In this assignment you will extend the Fun compiler, using the compiler generation tool ANTLR. The assignment itself consists of three stages (to be submitted altogether as one): syntactic analysis, contextual analysis, and code generation.

The assignment contributes 20% of the assessment for the PL(H) course.

Assignment: extending Fun with a repeat-until loop and a switch statement

The assignment is to design and implement two extensions to Fun.

(A) The first is a repeat-until command, which is a new form of loop.

(B) The second extension is a switch command, analogous to that of Java and C.

Each extension consists of three stages.

1.   Syntactic analysis: the definition of the grammar to specify the syntax of the new language feature.

2.   Contextual analysis: the definition of scope and type checking rules for the new feature, and the extension of the visitor to implement them.

3.   Code generation: the definition of code templates for the new feature, and the extension of the visitor to implement code generation.

Setup a new project, called FunExtended, and start by importing or copying the files from your version of Fun, which was extended with multiple parameters from the warm-up exercise.

Maintain the same structure of directory as it was given to you.

Add your own name and the date to the header comment in any file you modify.

Additionally, clearly highlight all your modifications in the code, using comments such as

“ // EXTENSION … //END OF EXTENSION”.

Extension A: the repeat-until command

Note the following points about the syntax, typing rules and semantics of the switch command.

•    The repeat-until command should contain any sequence of commands in its body.

•    The guard of until should be any general expression.

Extension B: the switch command

Note the following points about the syntax, typing rules and semantics of the switch command.

•    The expression being tested (i.e. the expression that appears after the switch keyword) can be any integer or boolean expression.

•    Each guard (i.e. the value or values that appear after a case keyword) has one of three possible forms. (1) An integer literal. (2) A boolean literal. (3) An integer range, such as 2..4 as in the first example; this case is selected when the value of n is either 2, 3 or 4. Note that only literal values, not arbitrary expressions, are allowed in guards.

•    All of the guards must have the same type as the expression being tested.

•    All of the guards must be non-overlapping. This means that guards of forms (1) and (2) must all have different values, and the ranges in guards of form (3) must not overlap with other ranges or with guards of form (1).

•    There can be any number of cases.

•    There must be exactly one default case.

•    The code for each case can be any sequence of commands.

•    There is no fall-through, therefore no need for a break keyword. At the end of the sequence of commands associated with a particular case, execution jumps to the end of the switch-command. This is true even if the sequence of commands is empty.

Assignment stage 1: syntactic analysis

Add a suitable grammar definition for the repeat-until and switch commands to Fun.g4. Remember to extend the lexicon as necessary. Make sure your grammar is general enough.

Write at least two test Fun programs containing repeat-until and at least two tests containing switch. Test your extended syntactic analyser by running the simplified driver program FunParse(not FunRun) with each of these test programs. You can also use the ANTLR visualisation tool to check the syntax trees.

Assignment stage 2: contextual analysis

Add implementations of the missing methods in the  FunCheckerVisitor class, so that all the necessary type checking is done.

For the switch command, you need to implement a check that there are no repeated or overlapping guards.

Test your extended contextual analyser by running FunCheck with each of your test programs, and see whether it correctly performs type checks and other analysis (for switch, including no repeated or overlapping guards). Your test programs should include one that violates the commands’ rules.

Assignment stage 3: code generation

Extend the Fun code generator.

Start by designing a code template for a repeat-until and a switch command. Note: Include your code template as a comment in FunEncoderVisitor.java. This should be in a separate part of the file (e.g., top of the corresponding method), not interspersed between other code. You will receive marks for a reasonable code template even if your code generator does not work as intended. Note that the code template does not mean the Java code of a visit method. It means a schematic outline of the code that will be generated, showing the structure of tests and jumps. The lecture slides show code templates for the if statement and the while statement, for reference.

After the code template, define the missing methods in the FunEncoderVisitor class.

Test your extended contextual analyser and code generator by running FunRun with each of your test programs, and see whether it performs proper scope/type checks and generates correct object code.

There are two ways to verify whether the compiler generates correct object code – use both!

1.  Visually inspect the object code.

2.  See what happens when the object code is interpreted. If the object code’s behaviour is unexpected, your compiler must be generating incorrect object code.