2009-11-20 Marek Safar <marek.safar@gmail.com>

[mcs.git] / docs / compiler.txt

                       The Internals of the Mono C# Compiler
        
                                Miguel de Icaza
                              (miguel@ximian.com)
                               2002, 2007, 2009

* Abstract

        The Mono C# compiler is a C# compiler written in C# itself.
        Its goals are to provide a free and alternate implementation
        of the C# language.  The Mono C# compiler generates ECMA CIL
        images through the use of the System.Reflection.Emit API which
        enable the compiler to be platform independent.
        
* Overview: How the compiler fits together

        The compilation process is managed by the compiler driver (it
        lives in driver.cs).

        The compiler reads a set of C# source code files, and parses
        them.  Any assemblies or modules that the user might want to
        use with his project are loaded after parsing is done.

        Once all the files have been parsed, the type hierarchy is
        resolved.  First interfaces are resolved, then types and
        enumerations.

        Once the type hierarchy is resolved, every type is populated:
        fields, methods, indexers, properties, events and delegates
        are entered into the type system.  

        At this point the program skeleton has been completed.  The
        next process is to actually emit the code for each of the
        executable methods.  The compiler drives this from
        RootContext.EmitCode.

        Each type then has to populate its methods: populating a
        method requires creating a structure that is used as the state
        of the block being emitted (this is the EmitContext class) and
        then generating code for the topmost statement (the Block).

        Code generation has two steps: the first step is the semantic
        analysis (Resolve method) that resolves any pending tasks, and
        guarantees that the code is correct.  The second phase is the
        actual code emission.  All errors are flagged during in the
        "Resolution" process. 

        After all code has been emitted, then the compiler closes all
        the types (this basically tells the Reflection.Emit library to
        finish up the types), resources, and definition of the entry
        point are done at this point, and the output is saved to
        disk. 

        The following list will give you an idea of where the
        different pieces of the compiler live:

        Infrastructure:

            driver.cs:
                This drives the compilation process: loading of
                command line options; parsing the inputs files;
                loading the referenced assemblies; resolving the type
                hierarchy and emitting the code. 

            codegen.cs:
                
                The state tracking for code generation. 

            attribute.cs:

                Code to do semantic analysis and emit the attributes
                is here.

            rootcontext.cs:

                Keeps track of the types defined in the source code,
                as well as the assemblies loaded.  

            typemanager.cs:

                This contains the MCS type system.

            report.cs:

                Error and warning reporting methods.

            support.cs:

                Assorted utility functions used by the compiler.
                
        Parsing

            cs-tokenizer.cs:

                The tokenizer for the C# language, it includes also
                the C# pre-processor.

            cs-parser.jay, cs-parser.cs:

                The parser is implemented using a C# port of the Yacc
                parser.  The parser lives in the cs-parser.jay file,
                and cs-parser.cs is the generated parser.

            location.cs:

                The `location' structure is a compact representation
                of a file, line, column where a token, or a high-level
                construct appears.  This is used to report errors.

        Expressions:
          
            ecore.cs
        
                Basic expression classes, and interfaces most shared
                code and static methods are here.

            expression.cs:

                Most of the different kinds of expressions classes
                live in this file.

            assign.cs:

                The assignment expression got its own file.

            constant.cs:

                The classes that represent the constant expressions.

            literal.cs
                
                Literals are constants that have been entered manually
                in the source code, like `1' or `true'.  The compiler
                needs to tell constants from literals apart during the
                compilation process, as literals sometimes have some
                implicit extra conversions defined for them. 

            cfold.cs:

                The constant folder for binary expressions.

        Statements

            statement.cs:

                All of the abstract syntax tree elements for
                statements live in this file.  This also drives the
                semantic analysis process.

            iterators.cs:

                Contains the support for implementing iterators from
                the C# 2.0 specification.

        Declarations, Classes, Structs, Enumerations

            decl.cs

                This contains the base class for Members and
                Declaration Spaces.   A declaration space introduces
                new names in types, so classes, structs, delegates and
                enumerations derive from it.

            class.cs:
                
                Methods for holding and defining class and struct
                information, and every member that can be in these
                (methods, fields, delegates, events, etc).

                The most interesting type here is the `TypeContainer'
                which is a derivative of the `DeclSpace' 

            delegate.cs:

                Handles delegate definition and use. 

            enum.cs:

                Handles enumerations.

            interface.cs:

                Holds and defines interfaces.  All the code related to
                interface declaration lives here.

            parameter.cs:

                During the parsing process, the compiler encapsulates
                parameters in the Parameter and Parameters classes.
                These classes provide definition and resolution tools
                for them.

            pending.cs:

                Routines to track pending implementations of abstract
                methods and interfaces.  These are used by the
                TypeContainer-derived classes to track whether every
                method required is implemented.

