CS 550 Project 1 Part 3: Expanding Mini Languagesknowak/cs550_spring_2016_o/project_pa… · Justin Buckley: Group 2 CS 550 Project 1 Part 3: Expanding Mini Languages. Introduction

IntroductionExisting Interpreters

Additional Enhancements

CS 550 Project 1 Part 3: Expanding MiniLanguages

Justin Buckley: Group 2

May 31, 2016

Justin Buckley: Group 2 CS 550 Project 1 Part 3: Expanding Mini Languages



Layout

1 Introduction

2 Existing Interpreters

3 Additional Enhancements




The Mini Language

A simple language which allowing:

Arithmetic on integersAssignment of values to variablesSimple conditional statementsSimple looping (while loop)

Some extensions and modifications exist as described inassignments from previous courses

The addition of definable functional proceduresBoolean operatorsThe addition of list structuresA Scheme based interpreter using S-expressions and prefixoperations




EBNF-ish Grammar

Mini Language as defined in lecture on Mini Language fromCS 550 Spring 2013:

program → stmt-liststmt-list → stmt ; stmt-list | stmtstmt → assign-stmt | if -stmt | while-stmtassign-stmt → identifier := exprif -stmt → if expr then stmt-list else stmt-list fiwhile-stmt → while expr do stmt-list odexpr → expr + term | expr − term | termterm → term + factor | factorfactor → (expr) | number | identifiernumber → number digit | digitdigit → 0|1|2|3|4|5|6|7|8|9identifier → identifier letter | letterletter → a|b|c| . . . |z




Syntax and Semantic notes

In the if statement, the expression is considered:True if eval(expr) > 0False otherwise

This is an explicit deviation from the behavior of manylanguages

Some have a distinct boolean type and require that ifstatement expressions evaluate to a boolean valueC and C++ assign the meaning of false to a specific value ofvarious types, such as 0 for integers while other values areconsidered to be trueMini’s unique mapping of integers to boolean values may causeissues when translating Mini programs to other languagesMini’s unique mapping of integers may cause issues whentrying to define functions or operators intended to work onboolean values

Conditional and looping structures have endcap tokens toprevent ambiguities




Sources of Mini Language Information and Exercises

Code for parsers and interpreters, as well as languagedefinition available on previous course websites

CS 550 Spring 2013 has C++ bison and flex sources forstandard Mini and Mini with proceduresCS 360 Spring 2015 has ply code for a Mini interpreter withProcedures

Exercises without provided solutions:

Adding boolean operators (Python)Adding lists to the mini language (Python)Implementing Scheme based Mini language interpreterImplementing Mini language interpreter with procedures withalternative scoping rules




Layout

1 Introduction






C++ Interpreter for Standard Mini

Yacc and lex grammar provided in notes for CS 550 Spring2013

Uses C++ Yacc options

Major syntactic components represented by classesEach of these classes contains an eval methodClasses allow for encapsulation of language semantics.

Identifiers referenced before assignment evaluate to 0

Bison grammar allows for 5 types:

int (used for numbers)char* (used for identifiers)Expr* (expressions)Stmt* (statements)StmtList* (statement lists)




C++ Interpreter for Standard Mini (Cont.)

Expr* type is used for:

exprfactorterm

Stmt* type is used for:

assignment statementsif statementswhile statements

StmtList* is used for stmt list




C++ Interpreter for Standard Mini (Cont.)

