- Timestamp:
- Apr 6, 2021, 9:16:52 PM (4 years ago)
- Branches:
- ADT, arm-eh, ast-experimental, enum, forall-pointer-decay, jacob/cs343-translation, master, new-ast-unique-expr, pthread-emulation, qualifiedEnum
- Children:
- e07b589
- Parents:
- 7039ab9
- File:
-
- 1 edited
Legend:
- Unmodified
- Added
- Removed
-
doc/theses/andrew_beach_MMath/features.tex
r7039ab9 r8483c39a 3 3 This chapter covers the design and user interface of the \CFA 4 4 exception-handling mechanism (EHM). % or exception system. 5 While an EHM is free to add many features, 6 the following overview covers the basic features that all EHMs use, but it is not an 7 exhaustive list of everything an EHM can do. 5 8 6 9 % We should cover what is an exception handling mechanism and what is an … … 9 12 \paragraph{Raise / Handle} 10 13 An exception operation has two main parts: raise and handle. 11 These are the two parts that the user will write themselves and so12 might be the only two pieces of the EHM that have any syntax.13 14 These terms are sometimes also known as throw and catch but this work uses 14 15 throw/catch as a particular kind of raise/handle. 16 These are the two parts a programmer writes and so 17 are the only two pieces of the EHM that have language syntax. 15 18 16 19 \subparagraph{Raise} 17 The raise is the starting point for exception handling and usually how 18 Some well known examples include the throw statementsof \Cpp and Java and19 the raisestatement from Python.20 21 For this overview a raise does nothing more kick offthe handling of an22 exception, which is called raising the exception. This is inexact but close23 enough for the broad strokes ofthe overview.20 The raise is the starting point for exception handling and usually how \PAB{This sentence is cut off.} 21 Some well known examples include the @throw@ statement of \Cpp and Java and 22 the \lstinline[language=Python]{raise} statement from Python. 23 24 For this overview, a raise starts the handling of an 25 exception, which is called \newterm{raising} an exception. This simple description is sufficient 26 for the overview. 24 27 25 28 \subparagraph{Handle} 26 The purpose of most exception operations is to run some sort of handler that27 contains user code.28 The try statement of \Cpp illistrates the common features29 Handlers have three common features: a region of code they apply to, an30 exception label that describes what exceptions they handleand code to run31 when they handle an exception.32 Each handler can handle exceptions raised in th at region that match their33 exception label. Different EHMs will have different rules to pick a handler34 if multipe handlers could be usedsuch as ``best match" or ``first found".29 The purpose of raising an exception is to run user code to address (handle) the 30 issue found at the raise point. 31 The @try@ statement of \Cpp illustrates a common approach for specifying multiple handlers. 32 A handler has three common features: the scope in which it applies, an 33 exception label that describes what exceptions it can handle, and code to run 34 that deals with the raised issue. 35 Each handler can handle exceptions raised in the region matching its 36 exception label. For multiple matches, different EHMs have different rules for matching an exception to a handler label, 37 such as ``best match" or ``first found". 35 38 36 39 \paragraph{Propagation} 37 After an exception is raised comes what is usually the biggest step for the 38 EHM, finding and setting up the handler. This can be broken up into three 39 different tasks: searching for a handler, matching against the handler and 40 installing the handler. 41 42 First the EHM must search for possible handlers that could be used to handle 43 the exception. Searching is usually independent of the exception that was 44 thrown and instead depends on the call stack, the current function, its caller 40 After an exception is raised, comes the most complex step for the 41 EHM: finding and setting up the handler. This propagation of exception from raise to handler can be broken up into three 42 different tasks: searching, matching, and 43 installing the handler so it can execute. 44 45 \subparagraph{Searching} 46 The EHM searches for possible handlers that can be used to handle 47 the exception. Searching is usually independent of the exception that is 48 thrown and instead depends on the call stack: current function, its caller 45 49 and repeating down the stack. 46 50 47 Second it much match the exception with each handler to see which one is the 48 best match and hence which one should be used to handle the exception. 49 In languages where the best match is the first match these two are often 50 intertwined, a match check is preformed immediately after the search finds 51 \subparagraph{Matching} 52 For each handler found, it compares the raised exception with the handler label to see which one is the 53 best match, and hence, which one should be used to handle the exception. 54 In languages where the best match is the first match, these two steps are often 55 intertwined, \ie a match check is performed immediately after the search finds 51 56 a possible handler. 52 57 53 Third, after a handler is chosen it must be made ready to run. 54 What this actually involves can vary widely to fit with the rest of the 55 design of the EHM. The installation step might be trivial or it could be 58 \subparagraph{Installing} 59 After a handler is chosen, it must be made ready to run. 60 This step varies widely to fit with the rest of the 61 design of the EHM. The installation step might be trivial or it can be 56 62 the most expensive step in handling an exception. The latter tends to be the 57 63 case when stack unwinding is involved. 58 59 As an alternate third step if no appropriate handler is found then some sort 60 of recovery has to be preformed. This is only required with unchecked 61 exceptions as checked exceptions can promise that a handler is found. It also 62 is also installing a handler but it is a special default that may be 64 An alternate action occurs if no appropriate handler is found, then some implicit action 65 is performed. This step is only required with unchecked 66 exceptions as checked exceptions (Java) promise a handler is always found. The implicit action 67 also installs a handler but it is a default handle that may be 63 68 installed differently. 