SB Syntax analysis How ScriptBasic performs Syntax analysis

SB Contents What is the role of syntax analysis What is the assumed syntax of a program What tables drive syntax analysis Creating the tables Pseudo terminals

SB Who this presentation is for Curious (why things happen?) Want to learn and understand how ScriptBasic works Want to modify ScriptBasic NOT for those, who just want to program in scriba

SB Role of syntax analysis Reader Lexer SYNTAX ANALYSIS Builder Execution

SB General purpose The syntax analyzer is a general purpose table driven syntax analyzer that could be used for other syntaxes not hard coded for ScriptBasic The first definition that fits is used. Easy to maintain but slower than special purpose analyzer

SB Syntax assumed Program is series of lines Lines are (usually) terminated by new line A line is series of terminal and pseudo- terminal symbols No block of code { } or BEGIN END –handled by go|come_forward|backward pseudo terminals Predefined pseudo terminal symbols, like ‘expression’

SB Tables defining syntax Binary operators Unary operators Built-in functions Commands –List of commands in the order syntax analyzer checks them against the lines

SB Creating the tables syntax.def is a readable format syntaxer.pl creates the syntax.c headerer.pl creates the syntax.h files.

SB Terminals and pseudo terminals ‘module’ is a terminal symbol (keyword) nl is terminal symbol (character) float is terminal symbol lval, expression are pseudo terminals –can create complex code ( one way street with dead end effect! ) –can have side effect

SB Code generated Code is array of nodes A node contains OpCode –list of command parameters (each is a node) –operands list (operators or function) –value (constant node) –Serial number (variable node) –start point and arguments (user defined function) –car and cdr values for list head nodes See builder.c struct _cNODE;

SB Example GOTO: 'goto' label nl OpCode: CMD_GOTO –Parameter.CommandArgument.Argument.pNode contains the node id of the line for label

SB Example ELSIF: 'else' 'if' * expression 'then' come_forward(IF) go_forward(IF) nl OpCode: CMD_ELSIF –Parameter.CommandArgument.Argument.pNode contains the node id of the expression –Parameter.CommandArgument.next next paremeter node: Parameter.CommandArgument.Argument.pNode where to go if expression fails

SB Pseudo terminals 1/10 nl tab These are the simplest pseudo-terminal symbols, because they are real terminals, just hard to write into a text file.

SB Pseudo terminals 2/10 expression expression_list Handles an expression or a comma separated list of expressions and creates nodes. OWSWDE

SB Pseudo terminals 3/10 string integer float These are simple terminals.

SB Pseudo terminals 4/10 symbol a symbol (in name space) absolute_symbol an absolute symbol name_space an absolute symbol that sets the name space (no code) end_name_space end a name space (no code)

SB Pseudo terminals 5/10 lval a left value lval_list a left value list separated by commas local local variable local_list list of local variables

SB Pseudo terminals 6/10 function a function name where it is defined thisfn the name of the currently defined function or procedure arg_num placeholder to store the number of arguments local_start starts a local scope local_end ends of local scope

SB Pseudo terminals 7/10 label a label used in goto label_def a label when defined

SB Pseudo terminals 8/10 cname constant name cval constant value corresponding to constant name These do not generate code, have only side effects defining a constant. ( cval defines a constant for the name last appeared for cname )

SB Pseudo terminals 9/10 go_back creates code go_forward creates code come_back has side effect come_forward has side effect These are used instead of code bracketing. There is a jump stack where node pointers are pushed and taken from.

SB Pseudo terminals 10/10 * star character OWSWDE noexec no code is generated from the line

SB Special commands All lines are analyzed by the C function ex_IsCommandThis except those defining special analysis function CALL/CALL ( ex_IsCommandCALL ) OPEN/OPEN ( ex_IsCommandOPEN )

SB Thank you for listening