Reserved Sequences (Keywords)

The following character sequences are considered reserved, and should be yield a distinct token (rather than be counted as an identifier).

and

bool

fn

else

false

file

if

int

or

print

read

return

true

void

while

Identifiers

Any sequence of one or more letters and/or digits, and/or underscores, starting with a letter or underscore, should be treated as an identifier token. Identifiers must not be reserved sequences, but may include a proper substring that is a reserved word, e.g. while1 is an identifier but while is not.

Integer Literals

Any sequence of of one or more digits yields an integer literal token as long as it is not part of an identifer or string.

String Literals

Any string literal (a sequence of zero or more string characters surrounded by double quotes) should yield a string literal token. A string character is either

• an escaped character: a backslash followed by any one of the following characters:
  1. n
  2. t
  3. a double quote
  4. another backslash
  or
• a single character other than newline or double quote or backslash.

Examples of legal string literals:

""
"&!88"
"use \n to denote a newline character"
"use \" to  for a quote and \\ for a backslash"

Examples of things that are not legal string literals:

"unterminated
"also unterminated \"
"backslash followed by space: \ is not allowed"
"bad escaped character: \a AND not terminated

Symbol Operators

Any of the following character symbol sequences constitute a distinct token:

=
->
:
,
+


-
==
>
>=
[
{


<
<=
(
!
!=
--


++
?
]
}
)
;


/
*

Comments

Text starting with # up to the end of the line is a comment (except of course if those characters are inside a string literal). For example:

 
# this is a comment
# and so is # this
# and so is # this %$!#

The scanner should recognize and ignore comments (there is no COMMENT token).

Whitespace

Spaces, tabs, and newline characters are whitespace. Whitespace separates tokens and changes the character counter, but should otherwise be ignored (except inside a string literal).

Illegal Characters

Any character that is not whitespace and is not part of a token or comment is illegal.

Length Limits

No limit may be assumed on the lengths of identifiers, string literals, integer literals, comments, etc. other than those limits imposed by the underlying implementation of the compiler's host language.

For the most part, the token to produce should be self-explanatory. For example, the + symbol should produce the CROSS token, the -  symbol should produce the DASH token, etc. The set of tokens can be found in frontend.hh or in the switch in tokens.cpp. The LCURLY token refers to a left curly brace, {. the RCURLY refers to a right curly brace, }.

The lexical structure of DrewScript is fairly straightforward, and largely similar to C. There are a few notable details, some of which are departures from C:

• The sequence file produces the FILE token
• The string && and || are NOT in the language . Instead "logical and" is represented by the string and and "logical or" is represented by the string or.

Notable Differences from C

While the canonical reference for DrewScript syntax is its context-free grammar, there are a couple of "standout" points which deserve special attention for their deviation from C:

int i;
i = 4;

int i;
void f() {
  i = 4;
}

     print "hello";

     read var;

/*********************************************************************
 Grammar for programs in the Drewscript language
 ********************************************************************/
program		: globals

globals		: globals decl
			| ε

decl		: varDecl SEMICOL
			| fnDecl

varDecl		: type name
			| type name ARROW initializer

type		: primType LBRACKET INTLITERAL RBRACKET
			| primType
			| VOID

primType	: INT
			| BOOL
			| FILE

fnDecl 		: type name LPAREN maybeFormals RPAREN LCURLY stmtList RCURLY
			| type name LPAREN maybeFormals RPAREN ARROW exp

maybeFormals	: ε
				| formalList

formalList	: formalDecl
			| formalList COMMA formalDecl

formalDecl	: name COLON type
			| name COLON type ASSIGN initializer

stmtList	: ε
			| stmtList stmt SEMICOL
			| stmtList blockStmt

blockStmt	: WHILE LPAREN exp RPAREN LCURLY stmtList RCURLY
		| IF LPAREN exp RPAREN LCURLY stmtList RCURLY
		| IF LPAREN exp RPAREN LCURLY stmtList RCURLY ELSE LCURLY stmtList RCURLY

stmt	: varDecl
		| loc ASSIGN exp
		| callExp
		| loc POSTDEC
		| loc POSTINC
		| READ loc
		| READ loc ARROW exp
		| PRINT exp
		| PRINT exp ARROW loc
		| RETURN exp
		| RETURN

exp		: exp DASH exp
		| exp CROSS exp
		| exp STAR exp
		| exp SLASH exp
		| exp AND exp
		| exp OR exp
		| exp EQUALS exp
		| exp NOTEQUALS exp
		| exp GREATER exp
		| exp GREATEREQ exp
		| exp LESS exp
		| exp LESSEQ exp
		| NOT exp
		| DASH term
		| term


callExp		: loc LPAREN RPAREN
			| loc LPAREN actualsList RPAREN

actualsList	: exp
			| actualsList COMMA exp

term 		: loc
			| literal
			| LPAREN exp RPAREN
			| LBRACKET exp QUESTION exp COLON exp RBRACKET
			| callExp

initializer	: literal
			| LBRACKET litList RBRACKET

litList		: literal
			| literal COMMA litList

literal		: TRUE
			| FALSE
			| INTLITERAL
			| STRINGLITERAL

loc			: name
			| loc LBRACKET exp RBRACKET

name		: ID