64 69 65 70 \subparagraph{Hierarchy} 66 In \CFA the EHM uses a hierarchial system to organise its exceptions.67 This strat agy is borrowed from object-orientated languages where the71 Some EHM (\CFA, Java) organize exceptions in a hierarchical structure. 72 This strategy is borrowed from object-orientated languages where the 68 73 exception hierarchy is a natural extension of the object hierarchy. 69 74 70 75 Consider the following hierarchy of exceptions: 71 76 \begin{center} 72 \setlength{\unitlength}{4000sp}% 73 \begin{picture}(1605,612)(2011,-1951) 74 \put(2100,-1411){\vector(1, 0){225}} 75 \put(3450,-1411){\vector(1, 0){225}} 76 \put(3550,-1411){\line(0,-1){225}} 77 \put(3550,-1636){\vector(1, 0){150}} 78 \put(3550,-1636){\line(0,-1){225}} 79 \put(3550,-1861){\vector(1, 0){150}} 80 \put(2025,-1490){\makebox(0,0)[rb]{\LstBasicStyle{exception}}} 81 \put(2400,-1460){\makebox(0,0)[lb]{\LstBasicStyle{arithmetic}}} 82 \put(3750,-1460){\makebox(0,0)[lb]{\LstBasicStyle{underflow}}} 83 \put(3750,-1690){\makebox(0,0)[lb]{\LstBasicStyle{overflow}}} 84 \put(3750,-1920){\makebox(0,0)[lb]{\LstBasicStyle{zerodivide}}} 85 \end{picture}% 77 \input{exceptionHierarchy} 86 78 \end{center} 87 88 79 A handler labelled with any given exception can handle exceptions of that 89 80 type or any child type of that exception. The root of the exception hierarchy 90 (here \ texttt{exception}) acts as a catch-all, leaf types catch single types81 (here \lstinline[language=C++]{exception}) acts as a catch-all, leaf types catch single types 91 82 and the exceptions in the middle can be used to catch different groups of 92 83 related exceptions. 93 84 94 85 This system has some notable advantages, such as multiple levels of grouping, 95 the ability for libraries to add new exception types and the isolation96 between different sub-hierarchies. So the design was adapted fora86 the ability for libraries to add new exception types, and the isolation 87 between different sub-hierarchies. This capability had to be adapted for \CFA, which is a 97 88 non-object-orientated language. 98 89 … … 100 91 101 92 \paragraph{Completion} 102 After the handler has finished the entire exception operation has to complete 103 and continue executing somewhere else. This step is usually very simple 104 both logically and in its implementation as the installation of the handler 105 usually does the heavy lifting. 106 107 The EHM can return control to many different places. 108 However, the most common is after the handler definition and the next most 109 common is after the raise. 93 After the handler has returned, the entire exception operation has to complete 94 and continue executing somewhere. This step is usually simple, 95 both logically and in its implementation, as the installation of the handler 96 usually does the preparation. 97 The EHM can return control to different places, 98 where the most common are after the handler definition or after the raise. 110 99 111 100 \paragraph{Communication} 112 For effective exception handling, additional information is usually required 113 as this base model only communicates the exception's identity. Common 114 additional methods of communication are putting fields on an exception and 115 allowing a handler to access the lexical scope it is defined in (usually 116 a function's local variables). 117 118 \paragraph{Other Features} 119 Any given exception handling mechanism is free at add other features on top 120 of this. This is an overview of the base that all EHMs use but it is not an 121 exaustive list of everything an EHM can do. 101 For effective exception handling, additional information is usually passed from the raise, 102 where this basic model only communicates the exception's identity. A common 103 methods for communication is putting fields into an exception and 104 allowing a handler to access these fields via an exception instance in the handler's scope. 122 105 123 106 \section{Virtuals} 124 Virtual types and casts are not part of the exception systemnor are they125 required for an exception system. But an object-oriented style hierarchy is a126 great way of organizing exceptions soa minimal virtual system has been added127 to \CFA .107 Virtual types and casts are not part of an EHM nor are they 108 required for an EHM. But as pointed out, an object-oriented-style hierarchy is an 109 excellent way of organizing exceptions. Hence, a minimal virtual system has been added 110 to \CFA to support hierarchical exceptions. 128 111 129 112 The virtual system supports multiple ``trees" of types. Each tree is 130 113 a simple hierarchy with a single root type. Each type in a tree has exactly 131 one parent - except for the root type which has zero parents- and any114 one parent -- except for the root type with zero parents -- and any 132 115 number of children. 133 116 Any type that belongs to any of these trees is called a virtual type. … … 135 118 % A type's ancestors are its parent and its parent's ancestors. 136 119 % The root type has no ancestors. 137 % A type's decendents are its children and its children's decendents. 138 139 Every virtual type also has a list of virtual members. Children inherit 140 their parent's list of virtual members but may add new members to it. 141 It is important to note that these are virtual members, not virtual methods. 142 However as function pointers are allowed they can be used to mimic virtual 143 methods as well. 144 145 The unique id for the virtual type and all the virtual members are combined 146 into a virtual table type. Each virtual type has a pointer to a virtual table 120 % A type's descendents are its children and its children's descendents. 121 122 Every virtual type has a list of virtual members. Children inherit 123 their parent's virtual members but may add new members to it. 124 It is important to note that these are virtual members, not virtual methods of an object type. 125 However, as \CFA has function pointers, they can be used to mimic virtual 126 methods. 127 128 Each virtual type has a unique id. 129 The unique id for the virtual type and all its virtual members are combined 130 into a virtual-table type. Each virtual type has a pointer to a virtual table 147 131 as a hidden field. 