Interpreter for a made up language MyPL

Description

MyPL supported elements are described below.

1. Primitive Data Types

   The data types that MyPL uses are integers, strings (denoted with double quotes), and booleans (either true or false).

2. Assignment Statements

   An assignment statement takes the form “var = expr;” where var is a valid identifier (a letter followed by zero or more letters, digits, or underscores) and expr is a valid expression. Assignment statements bind the variable to the value that results from evaluating the expression.

3. Output Statements

   MyPL uses pretty generic print statements. Some examples of print statements are “print(expr);” and “println(expr);”

4. Math Operators

   Math operators usch as +, -, *, /, and % are all supported.

5. Relational Operators

   Relational operators such as ==, <, >, <=, >=, and != are all supported.

6. Boolean Connectives

   Connectives such as and, or, and not are all supported.

7. Input Expressions

   Valid user input prompts are readint(msg) and readstr(msg) where msg is of type string.

8. While Statements

   A while statement takes the form “while bool-expr do stmts end”, where bool-expr is a Boolean expression and stmts is a list of statements.

9. Conditional Statements

   A condition statement takes the form “if bool-expr then stmts elseif bool-expr then stmts else stmts end”. A conditional statement can have zero or more elseif clauses and zero or one else clause. A conditional statement always ends with an “end” reserved word. Note that elseif is a distinct reserved word and should be used instead of an else followed by an if.

10. Comments

    MyPL uses Python-like comments by using a #

Grammar to construct a program

The MyPL language is based upon the following grammar:

<stmts> ::= <stmt> <stmts> | empty
<stmt> ::= <output> | <assign> | <cond> | <loop>
<output> ::= PRINT LPAREN <expr> RPAREN SEMICOLON | PRINTLN LPAREN <expr> RPAREN SEMICOLON
<input> ::= READINT LPAREN STRING RPAREN | READSTR LPAREN STRING RPAREN
<assign> ::= ID <listindex> ASSIGN <expr> SEMICOLON
<listindex> ::= LBRACKET <expr> RBRACKET | empty
<expr> ::= <value> <exprt>
<exprt> ::= <math_rel> <expr> | empty
<value> ::= ID <listindex> | STRING | INT | BOOL | <input> | LBRACKET <exprlist> RBRACKET
<exprlist> ::= <expr> <exprtail> | empty
<exprtail> ::= COMMA <expr> <exprtail> | empty
<math_rel> ::= PLUS | MINUS | DIVIDE | MULTIPLY | MODULUS
<cond> ::= IF <bexpr> THEN <stmts> <condt> END
<condt> ::= ELSEIF <bexpr> THEN <stmts> <condt> | ELSE <stmts> | empty
<bexpr> ::= <expr> <bexprt> | NOT <expr> <bexprt>
<bexprt> ::= <bool_rel> <expr> <bconnct> | empty
<bconnct> ::= AND <bexpr> | OR <bexpr> | empty
<bool_rel> ::= EQUAL | LESS_THAN | GREATER_THAN | LESS_THAN_EQUAL | GREATER_THAN_EQUAL | NOT_EQUAL
<loop> ::= WHILE <bexpr> DO <stmts> END

Example of a Simple MyPL Program

# obligatory hello world program
println("Hello world!");


# simple conditional statement
x = readint("Enter an int: ");
y = readint("Enter an int: ");
if x > y then
    println("The first int was bigger than the second!");
elseif y > x then
    println("The second int was bigger than the first!");
else
    println("You entered the same value twice!");
end


# simple while statement
z = readint("Enter an int: ");
i = 0;
while z > 2 do
    z = z / 2;
    i = i + 1;
end
print("z = ");
print(z);
print(", i = ");
println(i);

Running the application

Run the program with this command in the terminal

python mypl.py ../programs/my_pl_file.txt

Example Output

Hello world!
Enter an int: 6
Enter an int: 5
The first int was bigger than the second!
Enter an int: 10
z = 2, i = 2