        
* The parsing process

        All the input files that make up a program need to be read in
        advance, because C# allows declarations to happen after an
        entity is used, for example, the following is a valid program:

        class X : Y {
                static void Main ()
                {
                        a = "hello"; b = "world";
                }
                string a;
        }
        
        class Y {
                public string b;
        }

        At the time the assignment expression `a = "hello"' is parsed,
        it is not know whether a is a class field from this class, or
        its parents, or whether it is a property access or a variable
        reference.  The actual meaning of `a' will not be discovered
        until the semantic analysis phase.

** The Tokenizer and the pre-processor

        The tokenizer is contained in the file `cs-tokenizer.cs', and
        the main entry point is the `token ()' method.  The tokenizer
        implements the `yyParser.yyInput' interface, which is what the
        Yacc/Jay parser will use when fetching tokens.  

        Token definitions are generated by jay during the compilation
        process, and those can be references from the tokenizer class
        with the `Token.' prefix. 

        Each time a token is returned, the location for the token is
        recorded into the `Location' property, that can be accessed by
        the parser.  The parser retrieves the Location properties as
        it builds its internal representation to allow the semantic
        analysis phase to produce error messages that can pin point
        the location of the problem. 

        Some tokens have values associated with it, for example when
        the tokenizer encounters a string, it will return a
        LITERAL_STRING token, and the actual string parsed will be
        available in the `Value' property of the tokenizer.   The same
        mechanism is used to return integers and floating point
        numbers. 

        C# has a limited pre-processor that allows conditional
        compilation, but it is not as fully featured as the C
        pre-processor, and most notably, macros are missing.  This
        makes it simple to implement in very few lines and mesh it
        with the tokenizer.

        The `handle_preprocessing_directive' method in the tokenizer
        handles all the pre-processing, and it is invoked when the '#'
        symbol is found as the first token in a line.  

        The state of the pre-processor is contained in a Stack called
        `ifstack', this state is used to track the if/elif/else/endif
        nesting and the current state.  The state is encoded in the
        top of the stack as a number of values `TAKING',
        `TAKEN_BEFORE', `ELSE_SEEN', `PARENT_TAKING'.

** Locations

        Locations are encoded as a 32-bit number (the Location
        struct) that map each input source line to a linear number.
        As new files are parsed, the Location manager is informed of
        the new file, to allow it to map back from an int constant to
        a file + line number.

        Prior to parsing/tokenizing any source files, the compiler
        generates a list of all the source files and then reserves the
        low N bits of the location to hold the source file, where N is
        large enough to hold at least twice as many source files as were
        specified on the command line (to allow for a #line in each file).
        The upper 32-N bits are the line number in that file.

        The token 0 is reserved for ``anonymous'' locations, ie. if we
        don't know the location (Location.Null).

        The tokenizer also tracks the column number for a token, but
        this is currently not being used or encoded.  It could
        probably be encoded in the low 9 bits, allowing for columns
        from 1 to 512 to be encoded.

* The Parser

        The parser is written using Jay, which is a port of Berkeley
        Yacc to Java, that I later ported to C#. 

        Many people ask why the grammar of the parser does not match
        exactly the definition in the C# specification.  The reason is
        simple: the grammar in the C# specification is designed to be
        consumed by humans, and not by a computer program.  Before
        you can feed this grammar to a tool, it needs to be simplified
        to allow the tool to generate a correct parser for it. 

        In the Mono C# compiler, we use a class for each of the
        statements and expressions in the C# language.  For example,
        there is a `While' class for the the `while' statement, a
        `Cast' class to represent a cast expression and so on.

        There is a Statement class, and an Expression class which are
        the base classes for statements and expressions. 

** Namespaces
        
        Using list.

* Internal Representation

** Expressions

        Expressions in the Mono C# compiler are represented by the
        `Expression' class.  This is an abstract class that particular
        kinds of expressions have to inherit from and override a few
        methods.

        The base Expression class contains two fields: `eclass' which
        represents the "expression classification" (from the C#
        specs) and the type of the expression.

        During parsing, the compiler will create the various trees of
        expressions.  These expressions have to be resolved before they
        are can be used.    The semantic analysis is implemented by
        resolving each of the expressions created during parsing and
        creating fully resolved expressions.