148 132 149 Up until this point the virtual system is a lot like ones found in object-150 orientated languages but this where they diverge. Objects encapsulate a133 Up to this point, a virtual system is similar to ones found in object-oriented 134 languages but this is where \CFA diverges. Objects encapsulate a 151 135 single set of behaviours in each type, universally across the entire program, 152 and indeed all programs that use that type definition. In this sense the136 and indeed all programs that use that type definition. In this sense, the 153 137 types are ``closed" and cannot be altered. 154 155 However in \CFA types do not encapsulate any behaviour. Traits are local and 156 types can begin to statify a trait, stop satifying a trait or satify the same 157 trait in a different way with each new definition. In this sense they are 158 ``open" as they can change at any time. This means it is implossible to pick 159 a single set of functions that repersent the type. 160 161 So we don't try to have a single value. The user can define virtual tables 162 which are filled in at their declaration and given a name. Anywhere you can 163 see that name you can use that virtual table; even if it is defined locally 164 inside a function, although in that case you must respect its lifetime. 165 166 An object of a virtual type is ``bound" to a virtual table instance which 138 However, \CFA types do not encapsulate any behaviour. Instead, traits are used and 139 types can satisfy a trait, stop satisfying a trait, or satisfy the same 140 trait in a different way depending on the lexical context. In this sense, the types are 141 ``open" as their behaviour can change in different scopes. This capability means it is impossible to pick 142 a single set of functions that represent the type's virtual members. 143 144 Hence, \CFA does not have a single virtual table for a type. A user can define different virtual tables, 145 which are filled in at their declaration and given a name. 146 That name is used as the virtual table, even if it is defined locally 147 inside a function, although lifetime issues must be considered. 148 Specifically, an object of a virtual type is ``bound" to a virtual table instance, which 167 149 sets the virtual members for that object. The virtual members can be accessed 168 150 through the object. … … 178 160 \Cpp syntax for special casts. Both the type of @EXPRESSION@ and @TYPE@ must be 179 161 a pointer to a virtual type. 180 The cast dynamically checks if the @EXPRESSION@ type is the same or a sub -type162 The cast dynamically checks if the @EXPRESSION@ type is the same or a subtype 181 163 of @TYPE@, and if true, returns a pointer to the 182 164 @EXPRESSION@ object, otherwise it returns @0p@ (null pointer). … … 196 178 \end{cfa} 197 179 The trait is defined over two types, the exception type and the virtual table 198 type. Th is should be one-to-one,each exception type has only one virtual180 type. These type should have a one-to-one relationship: each exception type has only one virtual 199 181 table type and vice versa. The only assertion in the trait is 200 182 @get_exception_vtable@, which takes a pointer of the exception type and 201 returns a reference to the virtual table typeinstance.183 returns a reference to the virtual-table type-instance. 202 184 203 185 The function @get_exception_vtable@ is actually a constant function. 204 Regardless of the value passed in (including the null pointer) it should205 return a reference to the virtualtable instance for that type.206 The reason it is a function instead of a constant is t hat itmake type207 annotations easier to write as you can use the exception type instead ofthe208 virtual table type; which usually has a mangled name.186 Regardless of the value passed in (including the null pointer) it 187 returns a reference to the virtual-table instance for that type. 188 The reason it is a function instead of a constant is to make type 189 annotations easier to write using the exception type rather than the 190 virtual-table type, which usually has a mangled name because it is an internal component of the EHM. 209 191 % Also \CFA's trait system handles functions better than constants and doing 210 192 % it this way reduce the amount of boiler plate we need. … … 212 194 % I did have a note about how it is the programmer's responsibility to make 213 195 % sure the function is implemented correctly. But this is true of every 214 % similar system I know of (except Agda's I guess) so I took it out. 215 216 There are two more traits for exceptions @is_termination_exception@ and 217 @is_resumption_exception@. They are defined as follows: 218 196 % similar system I know of (except Ada's I guess) so I took it out. 197 198 There are two more exception traits defined as follows: 219 199 \begin{cfa} 220 200 trait is_termination_exception( … … 228 208 }; 229 209 \end{cfa} 230 231 In other words they make sure that a given type and virtual type is an 232 exception and defines one of the two default handlers. These default handlers 233 are used in the main exception handling operations \see{Exception Handling} 234 and their use will be detailed there. 235 236 However all three of these traits can be tricky to use directly. 237 There is a bit of repetition required but 238 the largest issue is that the virtual table type is mangled and not in a user 239 facing way. So there are three macros that can be used to wrap these traits 240 when you need to refer to the names: 210 These traits ensure a given type and virtual type are an 211 exception type and defines one of the two default handlers. The default handlers 212 are used in the main exception-handling operations and discussed in detail in \VRef{s:ExceptionHandling}. 213 214 However, all three of these traits are tricky to use directly. 215 While there is a bit of repetition required, 216 the largest issue is that the virtual-table type is mangled and not in a user 217 facing way. So three macros are provided to wrap these traits 218 to simplify referring to the names: 241 219 @IS_EXCEPTION@, @IS_TERMINATION_EXCEPTION@ and @IS_RESUMPTION_EXCEPTION@. 