Introduction
Some may wonder why one should ever bother writing a compiler, when there are already dozens of great compilers out there. The answer is simple, you may never need to write a compiler, but most likely you will find yourself in a situation where you could apply many of the ideas and algorithms that come together to create a compiler. Besides, it's kind of cool to write programs in your own language (Job security).
Programming languages are called languages for a reason, that being, that they are in fact languages. A programming language is defined by a set of tokens (words in the English language) and productions (grammar rules). The main difference between a programming language and a natural language is that while the natural language can have multiple interpretations a programming language cannot. It wouldn't be much fun to have our programs compile differently ever time we build them. A language that can have multiple interpretations is called ambiguous, and is not useful to us.
Warning
Entire books are written on this stuff, and I cannot explain all of the 5000+ lines of this project in this article, and frankly I don't want to. It is strongly recommended that you have some background on grammars and bottom up parsing techniques as well as a little MSIL knowledge. What I am especially proud of is the code in Scanner.cs and Parser.cs this contains the heart of the entire project, the rest is just the grunt work that needs to be done. Don't bother looking at the Generator.cs file, that's where I got sloppy and tired of this school project.
A simple language
A set of tokens and a set of rules define a language. Some text is said to be grammatical if there is a way to derive it using the language's tokens and rules. A language that describes all binary numbers may be described as follows:
L = {Tokens(0,1),Rules(Any number of 0s or 1s)}
We can now ask ourselves if the following text is grammatical "1200011". The answer should be no, since we see a "2", which is not a token that is part of our language.
The topic of languages deserves a large discussion which I am not prepared to make, or know fully well to explain, so I invite you to strap on your favorite search engine.
Regular Expressions
Regular Expressions are used to describe simple languages, generally the tokens themselves, or things like phone numbers or social security numbers. The regular expression "A*BC", translates into a more verbose language definition L {Tokens(A,B,C),Rules(Zero or more As, followed by a B then a C)}. This is all great, but how do we write a program that uses a regular expression to identify whether a stream of characters is grammatical or not. The general solution to the problem described is to create a Non Deterministic Finite State Automaton (NFA) from the given regular expression and then convert this NFA to a Deterministic Finite State Automaton (DFA). If you have never heard of these things before, don't worry, you won't have to deal with them in the rest of the article. A DFA is simply a state machine, the fact that it's deterministic means that, from any one state you can only have one other state to go to based on an input character. Consider the following regular expression;
R = int|char|ch
This regular expression has the following DFA
Once we have a DFA we can easily write an algorithm to that checks whether a string of characters is grammatical. Consider the following strings;
interop
We start out in state 0 (the start state), we read the first character which is "i", this takes us to state 1. We then read the next character "n", this takes us to state 2, the next character "t" takes us to state 3, which is an accepting state, however we have not yet read the entire string. The next character "e" takes us nowhere, actually it take us to an error state, which tells us the string is not grammatical.
char
The first character "c" takes us to state 4, the next takes us to state 5, then "a" takes us to state 6 and finally "r" takes us to state 7. We have finished reading the entire string so we check if the current state (state 7) is an accepting state, and in this particular case it is, it accepts char. Thus this string is grammatical.
cin
The first character "c" takes us to state 4. The next character "i" doesn't take us anywhere, except to the error state. This string is not grammatical.
Grammars
Grammars, like Regular Expressions, are used to describe languages. However grammars are much more powerful then Regular Expressions. I will not go into depth about Grammars simply because it is too much to cover. Briefly, a Grammar is a set of Productions (Rules), composed of terminals and non-terminals. A terminal is a token. A non-terminal is a grammar element that is composed of terminals and non-terminals.
A typical example of a grammar that defines a language that describes the set of all enclosed parentheses is as follows:
1 : S -> (S)
2 : S ->
In this case "(" and ")" are the tokens of the language and S is the only non-terminal. Production 1 tells us that S can be a "(" followed by S and followed by a ")". Production 2 tells us that S can be null. To better visualize this, lets take for example the string "((()))". We can replace the most inner null space with S using production 2, we now have (((S))). Using production 1 we can replace the most inner (S) with S to get ((S)), repeating this step we get (S) and finally S. The difficulty arises in trying to determine what production to use and when. There is a lot of theoretical stuff that deals with this, which I don't fully understand hence I don't want to get into.