        A common pattern that you will notice in the compiler is this:

                  Expression expr;
                  ...
        
                  expr = expr.Resolve (ec);
                  if (expr == null)
                        // There was an error, stop processing by returning

        The resolution process is implemented by overriding the
        `DoResolve' method.  The DoResolve method has to set the `eclass'
        field and the `type', perform all error checking and computations
        that will be required for code generation at this stage.

        The return value from DoResolve is an expression.  Most of the
        time an Expression derived class will return itself (return
        this) when it will handle the emission of the code itself, or
        it can return a new Expression.

        For example, the parser will create an "ElementAccess" class
        for:

                a [0] = 1;

        During the resolution process, the compiler will know whether
        this is an array access, or an indexer access.  And will
        return either an ArrayAccess expression or an IndexerAccess
        expression from DoResolve.

        All errors must be reported during the resolution phase
        (DoResolve) and if an error is detected the DoResolve method
        will return null which is used to flag that an error condition
        has occurred, this will be used to stop compilation later on.
        This means that anyone that calls Expression.Resolve must
        check the return value for null which would indicate an error
        condition.

        The second stage that Expressions participate in is code
        generation, this is done by overwriting the "Emit" method of
        the Expression class.  No error checking must be performed
        during this stage.

        We take advantage of the distinction between the expressions that
        are generated by the parser and the expressions that are the
        result of the semantic analysis phase for lambda expressions (more
        information in the "Lambda Expressions" section).

        But what is important is that expressions and statements that are
        generated by the parser should implement the cloning
        functionality.  This is used lambda expressions require the
        compiler to attempt to resolve a given block of code with
        different possible types for parameters that have their types
        implicitly inferred. 

** Simple Names, MemberAccess

        One of the most important classes in the compiler is
        "SimpleName" which represents a simple name (from the C#
        specification).  The names during the resolution time are
        bound to field names, parameter names or local variable names.

        More complicated expressions like:

                Math.Sin

        Are composed using the MemberAccess class which contains a
        name (Math) and a SimpleName (Sin), this helps driving the
        resolution process.

** Types

        The parser creates expressions to represent types during
        compilation.  For example:

           class Sample {

                Version vers;

           }


        That will produce a "SimpleName" expression for the "Version"
        word.  And in this particular case, the parser will introduce
        "Version vers" as a field declaration.

        During the resolution process for the fields, the compiler
        will have to resolve the word "Version" to a type.  This is
        done by using the "ResolveAsType" method in Expression instead
        of using "Resolve".

        ResolveAsType just turns on a different set of code paths for
        things like SimpleNames and does a different kind of error
        checking than the one used by regular expressions. 

** Constants

        Constants in the Mono C# compiler are represented by the
        abstract class `Constant'.  Constant is in turn derived from
        Expression.  The base constructor for `Constant' just sets the
        expression class to be an `ExprClass.Value', Constants are
        born in a fully resolved state, so the `DoResolve' method
        only returns a reference to itself.

        Each Constant should implement the `GetValue' method which
        returns an object with the actual contents of this constant, a
        utility virtual method called `AsString' is used to render a
        diagnostic message.  The output of AsString is shown to the
        developer when an error or a warning is triggered.

        Constant classes also participate in the constant folding
        process.  Constant folding is invoked by those expressions
        that can be constant folded invoking the functionality
        provided by the ConstantFold class (cfold.cs).   

        Each Constant has to implement a number of methods to convert
        itself into a Constant of a different type.  These methods are
        called `ConvertToXXXX' and they are invoked by the wrapper
        functions `ToXXXX'.  These methods only perform implicit
        numeric conversions.  Explicit conversions are handled by the
        `Cast' expression class.

        The `ToXXXX' methods are the entry point, and provide error
        reporting in case a conversion can not be performed.

** Constant Folding

        The C# language requires constant folding to be implemented.
        Constant folding is hooked up in the Binary.Resolve method.
        If both sides of a binary expression are constants, then the
        ConstantFold.BinaryFold routine is invoked.  

        This routine implements all the binary operator rules, it
        is a mirror of the code that generates code for binary
        operators, but that has to be evaluated at runtime.

        If the constants can be folded, then a new constant expression
        is returned, if not, then the null value is returned (for
        example, the concatenation of a string constant and a numeric
        constant is deferred to the runtime). 

** Side effects

        a [i++]++ 
        a [i++] += 5;

** Statements

*** Invariant meaning in a block

        The seemingly small section in the standard entitled
        "invariant meaning in a block" has several subtleties
        involved, especially when we try to implement the semantics
        efficiently.