242 220 243 Alltake one or two arguments. The first argument is the name of the244 exception type. Its unmangled and mangled form are passedto the trait.221 These macros take one or two arguments. The first argument is the name of the 222 exception type. The macro passes the unmangled and mangled form to the trait. 245 223 The second (optional) argument is a parenthesized list of polymorphic 246 arguments. This argument should onlywith polymorphic exceptions and the247 list will bepassed to both types.248 In the current set-up the base name and the polymorphic arguments have to224 arguments. This argument is only used with polymorphic exceptions and the 225 list is passed to both types. 226 In the current set-up, the base name and the polymorphic arguments have to 249 227 match so these macros can be used without losing flexibility. 250 228 … … 252 230 defined arithmetic exception: 253 231 \begin{cfa} 254 forall(Num | IS_EXCEPTION(Arithmetic, (Num)))232 forall(Num | @IS_EXCEPTION(Arithmetic, Num)@) 255 233 void some_math_function(Num & left, Num & right); 256 234 \end{cfa} 235 where the function may raise exception @Arithmetic@ or any of its decedents. 257 236 258 237 \section{Exception Handling} 259 \CFA provides two kinds of exception handling, termination and resumption. 260 These twin operations are the core of the exception handling mechanism and 261 are the reason for the features of exceptions. 262 This section will cover the general patterns shared by the two operations and 263 then go on to cover the details each individual operation. 264 265 Both operations follow the same set of steps to do their operation. They both 266 start with the user preforming a throw on an exception. 267 Then there is the search for a handler, if one is found than the exception 268 is caught and the handler is run. After that control returns to normal 269 execution. 270 271 If the search fails a default handler is run and then control 272 returns to normal execution immediately. That is where the default handlers 273 @defaultTermiationHandler@ and @defaultResumptionHandler@ are used. 238 \label{s:ExceptionHandling} 239 \CFA provides two kinds of exception handling: termination and resumption. 240 These twin mechanisms are the core of the \CFA EHM and 241 multiple features are provided to support them. 242 This section covers the general patterns shared by the two kinds of exceptions and 243 then covers the individual detail operations. 244 245 Both mechanisms follow the same set of steps to do their operations. Both 246 start with the user performing an exception raise. 247 Then there is the handler search. If one is found, than the exception 248 is caught and the handler is run. When the handler returns, control returns to an 249 location appropriate for each kind of exception. 250 251 \begin{sloppypar} 252 If the search fails, an appropriate default handler, @defaultTermiationHandler@ 253 or @defaultResumptionHandler@, is run and control returns to the 254 appropriate location. 255 \end{sloppypar} 274 256 275 257 \subsection{Termination} 276 258 \label{s:Termination} 277 278 Termination handling is more familiar kind and used in most programming 259 Termination handling is familiar and used in most programming 279 260 languages with exception handling. 280 It is dynamic, non-local goto. If a throw is successful then the stack will281 be unwound and control will (usually) continue in a differentfunction on282 the call stack . They are commonly used when an error has occurred andrecovery283 is impossible in the current function.261 It is a dynamic, non-local goto. The raise starts searching, and if matched and handled, the stack is 262 unwound and control (usually) continues in the function on 263 the call stack containing the handler. Terminate is commonly used for an error where recovery 264 is impossible in the function performing the raise. 284 265 285 266 % (usually) Control can continue in the current function but then a different 286 267 % control flow construct should be used. 287 268 288 A termination throwis started with the @throw@ statement:269 A termination raise is started with the @throw@ statement: 289 270 \begin{cfa} 290 271 throw EXPRESSION; 291 272 \end{cfa} 292 273 The expression must return a reference to a termination exception, where the 293 termination exception is any type that satisfies @is_termination_exception@ 294 at the call site. 295 Through \CFA's trait system the functions in the traits are passed into the 296 throw code. A new @defaultTerminationHandler@ can be defined in any scope to 297 change the throw's behavior (see below). 298 299 The throw will copy the provided exception into managed memory. It is the 300 user's responsibility to ensure the original exception is cleaned up if the 301 stack is unwound (allocating it on the stack should be sufficient). 302 303 Then the exception system searches the stack using the copied exception. 304 It starts starts from the throw and proceeds to the base of the stack, 274 termination exception is any type that satisfies trait 275 @is_termination_exception@ at the call site. Through \CFA's trait system, the 276 trait functions are implicitly passed into the hidden throw code and available 277 to the exception system while handling the exception. A new 278 @defaultTerminationHandler@ can be defined in any scope to change the throw's 279 unhandled behaviour (see below). 280 281 The throw must copy the provided exception into managed memory because the stack is unwounded. 282 The lifetime of the exception copy is managed by the exception runtime. 283 It is the user's responsibility to ensure the original exception is cleaned up, where allocating it on the unwound stack is sufficient. 284 285 The exception search walks the stack matching with the copied exception. 286 It starts from the throwing function and proceeds to the base of the stack, 305 287 from callee to caller. 306 At each stack frame, a check is made for resumption handlers defined by the288 At each stack frame, a check is made for termination handlers defined by the 307 289 @catch@ clauses of a @try@ statement. 