The reason I am glancing over all the hard stuff is that there are compiler tools that create the DFA and LALR tables. So we don't need to concern ourselves too much with how this is done. Most compiler tools actually generate C++/Java/C# code, the tool that is used in this project generates a file that contains the tables. This tool is called Gold Parser. Don't feel like you are cheating either, it is practically impossible to write an bottom up parser without using a tool such as this.
Sharp Compiler
Honestly, since I don't want to spend days writing this article I will base the rest of the discussion on my school project, which I have named Sharp. Sharp is a very easy language that contains a few essential features; integer primitives, integer arrays, if for do while statements, complex expressions, functions, passing values by reference or value to functions, functions inside functions, a function for input and one for output and a few other neat things. Bellow is a glimpse of what the language looks like.
Bubble Sort implemented in Sharp
module Bubble_Sort{ /* Global Variables */ int size = Read(); int [] array = new [size]; int i; /* Initialize Array */ for(set i = 0;i < size;set i = i + 1) { set array[i] = Read(); } Sort(); /* Display sorted array */ for(set i = 0;i < size;set i = i + 1) { Write(array[i]); } Read(); return; /* Simple bubble sort routine */ void Sort() { int sorting = 1; int temp; while(sorting) { set sorting = 0; for(set i = 0; i < (size - 1); set i = i + 1) { if(array[i] > array[i+1]) { set temp = array[i]; set array[i] = array[i+1]; set array[i+1] = temp; set sorting = 1; } } } return; }}Originally when I designed the grammar for the language I took a larger bite then I could chew. Not everything that appears in the grammar is implemented. Things such as structs or other primitive types are not implemented, array passing is also not implemented.
Organization
In the above source code download you will notice two projects. Magic IDE and Core. Magic IDE is a simple IDE that provides project management, although the compiler can only build one file. Magic IDE uses two components which were not created by me, the IC#Code TextEditor and the wonderful Magic Library. Core contains the compiler source code, it provides 5 main classes. Language, Scanner, Parser, Semantics, Generator.
The language class is a wrapper around the file produced by the Gold Parser, it reads this file and makes its content more accessible. The Scanner class is responsible for tokenizing source code. The Parser class parses the source file using the Scanner class. The Semantics class defines the semantic actions to be taken every time a production is applied. Lastly the Generator class is responsible for building the actual executable. The generator uses the .NET namespace System.Reflection.Emit to produce IL code. Ildasm framework SDK tool can be used to see the contents of the generated executable.
Dissecting the Sharp Compiler Architecture
Every compiler has at least 3 parts to it. The scanner, the parser and code generator.
The Scanner
The scanner performs a very remedial job compared to the other sections, however it is crucial to the simplicity of the overall design. The scanner's job is to break up the source code into tokens. The Sharp statement int [] array = new [size]; will be broken up by the scanner into "int,[,],identifier,=,new,[,identifier,],;". The code for the scanner can be written in numerous ways, however the most elegant way is to base it on a DFA generated by some tool, in our case the Gold Parser. Using this approach it makes the code much much slimmer and elegant. The following C# method is responsible for getting the next token from the source file.
public Token GetNextToken(){ State currentState = m_Language.StartState; State lastAcceptingState = null; int tokenStart = m_Cursor + 1; int tokenEnd = tokenStart; int tokenStartColumn = m_Column; int tokenStartLine = m_Line; Token result = null; // // Retrieve one character at a time from the source input and walk through the DFA. // when we enter an accepting state save it as the lastAcceptingState and keep walking. // If we enter an error state (nextState == null) then return the lastAcceptingState, or // a null token if the lastAcceptingState is never set. // while(true) { // Don't advance the cursor. char nextChar = PeekNextChar(); // Return an EOF token. if(nextChar == (char)0 && (lastAcceptingState == null)) { result = new Token(m_Language.Symbols[0]); result.Column = tokenStartColumn; result.Line = tokenStartLine; break; } // Get next state from current state on the next character. State nextState = currentState.Move(nextChar); // If the next state is not an error state move to the next state. if(nextState != null) { // Save accepting state if its accepting. if(nextState.IsAccepting) { lastAcceptingState = nextState; tokenEnd = m_Cursor + 2; } // Move to the next state. currentState = nextState; // Advance cursor. nextChar = GetNextChar(); } else { // We have entered an error state. Thus either return the lastAcceptingState or // a null token. if(lastAcceptingState == null) { result = new Token(null); result.Column = tokenStartColumn; result.Line = tokenStartLine; result.Text = new string(m_Buffer,tokenStart,tokenEnd - tokenStart); } else { result = new Token(lastAcceptingState.Accepts); result.Column = tokenStartColumn; result.Line = tokenStartLine; result.Text = new string(m_Buffer,tokenStart,tokenEnd - tokenStart); } break; } } return result;}The Parser
The parser is the heart of the entire compiler. Since this parser uses a pre calculated table generated by Gold Parser, the only task left to do is write the algorithm that uses this table. The following C# method uses the parse table to parse the source file tokens constructed by the scanner. This is the classic algorithm for a table driven LR parser.
