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6.3. Contextual Analysis
6.3.1
Aalborg University
Choice of visitor pattern for the compiler
A way of traversing an AST must be chosen in order to decide if a given AST, which
represents a program, conforms to the contextual rules of PIMPL. There are different
ways to traverse an AST and some of them are described in this section. Lastly one of
these are chosen to be the method used for the contextual analysis part of the compiler
and later for the code generation part of the compiler.
The object oriented approach dictates that a new method must be added to each node
class for every type of traversal the AST must support. This approach is very intuitive
if the developer has experience with object-oriented programming languages. Each new
traversal will be placed in many different class files. This results in scattered code.
The procedural method uses switch statements for traversing the AST. The code for each
traversal in this approach is, unlike the object oriented approach, kept in one place.
Depending on the AST this might cause the programmer to write more code than with
other approaches.
A Visitor approach makes it easy to evoke different methods on the nodes in the AST.
A visitor pattern allows methods to be encapsulated in a visitor class, instead of having
each node class implement its own methods.
The project group decided that a visitor approach would be the best solution. Many
different types of visitors exists, but the project group decided to use a variation of the
Visitor pattern which exploits static overloading, because the chosen parser generator tool,
JavaCC, provides a Java interface that follows this static overloading visitor approach.
The PIMPL compiler uses three different visitors, one for AST transformation, one for
contextual analysis, and one for code generation.
The AST transformation visitor is a dynamic dispatch visitor variation which uses a single
method to the apply its transformation, to the AST, to a Select group of Nodes.
6.3.2
Type checking
Type checking is done in order to enforce the rules of the type system of the programming
language. The type rules are in turn designed to prevent runtime errors. Type rules check
the expected types of arguments, for both subprograms and operators, and specify what
the resulting type should be. This check is done by utilizing a visitor, that traverses the
trees nodes. An example of a visit method in the contextual analysis visitor of the PIMPL
compiler is shown in listing 6.9.
When the visitor reaches an IntConst node during the traversal of the AST, it will evoke a
method that tries to parse an Int from the string in value in the node, and report an error
if parseInt throws an exception.
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