        Most of the semantics are trivial, and basically prevent local
        variables from shadowing parameters and other local variables.
        However, this notion is not limited to that, but affects all
        simple name accesses within a block.  And therein lies the rub
        -- instead of just worrying about the issue when we arrive at
        variable declarations, we need to verify this property at
        every use of a simple name within a block.

        The key notion that helps us is to note the bi-directional
        action of a variable declaration.  The declaration together
        with anti-shadowing rules can maintain the IMiaB property for
        the block containing the declaration and all nested sub
        blocks.  But, the IMiaB property also forces all surrounding
        blocks to avoid using the name.  We thus need to maintain a
        blacklist of taboo names in all surrounding blocks -- and we
        take the expedient of doing so simply: actually maintaining a
        (superset of the) blacklist in each block data structure, which
        we call the 'known_variable' list.

        Because we create the 'known_variable' list during the parse
        process, by the time we do simple name resolution, all the
        blacklists are fully populated.  So, we can just enforce the
        rest of the IMiaB property by looking up a couple of lists.

        This turns out to be quite efficient: when we used a block
        tree walk, a test case took 5-10mins, while with this simple
        mildly-redundant data structure, the time taken for the same
        test case came down to a couple of seconds.

        The IKnownVariable interface is a small wrinkle.  Firstly, the
        IMiaB also applies to parameter names, especially those of
        anonymous methods.  Secondly, we need more information than
        just the name in the blacklist -- we need the location of the
        name and where it's declared.  We use the IKnownVariable
        interface to abstract out the parser information stored for
        local variables and parameters.

* The semantic analysis 

        Hence, the compiler driver has to parse all the input files.
        Once all the input files have been parsed, and an internal
        representation of the input program exists, the following
        steps are taken:

                * The interface hierarchy is resolved first.
                  As the interface hierarchy is constructed,
                  TypeBuilder objects are created for each one of
                  them. 

                * Classes and structure hierarchy is resolved next,
                  TypeBuilder objects are created for them.

                * Constants and enumerations are resolved.

                * Method, indexer, properties, delegates and event
                  definitions are now entered into the TypeBuilders. 

                * Elements that contain code are now invoked to
                  perform semantic analysis and code generation.

* Output Generation

** Code Generation

        The EmitContext class is created any time that IL code is to
        be generated (methods, properties, indexers and attributes all
        create EmitContexts).  

        The EmitContext keeps track of the current namespace and type
        container.  This is used during name resolution.

        An EmitContext is used by the underlying code generation
        facilities to track the state of code generation:

                * The ILGenerator used to generate code for this
                  method.

                * The TypeContainer where the code lives, this is used
                  to access the TypeBuilder.

                * The DeclSpace, this is used to resolve names through
                  RootContext.LookupType in the various statements and
                  expressions. 
        
        Code generation state is also tracked here:

                * CheckState:

                  This variable tracks the `checked' state of the
                  compilation, it controls whether we should generate
                  code that does overflow checking, or if we generate
                  code that ignores overflows.
                  
                  The default setting comes from the command line
                  option to generate checked or unchecked code plus
                  any source code changes using the checked/unchecked
                  statements or expressions.  Contrast this with the
                  ConstantCheckState flag.

                * ConstantCheckState
                  
                  The constant check state is always set to `true' and
                  cant be changed from the command line.  The source
                  code can change this setting with the `checked' and
                  `unchecked' statements and expressions.
                  
                * IsStatic
                  
                  Whether we are emitting code inside a static or
                  instance method
                  
                * ReturnType
                  
                  The value that is allowed to be returned or NULL if
                  there is no return type.
                  
                * ReturnLabel 

                  A `Label' used by the code if it must jump to it.
                  This is used by a few routines that deals with exception
                  handling.

                * HasReturnLabel

                  Whether we have a return label defined by the toplevel
                  driver.
                  
                * ContainerType
                  
                  Points to the Type (extracted from the
                  TypeContainer) that declares this body of code
                  summary>
                  
                  
                * IsConstructor
                  
                  Whether this is generating code for a constructor

                * CurrentBlock

                  Tracks the current block being generated.

                * ReturnLabel;
                
                  The location where return has to jump to return the
                  value

        A few variables are used to track the state for checking in
        for loops, or in try/catch statements:

                * InFinally
                
                  Whether we are in a Finally block

                * InTry

                  Whether we are in a Try block

                * InCatch
                  
                  Whether we are in a Catch block

                * InUnsafe
                  Whether we are inside an unsafe block

        Methods exposed by the EmitContext:

                * EmitTopBlock()

                  This emits a toplevel block. 

                  This routine is very simple, to allow the anonymous
                  method support to roll its two-stage version of this
                  routine on its own.

