COMP712 Programming Languages Interpreter Project
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COMP712 Programming Languages
Interpreter Implementation Project
This document specifies a programming language called AJS (A JavaScript) which is a subset of ECMAScript 2018. The goal of the project is to implement an interpreter for AJS using Racket with the help of the SLLgen lexer and parser generator and the Datatype macro extension introduced by D. Friedman and M Wand in their book Essentials of Programming Languages, third edition.
1. Syntax
The syntax is written in a variant of BNF described in A Human Engineered Variant of BNF, ACM SIGPLAN Notices, Vol. 15, issue 10, Oct. 1980, pp. 57-62. In the grammar below, bold font is used for keywords, italics for syntactic variables, ε for nothing, x … for zero or more repetitions of x.
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program |
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statement … |
Program |
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statement |
::= |
const name = expression ; |
Constant declaration |
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function name ( names ) block |
Function declaration |
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return expression ; |
Return statement |
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if-statement |
Conditional statement |
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block |
Block statement |
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expression ; |
Expression statement |
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names |
::= |
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name ( , name ) … |
Name list |
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if-statement |
::= |
if ( expression ) block |
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else ( block | if-statement ) |
Conditional statement |
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block |
::= |
{ statement } |
Block statement |
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expression |
::= |
number |
Primitive number |
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true | false |
Primitive Boolean |
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string |
Primitive string |
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null |
Primitive list |
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name |
Name expression |
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expression binary-operator expression |
Binary operator combination |
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unary-operator expression |
Unary operator combination |
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expression binary-logical expression |
Logical composition |
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expression ( expressions ) |
Function application |
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( name | ( names ) ) => expression |
Lambda expression |
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( name | ( names ) ) => block |
Lambda expression |
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expression ? expression : expression |
Conditional expression |
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( expression ) |
Parenthesized expression |
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binary-operator |
::= |
+ | - | * | / | % | === | !== |
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> | < | >= | <= |
Binary operator |
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Unary-operator |
::= |
! | - |
Unary operator |
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Binary-logical |
::= |
&& | || |
Logical composition |
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Expressions |
::= |
ε |
expression ( , expression ) … |
Argument expressions |
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Restrictions
(a) Return statements are only allowed in bodies of functions.
(b) There cannot be a newline character between return and expression in return statements.
(c) There cannot be any newline characters between ( name | ( parameters ) ) and => in function definition expressions.
(d) Implementations of AJS are allowed to treat function declaration as syntactic sugar for constant declaration. Programmers need to ensure that functions are not called before their corresponding function declaration is evaluated.
Names
Name (identifier) must start with _, $ or a letter and contain only _, $, letters or digits. Keywords are not allowed as names. Keywords include any of the words in bold in the syntax rules above.
Numbers
Only decimal numbers are used, with an optional decimal point. Scientific notation is not allowed. Example numbers are therefore, 1234, -3456, and 567.8.
Strings
String is a possibly empty sequence of characters within double-quotes. For example, “This is a string.” Newline and other special characters can be represented as follows:
. Newline: \n
. Single quote: \’
. Double quote: \”
Comments
Single-line comments start with // and therefore any characters until the next newline character is ignored.
Library Function
There is no need to provide any sort of library function other than the arithmetic and Boolean operators shown in the syntax.
2023-10-26