Exceptions An example of reflection Using objects to implement a language
Error-handling • How do you signal an error? • 1) Special return value • 2) Evaluate error block • (map at: key ifAbsent: [ ^map at: key put: (Set with: value)]) add: value • 3) Exceptions
Exceptions • Module will “raise an exception” • Client will “handle an exception” • In Smalltalk, exception is an object. • Implementation uses contexts.
Contexts • • context stores temporaries, method arguments, program counter, and return address • • thisContext gives method its own context • • used by debugger, exception handling, processes, backtracking
Contexts PolylineFigure displayOn: CompositeFigure displayOn: Drawing displayOn:
Key Idea • Program handles a particular error signal. • [ object doWork ] • on: Error • do: [ :theException | ... ] • Server raises the same class of exception. • Error signal
Exception Handling • [ x / y ] • on: ZeroDivide • do: [ :theException | theException return: 0] • / signals an error by • ZeroDivide signal
Exception Handling • The ”signal" method of Exception • 1) creates an Exception, • 2) looks on the stack for an on:do: message that uses the class or its subclass, and • 3) evaluates the second argument of the on:do: message with the Exception as the argument.
Exceptions • Exceptions can have an error string. • Some implementations give them other arguments, but that is not part of the standard. • Exception • signal • signal: aMessageString
Exception Protocol • resume: - return from message that signaled exception • retryUsing: - abort exception handler and reevaluate its exception block • return: - return from block protected by active exception handler
Multiple Handlers • Exception travels down stack of contexts, looking for a handler to the exception. It evaluates the handle block of the first match. E raise ... on: E do: ... on: E: do:
Exception Protocol • reject will cause an Exception to look for the next handler. • return will throw away all contexts above the handler.
Exception Hierarchy • Handler for one exception will catch all subclasses, as well. • Exception • Error • ArithmeticError • MessageNotUnderstood • KeyNotFoundError
Unwind Protection • Problem: program can get blown away by an exception while it is in the middle of making delicate changes. This is a problem with returns in general, even without exceptions.
Unwind Protection • Solution: • [self dangerousCode] ifCurtailed: [self cleanUp] • The cleanup block is used if the execution stack is cut back for any reason.
Unwind Protection • ensure: evaluates the cleanup block after the receiver is evaluated. • Semaphore critical: aBlock • self wait. • aBlock ensure: [self signal]
An example • processFile: aFile • "Read a catalog card from the first comment. Trap and report all errors." • aFile isReadable ifFalse: [^self]. • [self processWithErrorsFile: aFile] • on: Error • do: [:ex | ... ]
The Handle Block • [:ex | • Transcript show: ex errorString. • Transcript show: ' for ' , aFile asString. • Transcript cr. • Dialog confirm: ex errorString. • ex return ]
processWithErrorsFile: aFile • "Read a catalog card from the first comment." • | aStream string document | • aStream := aFile readStream. • [...] ensure: [aStream close]
If we encounter an error • [Transcript show: 'bad file ' , aFile asString. • ^Transcript cr]. • Also, system generates errors for files that are not readable.
Halt • halt • "This is the typical message to use for inserting breakpoints during debugging. It behaves like halt:, but does not call on halt: in order to avoid putting this message on the stack. Halt is especially useful when the breakpoint message is an arbitrary one." • Halt signal
Exception handling important in developing reusable libraries. • Squeaks reflective use of contexts makes it possible to implement exception handling as a class library.