                * NeedReturnLabel ():

                  This is used to flag during the resolution phase that 
                  the driver needs to initialize the `ReturnLabel'

* Anonymous Methods

        The introduction of anonymous methods in the compiler changed
        various ways of doing things in the compiler.  The most
        significant one is the hard split between the resolution phase
        and the emission phases of the compiler.

        For instance, routines that referenced local variables no
        longer can safely create temporary variables during the
        resolution phase: they must do so from the emission phase,
        since the variable might have been "captured", hence access to
        it can not be done with the local-variable operations from the
        runtime.

        The code emission is in:

                EmitTopBlock ()

        Which drives the process, it first resolves the topblock, then
        emits the required metadata (local variable definitions) and
        finally emits the code.

        A detailed description of anonymous methods and iterators is
        on the new-anonymous-design.txt file in this directory.

* Lambda Expressions

        Lambda expressions can come in two forms: those that have implicit
        parameter types and those that have explicit parameter types, for
        example:

                Explicit:       

                        Foo ((int x) => x + 1);

                Implicit:

                        Foo (x => x + 1)


        One of the problems that we faced with lambda expressions is
        that lambda expressions need to be "probed" with different
        types until a working combination is found.

        For example:

            x => x.i

        The above expression could mean vastly different things depending
        on the type of "x".  The compiler determines the type of "x" (left
        hand side "x") at the moment the above expression is "bound",
        which means that during the compilation process it will try to
        match the above lambda with all the possible types available, for
        example:

        delegate int di (int x);
        delegate string ds (string s);
        ..
        Foo (di x) {}
        Foo (ds x) {}
        ...
        Foo (x => "string")

        In the above example, overload resolution will try "x" as an "int"
        and will try "x" as a string.  And if one of them "compiles" thats
        the one it picks (and it also copes with ambiguities if there was
        more than one matching method).

        To compile this, we need to hook into the resolution process,
        but since the resolution process has side effects (calling
        Resolve can either return instances of the resolved expression
        type, or can alter field internals) it was necessary to
        incorporate a framework to "clone" expressions before we
        probe.

        The support for cloning was added into Statements and
        Expressions and is only necessary for objects of those types
        that are created during parsing.   It is not necessary to
        support these in the classes that are the result of calling
        Resolve.   This means that SimpleName needs support for
        Cloning, but FieldExpr does not need it (SimpleName is created
        by the parser, FieldExpr is created during semantic analysis
        resolution).   

        The work happens through the public method called "Clone" that
        clones the given Statement or Expression.  The base method in
        Statement and Expression merely does a MemberwiseCopy of the
        elements and then calls the virtual CloneTo method to complete
        the copy.    By default this method throws an exception, this
        is useful to catch cases where we forgot to override CloneTo
        for a given Statement/Expression. 

        With the cloning capability it became possible to call resolve
        multiple times (once for each Cloned copy) and based on this
        picking the one implementation that would compile and that
        would not be ambiguous.

        The cloning process is basically a deep copy that happens in the
        LambdaExpression class and it clones the top-level block for the
        lambda expression.    The cloning has the side effect of cloning
        the entire containing block as well. 

        This happens inside this method:

        public override bool ImplicitStandardConversionExists (Type delegate_type)

        This is used to determine if the current Lambda expression can be
        implicitly converted to the given delegate type.

        And also happens as a result of the generic method parameter
        type inferencing. 

** Lambda Expressions and Cloning

        All statements that are created during the parsing method should
        implement the CloneTo method:

                protected virtual void CloneTo (CloneContext clonectx, Statement target)

        This method is called by the Statement.Clone method after it has
        done a shallow-copy of all the fields in the statement, and they
        should typically Clone any child statements.

        Expressions should implement the CloneTo method as well:

                protected virtual void CloneTo (CloneContext clonectx, Expression target)

** Lambda Expressions and Contextual Return

        When an expression is parsed as a lambda expression, the parser
        inserts a call to a special statement, the contextual return.

        The expression:

            a => a+1

        Is actually compiled as:

            a => contextual_return (a+1)

        The contextual_return statement will behave differently depending
        on the return type of the delegate that the expression will be
        converted to.

        If the delegate return type is void, the above will basically turn
        into an empty operation.   Otherwise the above will become
        a return statement that can infer return types.

* Evaluation API

        The compiler can now be used as a library, the API exposed
        lives in the Mono.CSharp.Evaluator class and it can currently
        compile statements and expressions passed as strings and
        compile or compile and execute immediately.

        As of April 2009 this creates a new in-memory assembly for
        each statement evaluated.   

