5. Attributes are simply called from inside check () and from computed properties that check () relies on.

A few examples:

  • Vala.Class.check () (vala/valaclass.vala) reads has_attribute ("Compact") directly to reject a [Compact] class inheriting from a non-compact base class, reporting Compact class `%s' cannot inherit from non-compact class `%s'.
  • Vala.Class.is_compact and the equivalent properties on Vala.Struct (vala/valastruct.vala) for SimpleType, BooleanType, IntegerType and FloatingType are backed by has_attribute (), computed once and cached on first access.
  • Vala.SemanticAnalyzer (vala/valasemanticanalyzer.vala) reaches into individual CCode arguments in its own helper methods while deciding how a symbol must be treated - for example is_gobject_property () reads array_length, array_null_terminated and delegate_target to work out whether a property can be exposed as a GObject property, and is_gobject_property_type () reads has_type_id to work out whether a struct type qualifies.

3.4.4.1. Version and Deprecation Checks ​

[Version] (and its deprecated predecessors [Deprecated]/[Experimental]) are checked by a dedicated helper class, Vala.VersionAttribute (vala/valaversionattribute.vala), rather than being read directly. Every Vala.Symbol lazily creates and caches one through its version property (vala/valasymbol.vala), and that object is what reads the since, replacement, deprecated, deprecated_since, experimental and experimental_until arguments.

Its check () method is called wherever a symbol is used, rather than once for the whole file - from Vala.MemberAccess (vala/valamemberaccess.vala), Vala.ObjectCreationExpression, Vala.Field, Vala.Parameter, Vala.LocalVariable and Vala.Class. This is why a deprecation or experimental-API warning is reported once per call site or reference, not once globally for the declaration.