public Module CreateSyntaxTree(){ // Are we currently in a comment block. bool inCommentBlock = false; // Parser stack. Stack stack = new Stack(); // Syntax stack. SyntaxStack syntaxStack = new SyntaxStack(); m_Scanner.Reset(); ParserState currentState = m_Language.ParserStartState; // m_Language.ParserStartState is our parser start state. // Push start state. stack.Push(currentState); while(true) { // Get next token. Token nextToken = m_Scanner.PeekNextToken(); Symbol nextSymbol = nextToken.Symbol; // Ignore whitespace. if(nextSymbol.Type == SymbolType.Whitespace) { m_Scanner.GetNextToken(); continue; } // Ignore comment blocks if(nextSymbol.Type == SymbolType.CommentStart) { m_Scanner.GetNextToken(); inCommentBlock = true; continue; } // Ignore comment blocks if(nextSymbol.Type == SymbolType.CommentEnd) { m_Scanner.GetNextToken(); inCommentBlock = false; continue; } // Ignore stuff inside comments if(inCommentBlock) { m_Scanner.GetNextToken(); continue; } // Print(stack); //Uncomment this to see the parse stack. // PrintSyntax(syntaxStack.Stack); // Uncomment this to see the syntax stack. // Lookup action out of current state. Action action = currentState.Find(nextSymbol); // Do we have a parser error ? (Entered an invalid state.) if(action == null) { StringBuilder message = new StringBuilder("Token Unexpected, expecting [ "); for(int x = 0; x < currentState.Actions.Length; x++) { if(currentState.Actions[x].Symbol.Type == SymbolType.Terminal) message.Append(currentState.Actions[x].Symbol.Name + " "); } message.Append("]"); if(Error != null) Error(nextToken,message.ToString()); return null; } // Should we shift token and the next state on the stack. else if(action.Type == ActionType.Shift) { Token token = m_Scanner.GetNextToken(); stack.Push(token.Symbol); syntaxStack.Push(token.Text); stack.Push(action.ParserState); } // Should we reduce ? else if(action.Type == ActionType.Reduce) { // Pop off the stack however many state-symbol pairs as the Production // has Right Terminals,Nonterminals. int rightItems = action.Production.Right.Length; for(int i = 0;i < rightItems;i++) { stack.Pop(); stack.Pop(); } // Find the top of the stack. ParserState topState = (ParserState)stack.Peek(); // Push left hand side of the production. stack.Push(action.Production.Left); // Find next state by looking up the action for the top of the stack // on the Left hand side symbol of the production. stack.Push(topState.Find(action.Production.Left).ParserState); // Apply semantic rule. Applies the semantic rule associated // with the production used. This ultimately creates the // syntax tree. Semantics.Apply(action.Production,syntaxStack); } else if(action.Type == ActionType.Accept) { Debug.WriteLine("Accept"); return (Module)syntaxStack.Pop(); } currentState = (ParserState)stack.Peek(); } return null;}The Parse & Syntax Stacks
The parser uses a parse stack to push terminals and non terminals, once it figures out that it should use a production it pops the top elements that make up the right hand side of the production and it pushes the new non-terminal on the stack. A successfull parse is obtained if at the end of the parse there is only one non-terminal on the stack and that being the start non-terminal itself. The syntax stack is used for semantic operations, which give the real meaning to the source code. The following list is a print out of the contents of the parse and syntax stack after each token.