        To support this evaluator mode, the evaluator API primes the
        tokenizer with an initial character that would not appear in
        valid C# code and is one of:

            int EvalStatementParserCharacter = 0x2190;   // Unicode Left Arrow
            int EvalCompilationUnitParserCharacter = 0x2191;  // Unicode Arrow
            int EvalUsingDeclarationsParserCharacter = 0x2192;  // Unicode Arrow

        These character are turned into the following tokens:

          %token EVAL_STATEMENT_PARSER
          %token EVAL_COMPILATION_UNIT_PARSER
          %token EVAL_USING_DECLARATIONS_UNIT_PARSER

        This means that the first token returned by the tokenizer when
        used by the Evalutor API is a special token that helps the
        yacc parser go from the traditional parsing of a full
        compilation-unit to the interactive parsing:

        The entry production for the compiler basically becomes:

          compilation_unit
                //
                // The standard rules
                //
                : outer_declarations opt_EOF  
                | outer_declarations global_attributes opt_EOF
                | global_attributes opt_EOF
                | opt_EOF /* allow empty files */

                //
                // The rule that allows interactive parsing
                //
                | interactive_parsing  { Lexer.CompleteOnEOF = false; } opt_EOF
                ;
          
          //
          // This is where Evaluator API drives the compilation
          //
          interactive_parsing
                : EVAL_STATEMENT_PARSER EOF 
                | EVAL_USING_DECLARATIONS_UNIT_PARSER using_directives
                | EVAL_STATEMENT_PARSER 
                  interactive_statement_list opt_COMPLETE_COMPLETION 
                | EVAL_COMPILATION_UNIT_PARSER 
                  interactive_compilation_unit
                ;
        
        Since there is a little bit of ambiguity for example in the
        presence of the using directive and the using statement a
        micro-predicting parser with multiple token look aheads is
        used in eval.cs to resolve the ambiguity and produce the
        actual token that will drive the compilation.

        This helps this scenario:
             using System;
           vs
             using (var x = File.OpenRead) {}

        This is the meaning of these new initial tokens:

             EVAL_STATEMENT_PARSER
                Used to parse statements or expressions as statements.

             EVAL_USING_DECLARATIONS_UNIT_PARSER
                This instructs the parser to merely do using-directive
                parsing instead of statement parsing. 

             EVAL_COMPILATION_UNIT_PARSER
                Used to evaluate toplevel declarations like namespaces
                and classes.   

                The feature is currently disabled because later stages
                of the compiler are not yet able to lookup previous
                definitions of classes.   

                What happens is that between each call to Evaluate()
                we reset the compiler state and at this stage we drop
                also any existing definitions, so evaluating "class X
                {}" followed by "class Y : X {}" does not currently
                work. 

                We need to make sure that new type definitions used
                interactively are preseved from one evaluation to the
                next. 

        The evaluator the expression or statement `BODY' is hosted
        inside a wrapper class.   If the statement is a variable
        declaration then the declaration is split from the assignment
        into a DECLARATION and BODY.   

        This is what the code generated looks like:

             public class Foo : $InteractiveBaseClass {
                    DECLARATION

                    static void Host (ref object $retval)
                    {
                        BODY
                    }
             }

        Since both statements and expressions are mixed together and
        it is useful to use the Evaluator to compute expressions we
        return expressions for example for "1+2" in the `retval'
        reference object. 

        To support this, the reference retval parameter is set to a
        special internal value that means "Value was not set" before
        the method Host is invoked.    During parsing the parser turns
        expressions like "1+2" into:

                    retval = 1 + 2;

        This is done using a special OptionalAssign
        ExpressionStatement class. 

        When the Host method return, if the value of retval is still
        the special flag no value was set.  Otherwise the result of
        the expression is in retval.

        The `InteractiveBaseClass' is the base class for the method,
        this allows for embedders to provide different base classes
        that could expose new static methods that could be useful
        during expression evaluation.
        
        Our default implementation is InteractiveBaseClass and new
        implementations should derive from this and set the property
        in the Evaluator to it. 

        In the future we will move to creating dynamic methods as the
        wrapper for this code.

* Code Completion

        Support for code completion is available to allow the compiler
        to provide a list of possible completions at any given point
        int he parsing process.   This is used for Tab-completion in
        an interactive shell or visual aids in GUI shells for possible
        method completions. 

        This method is available as part of the Evaluator API where a
        special method GetCompletions returns a list of possible
        completions given a partial input.

        The parser and tokenizer work together so that the tokenizer
        upon reaching the end of the input generates the following
        tokens: GENERATE_COMPLETION followed by as many
        COMPLETE_COMPLETION token and finally the EOF token.

