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. module.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'. To debug problems in your grammar, you need to edit the Makefile and make sure that the -ct options are passed to jay. The current incarnation says: ./../jay/jay -c < ./../jay/skeleton.cs cs-parser.jay During debugging, you want to change this to: ./../jay/jay -cvt < ./../jay/skeleton.cs cs-parser.jay This generates a parser with debugging information and allows you to activate verbose parser output in both the csharp command and the mcs command by passing the "-v -v" flag (-v twice). When you do this, standard output will have a dump of the tokens parsed and how the parser reacted to those. You can look up the states with the y.output file that contains the entire parser state diagram in human readable form. ** 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 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 `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 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; ** Optimalizations Compiler does some limited high-level optimalizations when -optimize option is used *** Instance field initializer to default value Code to optimize: class C { enum E { Test } int i = 0; // Field will not be redundantly assigned int i2 = new int (); // This will be also completely optimized out E e = E.Test; // Even this will go out. } ** 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. * References loading Most programs use external references (assemblies and modules). Compiler loads all referenced top-level types from referenced assemblies into import cached. It imports initialy only C# valid top-level types all other members are imported on demand when needed. * Namespaces definition Before any type resolution can be done we define all compiled namespaces. This is mainly done to prepare using clauses of each namespace block before any type resolution takes a place. * Types definition The first step of type definition is to resolve base class or base interfaces to correctly setup type hierarchy before any member is defined. At this point we do some error checking and verify that the members inheritance is correct and some other members oriented checks. By the time we are done, all classes, structs and interfaces have been defined and all their members have been defined as well. * MemberCache MemberCache is one of core compiler components. It maintains information about types and their members. It tries to be as fast as possible because almost all resolve operations end up querying members info in some way. MemberCache is not definition but specification oriented to maintain differences between inflated versions of generic types. This makes usage of MemberCache simple because consumer does not need to care how to inflate current member and returned type information will always give correctly inflated type. However setting MemberCache up is one of the most complicated parts of the compiler due to possible dependencies when types are defined and complexity of nested types. * 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. * Debugger support Compiler produces .mdb symbol file for better debugging experience. The process is quite straightforward. For every statement or a block there is an entry in symbol file. Each entry includes of start location of the statement and it's starting IL offset in the method. For most statements this is easy but few need special handling (e.g. do, while). When sequence point is needed to represent original location and no IL entry is written for the line we emit `nop' instruction. This is done only for very few constructs (e.g. block opening brace). Captured variables are not treated differently at the moment. Debugger has internal knowledge of their mangled names and how to decode them. * IKVM.Reflection vs System.Reflection Mono compiler can be compiled using different reflection backends. At the moment we support System.Reflection and IKVM.Reflection they both use same API as official System.Reflection.Emit API which allows us to maintain only single version of compiler with few using aliases to specialise. The backends are not plug-able but require compiler to be compiled with specific STATIC define when targeting IKVM.Reflection. IKVM.Reflection is used for static compilation. This means the compiler runs in batch mode like most compilers do. It can target any runtime version and use any mscorlib. The mcs.exe is using IKVM.Reflection. System.Reflection is used for dynamic compilation. This mode is used by our REPL and Evaluator API. Produced IL code is not written to disc but executed by runtime (JIT). Mono.CSharp.dll is using System.Reflection and System.Reflection.Emit. * 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 and 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 (make sure that your parser has been compiled with the -v flag, see above for details). 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 class/Mono.CSharp; 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. If you pass a number to --fatal the exception will only be thrown when the error count reaches the specified count. 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.