308 290 \begin{cfa} 309 291 try { 310 292 GUARDED_BLOCK 311 } catch (EXCEPTION_TYPE$\(_1\)$ * NAME$\(_1\)$) {293 } catch (EXCEPTION_TYPE$\(_1\)$ [* NAME$\(_1\)$]) { 312 294 HANDLER_BLOCK$\(_1\)$ 313 } catch (EXCEPTION_TYPE$\(_2\)$ * NAME$\(_2\)$) {295 } catch (EXCEPTION_TYPE$\(_2\)$ [* NAME$\(_2\)$]) { 314 296 HANDLER_BLOCK$\(_2\)$ 315 297 } 316 298 \end{cfa} 317 When viewed on its own a try statement will simply execute the statements in 318 @GUARDED_BLOCK@ and when those are finished the try statement finishes. 319 320 However, while the guarded statements are being executed, including any 321 functions they invoke, all the handlers following the try block are now 322 or any functions invoked from those 323 statements, throws an exception, and the exception 324 is not handled by a try statement further up the stack, the termination 325 handlers are searched for a matching exception type from top to bottom. 326 327 Exception matching checks the representation of the thrown exception-type is 328 the same or a descendant type of the exception types in the handler clauses. If 329 it is the same of a descendant of @EXCEPTION_TYPE@$_i$ then @NAME@$_i$ is 299 When viewed on its own, a @try@ statement with @catch@ clauses simply executes the statements in 300 the @GUARDED_BLOCK@, and when those are finished, the try statement finishes. 301 302 However, while the guarded statements are being executed, including any invoked 303 functions, a termination exception may be thrown. If that exception is not handled by a try 304 statement further up the stack, the handlers following the try block are now 305 searched for a matching termination exception-type from top to bottom. 306 307 Exception matching checks each @catch@ clasue from top to bottom, if the representation of the thrown exception-type is 308 the same or a descendant type of the exception types in the @catch@ clauses. If 309 it is the same or a descendant of @EXCEPTION_TYPE@$_i$, then the optional @NAME@$_i$ is 330 310 bound to a pointer to the exception and the statements in @HANDLER_BLOCK@$_i$ 331 311 are executed. If control reaches the end of the handler, the exception is 332 freed and control continues after the try statement. 333 334 If no handler is found during the search then the default handler is run. 335 Through \CFA's trait system the best match at the throw sight will be used. 336 This function is run and is passed the copied exception. After the default 337 handler is run control continues after the throw statement. 338 339 There is a global @defaultTerminationHandler@ that cancels the current stack 340 with the copied exception. However it is generic over all exception types so 341 new default handlers can be defined for different exception types and so 342 different exception types can have different default handlers. 312 freed and control continues after the @try@ statement. 313 314 If no termination handler is found during the search, the default termination 315 handler visible at the raise is called. Through \CFA's trait-system the best 316 default-handler match at the throw sight is used. This function is 317 passed the copied exception given to the raise. After the default handler is 318 run, control continues after the @throw@ statement. 319 320 There is a global @defaultTerminationHandler@ function that that is polymorphic 321 over all exception types allowing new default handlers to be defined for 322 different exception types and so different exception types can have different 323 default handlers. The global default termination-handler performs a 324 cancellation \see{\VRef{s:Cancellation}} on the current stack with the copied 325 exception. 343 326 344 327 \subsection{Resumption} 345 328 \label{s:Resumption} 346 347 Resumption exception handling is a less common form than termination but is 348 just as old~\cite{Goodenough75} and is in some sense simpler. 349 It is a dynamic, non-local function call. If the throw is successful a 350 closure will be taken from up the stack and executed, after which the throwing 351 function will continue executing. 352 These are most often used when an error occurred and if the error is repaired 329 Resumption exception-handling is a less common counterpart to termination but is 330 just as old~\cite{Goodenough75} and is simpler to understand. 331 It is a dynamic, non-local function call (like Lisp). If the throw is successful, a 332 closure is taken from up the stack and executed, after which the throwing 333 function continues executing. 334 Resumption is used when an error occurred, and if the error is repaired, 353 335 then the function can continue. 354 336 337 An alternative approach is explicitly passing fixup functions with local 338 closures up the stack to be called when an error occurs. However, fixup 339 functions significantly expand the parameters list of functions, even when the 340 fixup function is not used by a function but must be passed to other called 341 functions. 342 355 343 A resumption raise is started with the @throwResume@ statement: 356 344 \begin{cfa} 357 345 throwResume EXPRESSION; 358 346 \end{cfa} 359 The semantics of the @throwResume@ statement are like the @throw@, but the 360 expression has return a reference a type that satisfies the trait 361 @is_resumption_exception@. The assertions from this trait are available to 347 Like termination, the expression must return a reference to a resumption 348 exception, where the resumption exception is any type that satisfies the trait 349 @is_termination_exception@ at the call site. 350 The assertions for this trait are available to 362 351 the exception system while handling the exception. 363 352 364 At run -time, no copies are made. As the stack is not unwoundthe exception and365 any values on the stack willremain in scope while the resumption is handled.366 367 The n the exception system searches the stack usingthe provided exception.368 It starts starts from the throwand proceeds to the base of the stack,353 At runtime, no exception copy is made, as the stack is not unwound. Hence, the exception and 354 any values on the stack remain in scope while the resumption is handled. 355 356 The exception searches walks the stack matching with the provided exception. 357 It starts from the resuming function and proceeds to the base of the stack, 369 358 from callee to caller. 