        GENERATE_COMPLETION needs to be handled in every production
        where the user is likely to press the TAB key in the shell (or
        in the future the GUI, or an explicit request in an IDE).
        COMPLETE_COMPLETION must be handled throughout the grammar to
        provide a way of completing the parsed expression.  See below
        for details.

        For the member access case, I have added productions that
        mirror the non-completing productions, for example:

          primary_expression DOT IDENTIFIER GENERATE_COMPLETION 
          {
                LocatedToken lt = (LocatedToken) $3;
                $$ = new CompletionMemberAccess ((Expression) $1, lt.Value, lt.Location);
          }
        
        This mirrors:

          primary_expression DOT IDENTIFIER opt_type_argument_list
          {
                LocatedToken lt = (LocatedToken) $3;
                $$ = new MemberAccess ((Expression) $1, lt.Value, (TypeArguments) $4, lt.Location);
          }
        
        The CompletionMemberAccess is a new kind of
        Mono.CSharp.Expression that does the actual lookup.  It
        internally mimics some of the MemberAccess code but has been
        tuned for this particular use.

        After this initial token is processed GENERATE_COMPLETION the
        tokenizer will emit COMPLETE_COMPLETION tokens.  This is done
        to help the parser basically produce a valid result from the
        partial input it received.  For example it is able to produce
        a valid AST from "(x" even if no parenthesis has been closed.
        This is achieved by sprinkling the grammar with productions
        that can cope with this "winding down" token, for example this
        is what parenthesized_expression looks like now:

          parenthesized_expression
                  : OPEN_PARENS expression CLOSE_PARENS
                    {
                          $$ = new ParenthesizedExpression ((Expression) $2);
                    }
                  //
                  // New production
                  //
                  | OPEN_PARENS expression COMPLETE_COMPLETION
                    {
                          $$ = new ParenthesizedExpression ((Expression) $2);
                    }
                  ;
          
          Once we have wrapped up everything we generate the last EOF token.

        When the AST is complete we actually trigger the regular
        semantic analysis process.  The DoResolve method of each node
        in our abstract syntax tree will compute the result and
        communicate the possible completions by throwing an exception
        of type CompletionResult.

        So for example if the user type "T" and the completion is
        "ToString" we return "oString".

** Enhancing Completion

        Code completion is a process that will be curated over time.  
        Just like producing good error reports or warnings is an
        iterative process to find a good balance, the code completion
        engine in the compiler will require tuning to find the right
        balance for the end user. 

        This section explains the basic process by which you can
        improve the code completion by using a real life sample.

        Once you add the GENERATE_COMPLETION token to your grammar
        rule, chances are, you will need to alter the grammar to
        support COMPLETE_COMPLETION all the way up to the toplevel
        production.

        To debug this, you will want to try the completion with either
        a sample program or with the `csharp' tool.

        I use this setup:

        $ csharp -v -v

        This will turn on the parser debugging output and will
        generate a lot of data when parsing its input.

        To start with a new completion scheme, type your C# code and
        then hit the tab key to trigger the completion engine.  In the
        generated output you will want to look for the first time that
        the parser got the GENERATE_COMPLETION token, it will look
        like this:

        lex     state 414       reading GENERATE_COMPLETION value {interactive}(1,35):

        The first word `lex' indicates that the parser called the
        lexer at state 414 (more on this in a second) and it got back
        from the lexer the token GENERATE_COMPLETION.   If this is a
        kind of completion chances are, you will get an error
        immediately as the rules at that point do not know how to cope
        with the stream of COMPLETE_COMPLETION tokens that will
        follow, they will look like this:

        error   syntax error
        pop     state 414       on error
        pop     state 805       on error
        pop     state 628       on error
        pop     state 417       on error
        
        The first line means that the parser has entered the error
        state and will pop states until it can find a production that
        can deal with the error.   At that point an error message will
        be displayed.

        Open the file `y.output' which describes the parser states
        generated by jay and search for the state that was reported
        previously in `lex' that got the GENERATE_COMPLETION:

        state 414
                object_or_collection_initializer : OPEN_BRACE . opt_member_initializer_list CLOSE_BRACE  (444)
                object_or_collection_initializer : OPEN_BRACE . member_initializer_list COMMA CLOSE_BRACE  (445)
                opt_member_initializer_list : .  (446)
        
        We now know that the parser was in the middle of parsing an
        `object_or_collection_initializer' and had alread seen the
        OPEN_BRACE token.

        The `.' after OPEN_BRACE indicates the current state of the
        parser, and this is where our parser got the
        GENERATE_COMPLETION token.   As you can see from the three
        rules in this sample, support for GENERATE_COMPLETION did not
        exist.

