Exception Handling, Modules, Names

1
Exception Handling,Modules, Names

• Per Brand

2
Exception Handling

• An exception is any expression E. To raise the
  exception , one executes the following statement
• raise E end
• proc Eval E ?R case E of plus(X Y) then
  Browse XY times(X Y) then Browse XY
  else raise illFormedExpression(E) end endend
• The basic exception handling statement is called
  a try-statement
• try S catch Pattern1 then S1 Pattern2 then
  S2 Patternn then Snend

3
Semantics

• Execution is equivalent to executing S if S does
  not raise an exception.
• If S raises exception E, and E matches one of the
  patterns Patterni, control is passed to the
  corresponding statement Si.
• If E does not match any pattern the exception is
  propagated outside the try-statement until
  eventually caught by the system, which catches
  all escaped exceptions.
• try S catch Pattern1 then S1 Pattern2 then
  S2 Patternn then Snend

4
Example

• try ForAll plus(5 10) times(6 11)
  min(7 10) Evalcatch illFormedExpression(X)
  then Browse X end

5
try catch finally

• A try-statement may also specify a final
  statement Sfinal, which is executed on normal as
  well as on exceptional exit. try S catch
  Pattern1 then S1 Pattern2 then S2
  Patternn then Snfinally Sfinalend
• Example
• Assume that F is an opened file the procedure
  Process/1 manipulates the file in some way and
  the procedure CloseFile/1 closes the file.
• The following program ensures that the F is
  closed upon normal or exceptional exit.try
  Process Fcatch illFormedExpression(X) then
  Browse X finally CloseFile F end
  

6
System Exceptions

• The exceptions raised by the Oz system are
  records with one of the labels failure, error,
  and system.
• failure indicates the attempt to perform an
  inconsistent equality operation on the store of
  Oz.
• error indicates a runtime error which should not
  occur such as applying a nonprocedure to some
  argument or adding an integer to an atom, etc.
• system indicates a runtime condition because of
  the environment of the Mozart operating system
  process, i.e., an unforeseeable situation like a
  closed file or window or failing to open a
  connection between two Mozart processes.

7

• proc One X X1 end 
• proc Two X X2 end 
• try OneTwo
• catch failure(...) then Show caughtFailure
• end
• Here the pattern failure(...) catches any record
  whose label is failure.
• When an exception is raised but not handled, an
  error message is printed in the emulator window
  (standard error), and the current thread
  terminates.
• In stand-alone applications the default behavior
  is that a message is printed on standard error
  and the whole application terminates. It is
  possible to change this behavior to something
  else that is more desirable for particular
  applications.

8
Modules and Interfaces

• A module is a software component that is
  first-class.
• A module consists of a collection of Oz entities
  (procedure, objects and other values) that are
  constructed together to provide certain related
  functionality.
• A module typically has a number of private
  entities that are not visible outside the module
  and a interface the external services of the
  module.
• The interface provides the services of the
  module.
• The lexical scoping of the language and the
  record data-type suffices to construct modules.

9
Homemade Module

• declare List local     proc Append ...   ... e
  nd  proc Partition ...   ... end    proc So
  rt ...  ... Partition ... ... end    proc Mem
  ber ... ... end  in    List  'export'(append 
  Append                   sort Sort             
        member Member                   ... )end
• Browse List.append 1 2 3 3 4

private
10
Functors, Modules and Interfaces
functor  export  appendAppendsortSortmember
Memberdefine proc Append ...   ... end proc 
Partition ... ... end proc Sort ...  
... Partition ... ... end proc Member ... ..
. end  end stored at /home/person/List.oz
compiled to a compiled-functor at file
/home/person/List.ozf

• Modules are software components that may be
  stateful
• Functors are specification of modules (stateless)
• A functor specifies how to create module from
  other modules (import), the interface of a module
  (export), and the initialization procedure
  (define).

11
Functors, Modules and Interfaces

• Functors as first class values

define functor LFexport  appendAppendsortSort
memberMemberdefine proc Append ...   ... end
 proc Partition ... ... end proc Sort ...  
... Partition ... ... end proc Member ..
. ... end   end declare List Module.link LF

12
Functors

• Linking functors into modules
• declare List Module.link /home/person/list.o
  zf
• When a functor is linked the statements between
  define and end are executed.
• Functors may have import declarations
• functor  import    Browser     FO at 'file///h
  ome/person/FileOperations.ozf'  define    Brows
  er.browse FO.countLines '/etc/passwd'  end 
• functors are values like any other entity.

13
Names for Security

• declare SensitiveNewNameproc Service In
  Out case In of Sensitive(X Y) then
  service1(X Y) then serviceN(X Y)then
  else raise unknownRequest end
  endend service1, serviceN are guessable
• Sensitive is not