CSE 428: Lecture notes

CSE 428: Lecture notes 4.1

The role of parse trees in the implementation of programming languages

Parse-trees are are used as an internal representation of the source program by the interpreter or the compiler. In very schematic terms, we can represent the various phases of the implementation as follows

Interpreter case

           _________     ________            __________     _____________  
          |         |   |        |          |          |   |             |  
 source ->| scanner |-->| parser |- parse ->| static   |-->| interpreter |-> results  
          |_________|   |________|  tree    | analyzer |   |_____________|  
                                            |__________|          ^  
                                                                  |  
                                                                 data

Compiler case

           _________     ________            __________     __________                _________  
          |         |   |        |          |          |   |          |              |         |  
 source ->| scanner |-->| parser |- parse ->| static   |-->| compiler |-> compiled ->| machine |-> results  
          |_________|   |________|  tree    | analyzer |   |__________|     code     |_________|  
                                            |__________|                                  ^  
                                                                                          |  
                                                                                         data

Scanner: takes the source file (sequence of keyboard characters) and generates the sequence of tokens
Parser: checks the syntactical correctness and generates the parse-tree
Static analyzer: check the static correctness and possibly makes some optimizations. The optimization can also be a separate phase. The static analyzer might enrich the parse tree with some information, typically informations about the type of the identifiers.

Actually, real implementations are often a combination of compilation and interpretation: the code gets compiled into an intermediate language (something in between the source langauge and the machine language) and then interpreted.