Source Code
module Test{ int x; void Foo(int x, int y) { return; } return;}Syntax and Parse Stacks
Stack : 0 Syntax : Stack : 0 module 1 Syntax : module Stack : 0 module 1 identifier 2 Syntax : module Test Stack : 0 module 1 identifier 2 { 3 Syntax : module Test { Stack : 0 module 1 identifier 2 { 3 int 96 Syntax : module Test { int Stack : 0 module 1 identifier 2 { 3 Primitive_Type 109 Syntax : module Test { Core.Type Stack : 0 module 1 identifier 2 { 3 Type 157 Syntax : module Test { Core.Type Stack : 0 module 1 identifier 2 { 3 Type 157 identifier 158 Syntax : module Test { Core.Type x Stack : 0 module 1 identifier 2 { 3 Type 157 identifier 158 ; 117 Syntax : module Test { Core.Type x ; Stack : 0 module 1 identifier 2 { 3 Variable_Declaration 173 Syntax : module Test { Core.Variable Stack : 0 module 1 identifier 2 { 3 Statement 152 Syntax : module Test { Core.Variable Stack : 0 module 1 identifier 2 { 3 Statements 153 Syntax : module Test { Core.StatementCollection Stack : 0 module 1 identifier 2 { 3 Statements 153 void 107 Syntax : module Test { Core.StatementCollection void Stack : 0 module 1 identifier 2 { 3 Statements 153 Primitive_Type 109 Syntax : module Test { Core.StatementCollection Core.Type Stack : 0 module 1 identifier 2 { 3 Statements 153 Type 157 Syntax : module Test { Core.StatementCollection Core.Type Stack : 0 module 1 identifier 2 { 3 Statements 153 Type 157 identifier 158 Syntax : module Test { Core.StatementCollection Core.Type Foo Stack : 0 module 1 identifier 2 { 3 Statements 153 Type 157 identifier 158 ... ( 159 Syntax : module Test { Core.StatementCollection Core.Type Foo ( Stack : 0 module 1 identifier 2 { 3 Statements 153 Type 157 identifier 158 ... ( 159 int 96 Syntax : module Test { Core.StatementCollection Core.Type Foo ( int Stack : 0 module 1 identifier 2 { 3 Statements 153 Type 157 identifier 158 ... ( 159 Primitive_Type 109 Syntax : module Test { Core.StatementCollection Core.Type Foo ( Core.Type Stack : 0 module 1 identifier 2 { 3 Statements 153 Type 157 identifier 158 ... ( 159 Type 171 Syntax : module Test { Core.StatementCollection Core.Type Foo ( Core.Type Stack : 0 module 1 identifier 2 { 3 Statements 153 Type 157 identifier 158 ... ( 159 Type 171 identifier 172 Syntax : module Test { Core.StatementCollection Core.Type Foo ( Core.Type x Stack : 0 module 1 identifier 2 { 3 Statements 153 Type 157 identifier 158 ... ( 159 Parameter 165 Syntax : module Test { Core.StatementCollection Core.Type Foo ( ... Core.Parameter Stack : 0 module 1 identifier 2 { 3 Statements 153 Type 157 identifier 158 ... ( 159 Parameters 166 Syntax : module Test { Core.StatementCollection Core.Type Foo ( ... Core.ParameterCollection Stack : 0 module 1 identifier 2 { 3 Statements 153 Type 157 identifier 158 ... ( 159 Parameters 166 , 169 Syntax : module Test { Core.StatementCollection Core.Type Foo ( ... Core.ParameterCollection , Stack : 0 module 1 identifier 2 { 3 Statements 153 Type 157 identifier 158 ... ( 159 Parameters 166 , 169 int 96 Syntax : module Test { Core.StatementCollection Core.Type Foo ( ... Core.ParameterCollection , int Stack : 0 module 1 identifier 2 { 3 Statements 153 Type 157 identifier 158 ... ( 159 Parameters 166 , 169 Primitive_Type 109 Syntax : module Test { Core.StatementCollection Core.Type Foo ( ... Core.ParameterCollection , Core.Type Stack : 0 module 1 identifier 2 { 3 Statements 153 Type 157 identifier 158 ... ( 159 Parameters 166 , 169 Type 171 Syntax : module Test { Core.StatementCollection Core.Type Foo ( ... Core.ParameterCollection , Core.Type Stack : 0 module 1 identifier 2 { 3 Statements 153 Type 157 identifier 158 ... ( 159 Parameters 166 , 169 Type 171 identifier 172 Syntax : module Test { Core.StatementCollection Core.Type Foo ( ... Core.ParameterCollection , Core.Type y Stack : 0 module 1 identifier 