Language Implementation by Source Transformation
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Compilation involves transforming a high level language source program into an equivalent assembly or machine language program. Programming language implementation can therefore be viewed as a source to source transformation from the original high level source code to the corresponding low level assembly language source code. This thesis presents an experiment in implementing an entire programming language system using declarative source transformation. To this end a complete compiler/interpreter is implemented using TXL, a source transformation system. The TXL-based PT Pascal compiler/interpreter is implemented in phases similar to those in a traditional compiler. In the lexical and syntactic analysis phase any lexical and syntactic errors present are detected when the source program is parsed according to the TXL grammar specified. The semantic analysis phase is then run in which semantic checks are performed on the source program and error messages are generated when semantic errors are detected. The source program is also annotated with type information. The typed intermediate code produced by the semantic analysis phase can be directly executed in the execution phase. Alternatively, the intermediate typed source can be transformed into a bytecode instruction sequence by running the code generation phase. This bytecode instruction sequence is then executed by a TXL implementation of an abstract stack machine in the code simulation phase. The TXL-based PT Pascal compiler/interpreter is compared against the traditional S/SL implementation of the PT Pascal compiler. The declarative style of TXL makes the rules and functions in the TXL-based PT Pascal compiler/interpreter easier to understand and the number of lines of code in the TXL implementation is less than in the S/SL implementation. The TXL implementation is however slower and less scalable. The implementation of the TXL-based PT Pascal compiler/interpreter and the advantages and disadvantages of this approach are discussed in greater detail in this thesis.