370 359 At each stack frame, a check is made for resumption handlers defined by the … … 373 362 try { 374 363 GUARDED_BLOCK 375 } catchResume (EXCEPTION_TYPE$\(_1\)$ * NAME$\(_1\)$) {364 } catchResume (EXCEPTION_TYPE$\(_1\)$ [* NAME$\(_1\)$]) { 376 365 HANDLER_BLOCK$\(_1\)$ 377 } catchResume (EXCEPTION_TYPE$\(_2\)$ * NAME$\(_2\)$) {366 } catchResume (EXCEPTION_TYPE$\(_2\)$ [* NAME$\(_2\)$]) { 378 367 HANDLER_BLOCK$\(_2\)$ 379 368 } 380 369 \end{cfa} 381 If the handlers are not involved in a search this will simply execute the 382 @GUARDED_BLOCK@ and then continue to the next statement. 383 Its purpose is to add handlers onto the stack. 384 (Note, termination and resumption handlers may be intermixed in a @try@ 385 statement but the kind of throw must be the same as the handler for it to be 386 considered as a possible match.) 387 388 If a search for a resumption handler reaches a try block it will check each 389 @catchResume@ clause, top-to-bottom. 390 At each handler if the thrown exception is or is a child type of 391 @EXCEPTION_TYPE@$_i$ then the a pointer to the exception is bound to 392 @NAME@$_i$ and then @HANDLER_BLOCK@$_i$ is executed. After the block is 393 finished control will return to the @throwResume@ statement. 394 395 Like termination, if no resumption handler is found, the default handler 396 visible at the throw statement is called. It will use the best match at the 397 call sight according to \CFA's overloading rules. The default handler is 398 passed the exception given to the throw. When the default handler finishes 399 execution continues after the throw statement. 400 401 There is a global @defaultResumptionHandler@ is polymorphic over all 402 termination exceptions and preforms a termination throw on the exception. 370 Termination and resumption handlers may be intermixed in a @try@ 371 statement but the kind of throw must match with kind of handler for it to be 372 considered as a possible match. 373 Like termination, when viewed on its own, a @try@ statement with 374 @catchResume@ clauses simply executes the statements in the @GUARDED_BLOCK@, 375 and when those are finished, the try statement finishes. 376 377 However, while the guarded statements are being executed, including any invoked 378 functions, a resumption exception may be thrown. If that exception is not handled by a try 379 statement further up the stack, the handlers following the try block are now 380 searched for a matching resumption exception-type from top to bottom. 381 382 Like termination, exception matching checks each @catch@ clasue from top to bottom, if the representation of the thrown exception-type is 383 the same or a descendant type of the exception types in the @catchResume@ clauses. If 384 it is the same or a descendant of @EXCEPTION_TYPE@$_i$, then the optional @NAME@$_i$ is 385 bound to a pointer to the exception and the statements in @HANDLER_BLOCK@$_i$ 386 are executed. If control reaches the end of the handler, the exception is 387 freed and control continues after the @throwResume@ statement. 388 389 Like termination, if no resumption handler is found during the search, the 390 default resumption handler visible at the raise is called, which is the best 391 match at the according to \CFA's overloading rules. This function is passed the 392 exception given to the raise. After the default handler is run, execution 393 continues after the @throwResume@ statement. 394 395 There is a global @defaultResumptionHandler@ that is polymorphic over all 396 resumption and preforms a termination throw on the exception. 403 397 The @defaultTerminationHandler@ for that throw is matched at the original 404 398 throw statement (the resumption @throwResume@) and it can be customized by 405 399 introducing a new or better match as well. 406 400 407 % \subsubsection? 408 401 \subsection{Resumption Marking} 409 402 A key difference between resumption and termination is that resumption does 410 not unwind the stack. A side effect thatis that when a handler is matched411 and run it 's try block (the guarded statements) and every try statement403 not unwind the stack. A side effect is that when a handler is matched 404 and run its try block (the guarded statements) and every try statement 412 405 searched before it are still on the stack. This can lead to the recursive 413 406 resumption problem. … … 423 416 } 424 417 \end{cfa} 425 When this code is executed the guarded @throwResume@ will throw, starta426 search and match the handler in the @catchResume@ clause. This will be427 call and placed on the stack on top of the try-block. The second throw then428 throws and will search the same try block and put callanother instance of the418 When this code is executed the guarded @throwResume@ starts a 419 search and matches the handler in the @catchResume@ clause. The handler is 420 called and placed on the stack on top of the try-block. The second throw in the handler 421 searches the same try block and calls another instance of the 429 422 same handler leading to an infinite loop. 430 423 431 This situation is trivial and easy to avoid, butmuch more complex cycles424 While this situation is trivial and easy to avoid, much more complex cycles 432 425 can form with multiple handlers and different exception types. 433 426 434 To prevent all of these cases we mask sections of the stack, or equivalently 435 the try statements on the stack, so that the resumption search skips over 436 them and continues with the next unmasked section of the stack. 437 438 A section of the stack is marked when it is searched to see if it contains 439 a handler for an exception and unmarked when that exception has been handled 440 or the search was completed without finding a handler. 427 To prevent this case, examined try statements on the stack are marked, so that 428 subsequent resumption searches skip over them and continue with the next unmarked section 429 of the stack. 430 Unmarking occurs when that exception is handled 431 or the search completes without finding a handler. 441 432 442 433 % This might need a diagram. But it is an important part of the justification 443 434 % of the design of the traversal order. 444 \begin{verbatim} 445 throwResume2 ----------. 446 | | 447 generated from handler | 448 | | 449 handler | 450 | | 451 throwResume1 -----. : 452 | | : 453 try | : search skip 454 | | : 455 catchResume <----' : 456 | | 457 \end{verbatim} 458 459 The rules can be remembered as thinking about what would be searched in 460 termination. So when a throw happens in a handler; a termination handler 435 436 \begin{center} 437 %\begin{verbatim} 438 % throwResume2 ----------. 