Description

• Lisp like syntax
• Interpreted
• Imperative, non-object oriented
• Static scope, static binding, strongly typed, …
• A few built-in types to promote exact arithmetic for various domains such as computational geometry
G++ Interpreter
• Starting G++ without an input file… $ g++
> _ \READ-EVAL-PRINT loop starts here…
• Starting G++ with an input file…
$ g++ myhelloword.g++
\READ-EVAL-PRINT everything in the file…
> _ \READ-EVAL-PRINT loop starts here…
G++ – Lexical Syntax
• Keywords: and, or, not, eq, gt, nil, set, defvar, deffun, while, if, load, disp, true, false
• Operators: + – / * ( ) ,
• Comment: Line or part of the line starting with ;; • Terminals:
• Keywords
• Operators
• Literals: There is only predefined type in this language.
– Unsigned fractions – two unsigned integers separated by the character “f”. E.g., 123f12 is the fraction
• Identifier: Any combination of alphabetical characters, digits and “_” with only leading alphabetical characters.
G++ Lexer Tokens
KW_NIL, DEFV, DEFF, KW_WHILE, KW_IF, KW_EXIT, KW_LOAD, KW_DISP, KW_TRUE, KW_FALSE
OP_PLUS, OP_MINUS, OP_DIV, OP_MULT, OP, CP, OP_SET
OP_COMMA, OP_AND, OP_OR, OP_NOT, OP_EQ, OP_GT
COMMENT
VALUEF
ID
G++ – Concrete Syntax
• Non-terminals:
– $START, $INPUT, $EXPLIST, $EXP, …
G++ – Concrete Syntax
• $START -> $INPUT
• $INPUT -> $FUNCTION | $EXP | $EXPLIST
G++ – Concrete Syntax
• An expression always returns a fraction
• An expression list returns the value of the last expression
• Expressions:
– $EXP -> OP_OP OP_PLUS $EXP $EXP OP_CP |
OP_OP OP_MINUS $EXP $EXP OP_CP | OP_OP OP_MULT $EXP $EXP OP_CP |
OP_OP OP_DIV $EXP $EXP OP_CP |
ID | VALUEF | $FCALL | $ASG
– $EXPLIST -> OP_OP $EXPLIST $EXP OP_CP
G++ – Syntax
• Assignment:
– $ASG -> OP OP_SET ID $EXP CP
– Imperative, therefore $EXP will be evaluated first…
G++ – Syntax
Extended syntax for four alternatives
• Functions:
– Definition:
$FUNCTION -> OP DEFF ID OP ( | ID | ID ID | ID ID ID ) CP OP $EXPLIST CP – Call:
$FCALL -> OP ID ( | $EXP| $EXP $EXP | $EXP $EXP $EXP ) CP
– Parameter passing by value (only up to 3 parameters allowed)
– Returning the value of the last expression
– Note that function definition is an expression always returning 0f1
G++ – Syntax

For easy writing, ‘(‘, ‘if’,
‘while’ etc. are used instead of their corresponding tokens

• Control Statements:
– $EXP -> (if $EXPB $EXPLISTI $EXPLISTI)
– $EXP -> (while $EXPB $EXPLISTI)
• Binary values and expressions
– $EXPB -> (eq $EXP $EXP) : returns true if equal
– $EXPB -> (gt $EXP $EXP) : returns true if greater
– $EXPB -> KW_TRUE | KW_FALSE
– $EXPB -> (and $EXPB $EXPB)
– $EXPB -> (or $EXPB $EXPB)
– $EXPB -> (not $EXPB)
G++ – Variables
• $EXP -> OP DEFV ID $EXP CP // delaring a variable • $EXP -> OP KW_SET ID $EXP CP // setting a variable
– Scope:
• Static, lexical scope (shadowing) – Binding:
• Static binding – Typing:
• Strong typing…
Example Programming in G++
$ g++ ;; helloworld.g++
> (load “helloworld.g++”) (deffun sumup (x)
> (sumup 4) (if (eq x 1f1) (1)
10 (+ x (sumup (- x 1f1)))
> (exit) )
$ _