2 { 3 Statements 153 Type 157 identifier 158 ... ( 159 Parameters 166 , 169 Parameter 170 Syntax : module Test { Core.StatementCollection Core.Type Foo ( ... Core.ParameterCollection , Core.Parameter Stack : 0 module 1 identifier 2 { 3 Statements 153 Type 157 identifier 158 ... ( 159 Parameters 166 Syntax : module Test { Core.StatementCollection Core.Type Foo ( ... Core.ParameterCollection Stack : 0 module 1 identifier 2 { 3 Statements 153 Type 157 identifier 158 ... ( 159 Parameters 166 ) 167 Syntax : module Test { Core.StatementCollection Core.Type Foo ( ... Core.ParameterCollection ) Stack : 0 module 1 identifier 2 { 3 Statements 153 Type 157 identifier 158 ... ( 159 Parameters 166 ) 167 { 3 Syntax : module Test { Core.StatementCollection Core.Type Foo ( ... Core.ParameterCollection ) { Stack : 0 module 1 identifier 2 { 3 Statements 153 Type 157 identifier 158 ... ( 159 Parameters 166 ) 167 { 3 return 98 Syntax : module Test { Core.StatementCollection Core.Type Foo ( ... Core.ParameterCollection ) { return Stack : 0 module 1 identifier 2 { 3 Statements 153 Type 157 identifier 158 ... ( 159 Parameters 166 ) 167 { 3 return 98 ; 99 Syntax : module Test { Core.StatementCollection Core.Type Foo ( ... Core.ParameterCollection ) { return ; Stack : 0 module 1 identifier 2 { 3 Statements 153 Type 157 identifier 158 ... ( 159 Parameters 166 ) 167 { 3 Statement 152 Syntax : module Test { Core.StatementCollection Core.Type Foo ( ... Core.ParameterCollection ) { Core.Return Stack : 0 module 1 identifier 2 { 3 Statements 153 Type 157 identifier 158 ... ( 159 Parameters 166 ) 167 { 3 Statements 153 Syntax : module Test { Core.StatementCollection Core.Type Foo ( ... Core.ParameterCollection ) { Core.StatementCollection Stack : 0 module 1 identifier 2 { 3 Statements 153 Type 157 identifier 158 ... ( 159 Parameters 166 ) 167 { 3 Statements 153 } 154 Syntax : module Test { Core.StatementCollection Core.Type Foo ( ... Core.ParameterCollection ) { Core.StatementCollection } Stack : 0 module 1 identifier 2 { 3 Statements 153 Type 157 identifier 158 ... ( 159 Parameters 166 ) 167 Body 168 Syntax : module Test { Core.StatementCollection Core.Type Foo ( ... Core.ParameterCollection ) Core.Body Stack : 0 module 1 identifier 2 { 3 Statements 153 Function_Declaration 150 Syntax : module Test { Core.StatementCollection Core.Function Stack : 0 module 1 identifier 2 { 3 Statements 153 Statement 155 Syntax : module Test { Core.StatementCollection Core.Function Stack : 0 module 1 identifier 2 { 3 Statements 153 Syntax : module Test { Core.StatementCollection Stack : 0 module 1 identifier 2 { 3 Statements 153 return 98 Syntax : module Test { Core.StatementCollection return Stack : 0 module 1 identifier 2 { 3 Statements 153 return 98 ; 99 Syntax : module Test { Core.StatementCollection return ; Stack : 0 module 1 identifier 2 { 3 Statements 153 Statement 155 Syntax : module Test { Core.StatementCollection Core.Return Stack : 0 module 1 identifier 2 { 3 Statements 153 Syntax : module Test { Core.StatementCollection Stack : 0 module 1 identifier 2 { 3 Statements 153 } 154 Syntax : module Test { Core.StatementCollection } Stack : 0 module 1 identifier 2 Body 183 Syntax : module Test Core.Body Stack : 0 Module 184 Syntax : Core.Module AcceptVisual Representation of Syntax and Parse Trees
Code Generator
Once we have a syntax tree in place the code generation is pretty straight forward. Since MSIL is a stack based instruction set the code generation ties in pretty easily. All the code for emitting a .NET assembly from a Module class resides inside of the Generator.cs source file.
Gold Parser - Grammar Source File
The following text file is the source file that Gold Parser uses to build the DFA and LALR state tables. The compiled grammar file this generates is used by the Language class.