439 % | | 440 % generated from handler | 441 % | | 442 % handler | 443 % | | 444 % throwResume1 -----. : 445 % | | : 446 % try | : search skip 447 % | | : 448 % catchResume <----' : 449 % | | 450 %\end{verbatim} 451 \input{stackMarking} 452 \end{center} 453 454 The resulting search can be understood by thinking about what is searched for 455 termination. When a throw happens in a handler, a termination handler 461 456 skips everything from the original throw to the original catch because that 462 part of the stack has been unwound, a resumption handler skips the same 463 section of stack because it has been masked. 464 A throw in a default handler will preform the same search as the original 465 throw because; for termination nothing has been unwound, for resumption 466 the mask will be the same. 467 468 The symmetry with termination is why this pattern was picked. Other patterns, 469 such as marking just the handlers that caught, also work but lack the 470 symmetry which means there is more to remember. 457 part of the stack is unwound. A resumption handler skips the same 458 section of stack because it is marked. 459 A throw in a resumption default-handler performs the same search as the original 460 @throwResume@ because for resumption nothing has been unwound. 461 462 The symmetry between resumption masking and termination searching is why this pattern was picked. Other patterns, 463 such as marking just the handlers that caught, also work but the 464 symmetry seems to match programmer intuition. 471 465 472 466 \section{Conditional Catch} 473 Both termination and resumption handler 474 condition to further control which exceptions they handle:475 \begin{cfa} 476 catch (EXCEPTION_TYPE * NAME ; CONDITION)467 Both termination and resumption handler-clauses can be given an additional 468 condition to further control which exceptions is handled: 469 \begin{cfa} 470 catch (EXCEPTION_TYPE [* NAME] @; CONDITION@) 477 471 \end{cfa} 478 472 First, the same semantics is used to match the exception type. Second, if the 479 473 exception matches, @CONDITION@ is executed. The condition expression may 480 reference all names in scope at the beginningof the try block and @NAME@474 reference all names in the scope of the try block and @NAME@ 481 475 introduced in the handler clause. If the condition is true, then the handler 482 476 matches. Otherwise, the exception search continues as if the exception type 483 477 did not match. 478 479 Conditional catch allows fine-gain matching based on object values as well as exception types. 480 For example, assume the exception hierarchy @OpenFailure@ $\rightarrow$ @CreateFailure@ and these exceptions are raised by function @open@. 481 \begin{cfa} 482 try { 483 f1 = open( ... ); // open raises CreateFailure/OpenFailure 484 f2 = open( ... ); // with the associate file 485 ... 486 } catch( CreateFailure * f ; @fd( f ) == f1@ ) { 487 // only handle IO failure for f1 488 } catch( OpenFailure * f ; @fd( f ) == f2@ ) { 489 // only handle IO failure for f2 490 } 491 \end{cfa} 492 Here, matching is very precise on the I/O exception and particular file with an open problem. 493 This capability cannot be easily mimiced within the handler. 484 494 \begin{cfa} 485 495 try { … … 487 497 f2 = open( ... ); 488 498 ... 489 } catch( IOFailure * f ; fd( f ) == f1 ) { 490 // only handle IO failure for f1 491 } 492 \end{cfa} 493 Note, catching @IOFailure@, checking for @f1@ in the handler, and re-raising the 494 exception if not @f1@ is different because the re-raise does not examine any of 495 remaining handlers in the current try statement. 496 497 \section{Rethrowing} 498 \colour{red}{From Andrew: I recomend we talk about why the language doesn't 499 } catch( CreateFailure * f ) { 500 if ( @fd( f ) == f1@ ) ... else // reraise 501 } catch( OpenFailure * f ) { 502 if ( @fd( f ) == f2@ ) ... else // reraise 503 } 504 \end{cfa} 505 When an exception @CreateFailure@ is raised, the first handler catches the 506 derived exception and reraises it if the object is inappropriate. The reraise 507 immediately terminates the current guarded block, which precludes the handler 508 for the base exception @OpenFailure@ from consideration for object 509 @f2@. Therefore, the ``catch first, then reraise'' approach is an incomplete 510 substitute for conditional catch. 511 512 \section{Reraise} 513 \label{s:Rethrowing} 514 \colour{red}{From Andrew: I recommend we talk about why the language doesn't 499 515 have rethrows/reraises instead.} 500 516 501 \label{s:Rethrowing}502 517 Within the handler block or functions called from the handler block, it is 503 518 possible to reraise the most recently caught exception with @throw@ or … … 518 533 is part of an unwound stack frame. To prevent this problem, a new default 519 534 handler is generated that does a program-level abort. 535 \PAB{I don't see how this is different from the normal throw/throwResume.} 520 536 521 537 \section{Finally Clauses} 522 Finally clauses are used to p reform unconditional clean-up when leaving a523 scope . They are placed at the end of a try statement:538 Finally clauses are used to perform unconditional clean-up when leaving a 539 scope and appear at the end of a try statement after any catch clauses: 524 540 \begin{cfa} 525 541 try { … … 532 548 The @FINALLY_BLOCK@ is executed when the try statement is removed from the 533 549 stack, including when the @GUARDED_BLOCK@ finishes, any termination handler 534 finishes or during an unwind.550 finishes, or during an unwind. 535 551 The only time the block is not executed is if the program is exited before 536 552 the stack is unwound. 