lexical analysis. the input read string input might be sequence of characters (unix) might be...
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Lexical Analysis
The Input
Read string input Might be sequence of characters (Unix) Might be sequence of lines (VMS) Character set:
ASCII ISO Latin-1 ISO 10646 (16-bit = unicode) Ada, Java Others (EBCDIC, JIS, etc)
The Output
A series of tokens: kind, location, name (if any) Punctuation ( ) ; , [ ] Operators + - ** := Keywords begin end if while try catch Identifiers Square_Root String literals “press Enter to continue” Character literals ‘x’ Numeric literals
Integer: 123 Floating_point: 4_5.23e+2 Based representation: 16#ac#
Free form vs Fixed form
Free form languages (all modern ones) White space does not matter. Ignore these:
Tabs, spaces, new lines, carriage returns Only the ordering of tokens is important
Fixed format languages (historical) Layout is critical
Fortran, label in cols 1-6 COBOL, area A B Lexical analyzer must know about layout to find
tokens
Punctuation: Separators
Typically individual special characters such as ( { } : .. (two dots) Sometimes double characters: lexical scanner
looks for longest token: (*, /* -- comment openers in various languages
Returned just as identity (kind) of token And perhaps location for error messages and
debugging purposes
Operators
Like punctuation No real difference for lexical analyzer Typically single or double special chars
Operators + - == <= Operations := =>
Returned as kind of token And perhaps location
Keywords
Reserved identifiers E.g. BEGIN END in Pascal, if in C, catch in C++ Maybe distinguished from identifiers
E.g. mode vs mode in Algol-68 Returned as kind of token
With possible location information Oddity: unreserved keywords in PL/1
IF IF THEN THEN = THEN + 1; Handled as identifiers (parser disambiguates)
Identifiers
Rules differ Length, allowed characters, separators
Need to build a names table Single entry for all occurrences of Var1
Language may be case insensitive: same entry for VAR1, vAr1, Var1
Typical structure: hash table Lexical analyzer returns token kind
And key (index) to table entry Table entry includes location information
Organization of names table
Most common structure is hash table With fixed number of headers Chain according to hash code Serial search on one chain Hash code computed from characters (e.g. sum
mod table size). No hash code is perfect! Expect collisions. Avoid any arbitrary limits on table or chain size.
String Literals Text must be stored Actual characters are important
Not like identifiers: must preserve casing Character set issues: uniform internal representation Table needed
Lexical analyzer returns key into table May or may not be worth hashing to avoid duplicates
Character Literals
Similar issues to string literals Lexical Analyzer returns
Token kind Identity of character
Cannot assume character set of host machine, may be different
Numeric Literals
need a table to store numeric value E.g. 123 = 0123 = 01_23 (Ada) But cannot use predefined type for values
Because may have different bounds
Floating point representations much more complex Denormals, correct rounding Very delicate to compute correct value. Host / target issues
Handling Comments
Comments have no effect on program Can be eliminated by scanner But may need to be retrieved by tools Error detection issues
E.g. unclosed comments Scanner skips over comments and returns
next meaningful token
Case Equivalence
Some languages are case-insensitive Pascal, Ada
Some are not C, Java
Lexical analyzer ignores case if needed This_Routine = THIS_RouTine Error analysis may need exact casing Friendly diagnostics follow user’s conventions
Performance Issues
Speed Lexical analysis can become bottleneck Minimize processing per character
Skip blanks fast I/O is also an issue (read large blocks)
We compile frequently Compilation time is important
Especially during development
Communicate with parser through global variables
General Approach
Define set of token kinds: An enumeration type (tok_int, tok_if, tok_plus,
tok_left_paren, tok_assign etc). Or a series of integer definitions in more primitive
languages… Some tokens carry associated data
E.g. key for identifier table May be useful to build tree node
For identifiers, literals etc
Interface to Lexical Analyzer
Either: Convert entire file to a file of tokens Lexical analyzer is separate phase
Or: Parser calls lexical analyzer to supply next token This approach avoids extra I/O Parser builds tree incrementally, using successive
tokens as tree nodes
Relevant Formalisms
Type 3 (Regular) Grammars Regular Expressions Finite State Machines Equivalent in expressive power Useful for program construction, even if
hand-written
Regular Grammars
Regular grammars Non-terminals (arbitrary names) Terminals (characters) Productions limited to the following:
Non-terminal ::= terminal Non-terminal ::= terminal Non-terminal Treat character class (e.g. digit) as terminal
Regular grammars cannot count: cannot express size limits on identifiers, literals
Cannot express proper nesting (parentheses)
Regular Grammars
grammar for real literals with no exponent digit :: = 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 REAL ::= digit REAL1 REAL1 ::= digit REAL1 (arbitrary size) REAL1 ::= . INTEGER INTEGER ::= digit INTEGER (arbitrary size) INTEGER ::= digit
Start symbol is REAL
Regular Expressions
Regular expressions (RE) defined by an alphabet (terminal symbols) and three operations: Alternation RE1 | RE2
Concatenation RE1 RE2
Repetition RE* (zero or more RE’s) Language of RE’s = regular grammars
Regular expressions are more convenient for some applications
Specifying RE’s in Unix Tools
Single characters a b c d \x Alternation [bcd] [b-z] ab|cd Any character . (period) Match sequence of characters x* y+ Concatenation abc[d-q] Optional RE [0-9]+(\.[0-9]*)?