Each production in the bison grammar helps build a treestructure using C++ classes

Some of the classes assigned to tokens/symbols in the bisongrammar have subclasses which actually generated by the rules

The semantic definition is probably closer to action routinesthan to a formal attribute grammar

The routine for the program production constructs a programobject and evaluates the program




C++ Interpreter for Mini with Proc Extension

Yacc and lex grammar provided in notes for CS 550 Spring2013

Adds types to those associated bison symbols:

list<string>*list<Expr*>*

Creates a new name table when the proc is run and uses it forevaluation (Enclosing environment is ignored)

Adds symbols to the grammar:

func calldefine stmtparam listexpr list




C++ Interpreter for Mini with Proc Extension

Additional productions:stmt → define stmtdefine stmt → define identifier proc ( param list ) stmt list endparam list → identifier , param list | identifierfactor → funcallfuncall → identifier ( expr list )expr list → expr , expr list | expr




Python interpreter for Mini

Covers the Proc extension to the Mini language

Uses ply for both scanning and parsing

Ply defines tokens with semantic value outside their functionin the syntax with functions defined with a prefix ‘t ’ whichtake an argument ‘t’ to which attributes can be added, noneed to define types

Productions in the yacc grammar are defined as functionswhose name starts with the prefix ‘p ’ which takes a parameterwhich allows us to add semantic meaning to the grammarClasses have been defined to wrap certain semantic ideas:

ExprNumberIdentTimesetc.




Python interpreter for Mini (Cont.)

Syntax is identical to C++ version

Throws an error when an undefined identifier is referenced

This includes when an identifier defined outside a procedure isreferenced inside it




Layout

1 Introduction






Scheme flavored Mini Language

Mini language adjusted for Scheme

Would utilize the skeleton for the Metacircular evaluator.

as previously mentioned, would use prefix notation.

Not achieved as part of this project




Mini language with Proc Scoping rules

Proc extended Mini language interpreter with alternativescoping

Assuming the extended mini language interpreters provideduse static scoping, this would have dynamic scoping.

Would have the semantic meaning of every procedure call alsotaking an environment which would evolve as executionprogresses

This would allow for access to global variables in the procedure

Could also explore other ways to ‘import’ enclosingenvironment

Embed into procedure a copy of the enclosing environment atthe time of definition.Implement a ‘capture’ syntax as in C++11




Mini language with Proc Scoping rules (Cont.)

Accessing variables in enclosing scope in C++ can be done byaltering the apply method of the Proc class.

This allows for global variables, but is less true to the‘functional’ nature of the procedures in this language

Another alternative scoping rule would be if a procedure coulduse the values of identifiers at the time the procedure isdefined

This would require some local state stored for proceduresThe name table can be copied when the define statement isevaluated




Boolean operators for Mini language

Boolean operators for Mini language.

Adding an additional operator precidence level would requireadditions to the yacc grammar operator precedence

Important to note how truth and falsehood are represented inMini (different from most modern programming languages)

Resultant grammar probably too large for a single slide.

Issues

Boolean encoding problem (eg. what := (3 < 4)− 2 == −1)




Boolean operators for Mini language (Implementation)

Boolean operators will result in the following values

1 if True-1 if False

Boolean operator precedence is identical to addition andsubtraction

Using the python version of the Mini interpreter as a base.

A new class for each operator was needed for evaluation




Boolean operators for Mini language (Results)

The Python interpreter’s test cases include a file which uses aless than operator within a procedure as the test for a whileloop

An additional test was run using a program which assignsvalues to variables based on the results of several booleantests, then uses one in an if statement, just for good measure.




List Structures and Operations for Mini

List operations will be the ones described in the assignment

Starting point will be the python implementation of the Minilanguage

Arithmetic operators will not be defined on lists




List Structures and Operations for Mini (Implementation)

Starting point was the unaltered python implementation ofthe Mini language

This caused problems, as will be explained shortly

Various list operations (cons, car, cdr, null) basically performevaluate their arguments then return the operation applied tothe results

This would have unexpected effects if expressions could haveside effects




List Structures and Operations for Mini (Results)

Tested primarily using the file ‘simple list.p’

Utilzed boolean operators in a while loop

Due to choice of starting points, boolean operators did notexistUsing the boolean operators described previously would nothave worked, since it assumed that false statement wouldresult in 0, not −1


Documents

CS 550 Project 1 Part 3: Expanding Mini Languagesknowak/cs550_spring_2016_o/project_pa… · Justin Buckley: Group 2 CS 550 Project 1 Part 3: Expanding Mini Languages. Introduction