"Name" = Sharp Grammar"Version" = 1.0"Author" = Michael Bebenita"Start Symbol" = <Module>"Case Sensitive" = True! Scanner Rules{LETTER} = {Letter}{DIGIT} = {Digit}{ALPHA_NUMERIC} = {DIGIT} + {LETTER}{HEX_DIGIT} = {DIGIT} + [a-fA-F]{String Char} = {Printable} - ["]! Identifiers must start with a letter followed any amount of letters, digits or underscores.identifier = {LETTER}({ALPHA_NUMERIC} | _)*! Literals may be either true or false, integers, real numbers, or characters.boolean_literal = (true)|(false)integer_literal = ({DIGIT}+) | (0x{HEX_DIGIT}+)real_literal = {DIGIT}*.{DIGIT}+fcharacter_literal = ''{LETTER}''string_literal = '"'{String Char}*'"'<Literal> ::= boolean_literal | integer_literal | real_literal | character_literal | string_literal!! Types<Type> ::= <Structure_Type> | <Primitive_Type> | <Array_Type><Structure_Type> ::= identifier<Primitive_Type> ::= bool | int | real | char | void<Array_Type> ::= <Structure_Type> '[' ']' | <Primitive_Type> '[' ']'!!!! Global stuff. Module and body declaration.!!<Module> ::= module identifier <Body><Body> ::= '{' <Statements> '}' | '{' '}'<Name> ::= identifier | <Name> . identifier!!!! Variable stuff.!!<Variable_Declarations> ::= <Variable_Declaration> | <Variable_Declarations> <Variable_Declaration><Variable_Declaration> ::= <Type> identifier ; | <Type> identifier = <Expression> ; | <Type> identifier = new '[' <Expression> ']' ; | <Type> identifier = '{' <Elements> '}' ;<Elements> ::= <Element> | <Elements> , <Element><Element> ::= <Expression> | '{' <Elements> '}'!!!! Function stuff.!!<Function_Declaration> ::= <Type> identifier '(' ')' <Body> | <Type> identifier '(' <Parameters> ')' <Body><Parameters> ::= <Parameter> | <Parameters> , <Parameter><Parameter> ::= <Type> identifier | ref <Type> identifier<Function_Call> ::= <Name> '(' ')' | <Name> '(' <Arguments> ')'<Arguments> ::= <Argument> | <Arguments> , <Argument> <Argument> ::= <Expression> | ref <Expression>!!!! Structure stuff.!!<Structure_Declaration> ::= struct identifier '{' <Variable_Declarations> '}' | struct identifier '{' '}'!!!! Statements!!<Statements> ::= <Statement> | <Statements> <Statement><Statement> ::= <Variable_Declaration> | <Function_Declaration> | <Structure_Declaration> | <Function_Call> ; | <Assignment> ; | return <Expression> ; | return ; | if '(' <Expression> ')' <Body> | if '(' <Expression> ')' <Body> else <Body> | while '(' <Expression> ')' <Body> | do <Body> while '(' <Expression> ')' | for '(' <Assignment> ; <Expression> ; <Assignment> ')' <Body>!! Expressions!<Assignment> ::= set <Name> = <Expression> | set <Name> '[' <Expression> ']' = <Expression> | set <Name> ++ | set <Name> --<Expression> ::= <Expression_Term> | <Expression> + <Expression_Term> | <Expression> - <Expression_Term> <Expression_Term> ::= <Expression_Factor> | <Expression_Term> * <Expression_Factor> | <Expression_Term> / <Expression_Factor><Expression_Factor> ::= <Expression_Binary> | <Expression_Factor> % <Expression_Binary> | <Expression_Factor> '>' <Expression_Binary> | <Expression_Factor> '<' <Expression_Binary> | <Expression_Factor> '>=' <Expression_Binary> | <Expression_Factor> '<=' <Expression_Binary> | <Expression_Factor> '==' <Expression_Binary> | <Expression_Factor> '!=' <Expression_Binary><Expression_Binary> ::= <Expression_Unary> | <Expression_Binary> && <Expression_Unary> | <Expression_Binary> || <Expression_Unary><Expression_Unary> ::= + <Expression_Primary> | - <Expression_Primary> | ! <Expression_Primary> | <Expression_Primary> | <Expression_Primary> '[' <Expression> ']'<Expression_Primary> ::= <Name> | <Function_Call> | <Literal> | '(' <Expression> ')' 
