PLC Project

Basic Description

Project will be composed from following steps:

1. Using ANTLR, implement a parser for the language specified bellow. If there is at least one syntax error, report this error (or errors) and stop the computations.
2. If there are no syntactic errors, perform the type checking. If there are some type errors, report all these errors and stop the computation.
3. If there are no type errors, generate appropriate target code. It will be a text file composed from stack-based instructions that are defined bellow.
4. Implement an interpreter, that gets a text file with these instructions and evaluates them.

Language Specification

Program's formatting

The program consists of a sequence of commands. Commands are written with free formatting. Comments, spaces, tabs, and line breaks serve only as delimiters and do not affect the meaning of the program. Comments are bounded by two slashes and the end of the line. Keywords are reserved. Identifiers and keywords are case sensitive.

Literals

There are following literals:

• integers - int - sequence of digits.
• floating point numbers - float - sequence of digits containing a '.' character.
• booleans - bool - values: true and false.
• strings - string - text given in quotation marks: "text". There are no escape sequences in our strings.

Variables

Variable's identifiers are composed from letters and digits, and it must start with a letter. Each variable must be declared before it is used. Repeated declaration of a variable with the same name is an error. Variables must have one of the following types: int, float, bool or string. After the variables are declared, they have initial values: 0, 0.0, "" respectively false.

Statements

Following statements are defined:

• ; - empty command.
• type variable, variable, ... ; - declaration of variables, all these variables have the same type type. It can be one of: int, float, bool, String
• expression ; - it evaluates given expression, the resulting value of the expression is ignored. Note, there can be some side effects like an assignment to a variable.
• read variable, variable, ... ; - it reads values ​​from standard input and then these values are assigned to corresponding variables. Each of these input values is on a separate line and it is verified, that have an appropriate type.
• write expression, expression, ... ; - it writes values of expressions to standard output. The "\n" character is written after the value of the last expression.
• { statement, statement, ... } - block of statements.
• if (condition) statement [else statement] - conditional statement - condition is an expression with a type: bool. The else part of the statement is optional.
• while (condition) statement - a cycle - condition must be a bool expression. This cycle repeats the given statement while the condition holds (it is true).

Expression

Lists in expressions trees are literals or variables. Types of operands must preserve the type of the operator. If necessary, int values are automatically cast to float. In other word, the type of 5 + 5.5 is float, and number 5 which type int is automatically converted to float. There is no conversion from float to int!

Following table defines operators in our expressions. Operator Signature is defined using letters: I, R, B, S which corespods to types: int, float, bool, string.

Description	Operator	Operator's Signature
unnary minus	-	$I\rightarrow I\lor F\rightarrow F$
binary arithmetic operators	+, -, *, /	$I\times I\rightarrow I\lor F\times F\rightarrow F$
modulo	%	$I\times I\rightarrow I$
concatenation of strings	.	$S\times S\rightarrow S$
relational operators	< <= > >= == !=	$x\times x\rightarrow B,~where~x\in \{I,F,S\}$
logic and, or	&&, ||	$B\times B\rightarrow B$
logic not	!	$B\rightarrow B$
assignment	=	$x\times x\rightarrow x,~where~x\in \{I,F,S,B\}$

In the assignment, left operand is strictly a variable and the right operand is expression. The type of the variable is the type of the left operand. A side effect is storing the value on the right side into the variable. The automatic conversion cannot change the type of the variable, i.e., it is impossible to store float value in int variable.

We can use parentheses in expressions.

All operators have left associativity, and their priority is (from lowest to highest):

1. =
2. ||
3. &&
4. < <= > >= == !=
5. + - .
6. * / %
7. !, unnary -

Our (Stack-based) Instructions Set

All instructions are stack based. The main memory is a stack and while evaluating the instructions, the input data are taken from stack and the results are put also in stack.

Instruction	Description
add	binary +
sub	binary -
mul	binary *
div	binary /
mod	binary %
uminus	unary -
concat	binary . - concatenation od strings
and	binary &&
or	binary ||
gt	binary >
lt	binary <
eq	binary == - campares two values
not	unary ! - negating boolean value
push T x	Instruction pushs the value x of type T. Where T represents I - int, F - float, S - string, B - bool. Example: push I 10