        So we must edit the grammar to add a production for this case,
        I made the code look like this:

        member_initializer
                [...]
                | GENERATE_COMPLETION 
                  {
                        LocatedToken lt = $1 as LocatedToken;
                        $$ = new CompletionElementInitializer (GetLocation ($1));
                  }
                [...]

        This new production creates the class
        CompletionElementInitializer and returns this as the value for
        this.   The following is a trivial implementation that always
        returns "foo" and "bar" as the two completions and it
        illustrates how things work:

        public class CompletionElementInitializer : CompletingExpression {
                public CompletionElementInitializer (Location l)
                {
                        this.loc = l;
                }

                public override Expression DoResolve (EmitContext ec)
                {
                        string [] = new string [] { "foo", "bar" };
                        throw new CompletionResult ("", result);
                }

                // 
                // You should implement CloneTo if your CompletingExpression
                // keeps copies to Statements or Expressions.   CloneTo
                // is used by the lambda engine, so you should always
                // implement this
                //
                protected override void CloneTo (CloneContext clonectx, Expression t)
                {
                        // We do not keep references to anything interesting
                        // so cloning is an empty operation.
                }
        }
        

        We then rebuild our compiler:

        (cd mcs/; make cs-parser.jay)
        (cd tools/csharplib; make install)

        And re-run csharp:

        (cd tools/csharp; csharp -v -v)

        Chances are, you will get another error, but this time it will
        not be for the GENERATE_COMPLETION, we already handled that
        one.   This time it will be for COMPLETE_COMPLETION.  

        The remaining of the process is iterative: you need to locate
        the state where this error happens.   It will look like this:

        lex     state 623       reading COMPLETE_COMPLETION     value {interactive}(1,35):
        error   syntax error
        
        And make sure that the state can handle at this point a
        COMPLETE_COMPLETION.   When receiving COMPLETE_COMPLETION the
        parser needs to complete constructing the parse tree, so
        productions that handle COMPLETE_COMPLETION need to wrap
        things up with whatever data they have available and just make
        it so that the parser can complete.

        To avoid rule duplication you can use the
        opt_COMPLETE_COMPLETION production and append it to an
        existing production:

        foo : bar opt_COMPLETE_COMPLETION {
            ..
        }

* Miscellaneous

** Error Processing.

        Errors are reported during the various stages of the
        compilation process.  The compiler stops its processing if
        there are errors between the various phases.  This simplifies
        the code, because it is safe to assume always that the data
        structures that the compiler is operating on are always
        consistent.

        The error codes in the Mono C# compiler are the same as those
        found in the Microsoft C# compiler, with a few exceptions
        (where we report a few more errors, those are documented in
        mcs/errors/errors.txt).  The goal is to reduce confusion to
        the users, and also to help us track the progress of the
        compiler in terms of the errors we report. 

        The Report class provides error and warning display functions,
        and also keeps an error count which is used to stop the
        compiler between the phases.  

        A couple of debugging tools are available here, and are useful
        when extending or fixing bugs in the compiler.  If the
        `--fatal' flag is passed to the compiler, the Report.Error
        routine will throw an exception.  This can be used to pinpoint
        the location of the bug and examine the variables around the
        error location.

        Warnings can be turned into errors by using the `--werror'
        flag to the compiler. 

        The report class also ignores warnings that have been
        specified on the command line with the `--nowarn' flag.

        Finally, code in the compiler uses the global variable
        RootContext.WarningLevel in a few places to decide whether a
        warning is worth reporting to the user or not.  

** Debugging the compiler

        Sometimes it is convenient to find *how* a particular error
        message is being reported from, to do that, you might want to use
        the --fatal flag to mcs.  The flag will instruct the compiler to 
        abort with a stack trace execution when the error is reported.

        You can use this with -warnaserror to obtain the same effect
        with warnings. 

** Debugging the Parser.

        A useful trick while debugging the parser is to pass the -v
        command line option to the compiler.

        The -v command line option will dump the various Yacc states
        as well as the tokens that are being returned from the
        tokenizer to the compiler.

        This is useful when tracking down problems when the compiler
        is not able to parse an expression correctly.

        You can match the states reported with the contents of the
        y.output file, a file that contains the parsing tables and
        human-readable information about the generated parser.

* Editing the compiler sources

        The compiler sources are intended to be edited with 134
        columns of width.

* Quick Hacks

        Once you have a full build of mcs, you can improve your
        development time by just issuing make in the `mcs' directory or
        using `make qh' in the gmcs directory.