537 553 538 554 Execution of the finally block should always finish, meaning control runs off 539 the end of the block. This requirement ensures always continues as if the540 finally clause is not present, \ie finallyis for cleanup not changing control555 the end of the block. This requirement ensures execution always continues as if the 556 finally clause is not present, \ie @finally@ is for cleanup not changing control 541 557 flow. Because of this requirement, local control flow out of the finally block 542 558 is forbidden. The compiler precludes any @break@, @continue@, @fallthru@ or 543 559 @return@ that causes control to leave the finally block. Other ways to leave 544 560 the finally block, such as a long jump or termination are much harder to check, 545 and at best requir ing additional run-time overhead, and so are mealy561 and at best require additional run-time overhead, and so are 546 562 discouraged. 547 563 548 564 Not all languages with exceptions have finally clauses. Notably \Cpp does 549 without it as des cructors serve a similar role. Although destructors and550 finally clauses can be used in many of the same areas they have their own565 without it as destructors serve a similar role. Although destructors and 566 finally clauses can be used in many of the same areas, they have their own 551 567 use cases like top-level functions and lambda functions with closures. 552 568 Destructors take a bit more work to set up but are much easier to reuse while 553 finally clauses are good for on ce offs and caninclude local information.569 finally clauses are good for one-off situations and can easily include local information. 554 570 555 571 \section{Cancellation} 556 Cancellation is a stack-level abort, which can be thought of as as an 557 uncatchable termination. It unwinds the entirety of the current stack, and if 558 possible forwards the cancellation exception to a different stack. 572 \label{s:Cancellation} 573 Cancellation is a stack-level abort, which can be thought of as an 574 uncatchable termination. It unwinds the entire stack, and when 575 possible, forwards the cancellation exception to a different stack. 559 576 560 577 Cancellation is not an exception operation like termination or resumption. … … 563 580 throw, this exception is not used in matching only to pass information about 564 581 the cause of the cancellation. 565 (This also means matching cannot fail so there is no default handler either.)566 567 After @cancel_stack@ is called the exception is copied into the exception568 handling mechanism's memory. Then the entirety ofthe current stack is582 (This semantics also means matching cannot fail so there is no default handler.) 583 584 After @cancel_stack@ is called, the exception is copied into the EHM's 585 memory and the current stack is 569 586 unwound. After that it depends one which stack is being cancelled. 570 587 \begin{description} 571 588 \item[Main Stack:] 572 589 The main stack is the one used by the program main at the start of execution, 573 and is the only stack in a sequential program. Even in a concurrent program 574 the main stack is o nly dependent on the environment that started the program.590 and is the only stack in a sequential program. Even in a concurrent program, 591 the main stack is often used as the environment to start the concurrent threads. 575 592 Hence, when the main stack is cancelled there is nowhere else in the program 576 to notify. After thestack is unwound, there is a program-level abort.593 to go. Hence, after the main stack is unwound, there is a program-level abort. 577 594 578 595 \item[Thread Stack:] 579 A thread stack is created for a @thread@ object or object that satisfies the596 A thread stack is created for a \CFA @thread@ object or object that satisfies the 580 597 @is_thread@ trait. A thread only has two points of communication that must 581 happen: start and join. As the thread must be running to perform a 582 cancellation, it must occur after start and before join, so join is used 583 for communication here. 598 happen: start and join. A thread must be running to perform a 599 cancellation (a thread cannot cancel another thread). Therefore, a cancellation must 600 occur after start and before join, so join is used 601 for cancellation communication. 584 602 After the stack is unwound, the thread halts and waits for 585 603 another thread to join with it. The joining thread checks for a cancellation, 586 604 and if present, resumes exception @ThreadCancelled@. 587 605 606 \begin{sloppypar} 588 607 There is a subtle difference between the explicit join (@join@ function) and 589 implicit join (from a destructor call). The explicit join takes the default608 implicit join (from a @thread@'s destructor call). The explicit join takes the default 590 609 handler (@defaultResumptionHandler@) from its calling context, which is used if 591 610 the exception is not caught. The implicit join does a program abort instead. 592 611 \end{sloppypar} 612 613 \PAB{uC++ does not have these issues, but catch(...) is not working.} 614 \begin{lstlisting}[language=uC++] 615 #include <iostream> 616 using namespace std; 617 618 struct Cl { 619 ~Cl() { cout << "C" << endl; } 620 }; 621 _Coroutine C { 622 void main() { 623 Cl c; 624 try { 625 cancel(); 626 } catch( ... ) { 627 cout << "..." << endl; 628 } _Finally { 629 cout << "F" << endl; 630 } 631 } 632 public: 633 void mem() { resume(); } 634 }; 635 _Task T { 636 void main() { 637 Cl c; 638 try { 639 cancel(); 640 } catch( ... ) { 641 cout << "..." << endl; 642 } _Finally { 643 cout << "F" << endl; 644 } 645 } 646 }; 647 int main() { 648 C c; 649 cout << "here1" << endl; 650 c.mem(); 651 cout << "here2" << endl; 652 { 653 T t; 654 } 655 cout << "here3" << endl; 656 } 657 \end{lstlisting} 658 659 \PAB{This discussion should be its own section.} 593 660 This semantics is for safety. If an unwind is triggered while another unwind 594 is underway only one of them can proceed as they both want to ``consume ''the661 is underway only one of them can proceed as they both want to ``consume" the 595 662 stack. Letting both try to proceed leads to very undefined behaviour. 596 663 Both termination and cancellation involve unwinding and, since the default … … 598 665 happen in an implicate join inside a destructor. So there is an error message 599 666 and an abort instead. 667 600 668 \todo{Perhaps have a more general disucssion of unwind collisions before 601 669 this point.} … … 620 688 for the exception. So it will use the default handler like a regular throw. 621 689 \end{description} 690 691 \PAB{You should have more test programs that compare \CFA EHM to uC++ EHM.}
Note: See TracChangeset
for help on using the changeset viewer.