Finite State Machines
A language defined by a grammar is a (possibly infinite) set of strings
An automaton is a computation that determines whether a given string belongs to a specified language
A finite state machine (FSM) is an automaton that recognize regular languages (regular expressions)
Simplest automaton: memory is single number (state)
Specifying an FSM A set of labeled states Directed arcs between states labeled with character One or more states may be terminal (accepting) A distinguished state is start Automaton makes transition from state S1 to S2
If and only if arc from S1 to S2 is labeled with next character in input
Token is legal if automaton stops on terminal state
Building FSM from Grammar
One state for each non-terminal A rule of the form
Nt1 ::= terminal Generates transition from S1 to final state
A rule of the form Nt1 ::= terminal Nt2 Generates transition from S1 to S2 on an arc
labeled by the terminal
Graphic representation
Sdigit
digit
letterletter lette
r
digitdigit
underscore
Int
id
Building FSM’s from RE’s
Every RE corresponds to a grammar For all regular expressions
A natural translation to FSM exists Alternation often leads to non-deterministic
machines
Non-Deterministic FSM
A non-deterministic FSM Has at least one state
With two arcs to two distinct states Labeled with the same character
Example: from start state, a digit can begin an integer literal or a real literal
Implementation requires backtracking Nasty
Deterministic FSM
For all states S For all characters C:
There is at most one arc from any state S that is labeled with C
Much easier to implement No backtracking
From NFSM to DFSM
There is an algorithm for converting a non-deterministic machine to a deterministic one
Result may have exponentially more states Intuitively: need new states to express uncertainty
about token: int or real Algorithm is efficient in practice (e.g. grep)
Other algorithms for minimizing number of states of FSM, for showing equivalence, etc.
Implementing the Scanner
Three methods Hand-coded approach:
draw DFSM, then implement with loop and case statement Hybrid approach :
define tokens using regular expressions, convert to NFSM, apply algorithm to obtain minimal DSFM
Hand-code resulting DFSM Automated approach:
Use regular grammar as input to lexical scanner generator (e.g. LEX)
Hand-coding
Normal coding techniques Scan over white space and comments till non-blank character
found. Branch depending on first character:
If digit, scan numeric literal If character, scan identifier or keyword If operator, check next character (++, etc.) Need table to determine character type efficiently
Return token found Write aggressive efficient code: goto’s, global
variables
Using grammar and FSM
Start with regular grammar or RE Typically found in the language reference
example (Ada): Chapter 2. Lexical Elements
Digit ::= 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 decimal-literal ::= integer [.integer][exponent] integer ::= digit {[underline] digit} exponent ::= E [+] integer | E - integer
Using grammar and FSM
Create one state for each non-terminal Label edges according to productions in grammar Each state becomes a label in the program Code for each state is a switch on next character,
corresponding to edges out of current state If no possible transition on next character, then:
If state is accepting, return the corresponding token If state is not accepting, report error
Hand-coded version:
Each state is encoded as follows: <<state1>>
case Next_Character iswhen ‘a’ => goto state3;when ‘b’ => goto state1;when others => End_of_token_processing;
end case; <<state2>>
… No explicit mention of state of automaton
Translating from FSM to code variable holds current state:
loop case State is when state1 =>
<<state1>> case Next_Character is
when ‘a’ => State := state3; when ‘b’ => State := state1; when others => End_token_processing;
end case; when state2 …
… end case;
end loop;
Automatic scanner construction
LEX builds a transition table, indexed by state and by character.
Code gets transition from table: Tab : array (State, Character) of State := …
begin
while More_Input loop
Curstate := Tab (Curstate, Next_Char);
if Curstate = Error_State then … end loop;
Automatic FSM Generation
Our example, FLEX See home page for manual in HTML
FLEX is given A set of regular expressions Actions associated with each RE
It builds a scanner Which matches RE’s and executes actions
Flex General Format
Input to Flex is a set of rules: Regexp actions (C statements) Regexp actions (C statements) …
Flex scans the longest matching Regexp And executes the corresponding actions
An Example of a Flex scanner DIGIT [0-9]
ID [a-z][a-z0-9]*%%{DIGIT}+ {
printf (“an integer %s (%d)\n”, yytext, atoi (yytext));
}
{DIGIT}+”.”{DIGIT}* { printf (“a float %s (%g)\n”, yytext, atof (yytext));
if|then|begin|end|procedure|function { printf (“a keyword: %s\n”, yytext));
Flex Example (continued)
{ID} printf (“an identifier %s\n”, yytext);
“+”|“-”|“*”|“/” { printf (“an operator %s\n”, yytext); }
“--”.*\n /* eat Ada style comment */
[ \t\n]+ /* eat white space */
. printf (“unrecognized character”);%%
Assembling the flex program
%{#include <math.h> /* for atof */%}
<<flex text we gave goes here>>
%%main (argc, argv)int argc;char **argv;{
yyin = fopen (argv[1], “r”);yylex();
}
Running flex
flex is an executable program The input is lexical grammar as described The output is a running C program
For Ada fans Look at aflex (www.adapower.com)
For C++ fans flex can run in C++ mode
Generates appropriate classes
Choice Between Methods?
Hand written scanners Typically much faster execution Easy to write (standard structure) Preferable for good error recovery
Flex approach Simple to Use Easy to modify token language
The GNAT Scanner
Hand written (scn.adb/scn.ads) Each call does:
Optimal scan past blanks/comments etc. Processing based on first character Call special routines for major classes:
Namet.Get_Name for identifier (hashing) Keywords recognized by special hash Strings (scn-slit.adb):
complication with “+”, “and”, etc. (string or operator?) Numeric literals (scn-nlit.adb):
complication with based literals: 16#FFF#
Historical oddities
Because early keypunch machines were unreliable, FORTRAN treats blanks as optional: lexical analysis and parsing are intertwined. DO10I=1.6 3 tokens:
identifier operator literal DO10I = 1.6
DO10I=1,6 7 tokens: Keyword stmt id operator literal comma literal DO 10 I = 1 , 6
Celebrated NASA failure caused by this bug (?)