VDM-10 Language Manual

VDM-10 Language Manual
#act: operation name → N
#fin: operation name → N
Furthermore, a derived function #active is available such that #active(A) = #act(A)
- #fin(A), giving the number of currently active instances of A. Another history function
– #req – is defined in section 15.1.3.
Examples: Consider a Web server that is capable of supporting 10 simultaneous connections and
can buffer a further 100 requests. In this case we have one instance variable, representing
the mapping from URLs to local filenames:
✞
instance variables
site_map : map URL to Filename := {|->}
✡✝
The following operations are defined in this class (definitions omitted for brevity):
ExecuteCGI: URL ==> File Execute a CGI script on the server
RetrieveURL: URL ==> File Transmit a page of html
UploadFile: File * URL ==> () Upload a file onto the server
ServerBusy: () ==> File Transmit a “server busy” page
DeleteURL: URL ==> () Remove an obsolete file
Since the server can support only 10 simultaneous connects, we can only permit an execute
or retrieve operation to be activated if the number already active is less than 10:
✞
✡✝
per RetrieveURL => #active(RetrieveURL) +
#active(ExecuteCGI) < 10;
per ExecuteCGI => #active(RetrieveURL) +
#active(ExecuteCGI) < 10;
✆
✆
15.1.2 The object state guard
Semantics: The object state guard is a boolean expression which depends on the values of one
(or more) instance variable(s) of the object itself. Object state guards differ from operation
pre-conditions in that a call to an an operation whose permission predicate is false results
in the caller blocking until the predicate is satisfied, whereas a call to an operation whose
pre-condition is false means the operation’s behaviour is unspecified.
Examples: Using the web server example again, we can only allow file removal if some files
already exist:
✞
✡✝
per DeleteURL => dom site_map {}
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✆