S B

Syntax analysis

How ScriptBasic performs Syntax analysis

S B 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

S B Who this presentation is for

• Curious (why things happen?)• Want to learn and understand how

ScriptBasic works• Want to modify ScriptBasicNOT for those, who just• want to program in scriba

S B Role of syntax analysis

• Reader• Lexer• SYNTAX ANALYSIS• Builder• Execution

S B 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

S B 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’

S B Tables defining syntax

• Binary operators• Unary operators• Built-in functions• Commands

– List of commands in the order syntax analyzer checks them against the lines

S B Creating the tables

• syntax.def is a readable format• syntaxer.pl creates the syntax.c• headerer.pl creates the syntax.h

files.

S B 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

S B 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;

S B Example

• GOTO: 'goto' label nl

• OpCode: CMD_GOTO– Parameter.CommandArgument.Argument.pNode

contains the node id of the line for label

S B 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

S B 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.

S B Pseudo terminals 2/10

• expression• expression_list

Handles an expression or a comma separated list of expressions and

creates nodes.

OWSWDE

S B Pseudo terminals 3/10

• string• integer• float

These are simple terminals.

S B 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)

S B Pseudo terminals 5/10

• lval a left value• lval_list a left value list separated

bycommas

• local local variable• local_list list of local variables

S B Pseudo terminals 6/10

• function a function name where it isdefined

• thisfn the name of the currentlydefined function or

procedure• arg_num placeholder to store the

number of arguments• local_start starts a local scope• local_end ends of local scope

S B Pseudo terminals 7/10

• label a label used in goto• label_def a label when defined

S B Pseudo terminals 8/10

• cname constant name• cvalconstant value corresponding

to constant nameThese do not generate code, have only

side effects defining a constant. (cval defines a constant for the name last

appeared for cname)

S B 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.

S B Pseudo terminals 10/10

• * star character OWSWDE• noexec no code is generated from

the line

S B Special commands

• All lines are analyzed by the C function

ex_IsCommandThisexcept those defining special analysis function

• CALL/CALL (ex_IsCommandCALL)• OPEN/OPEN (ex_IsCommandOPEN)

S B

Thank you for listening