"

 COPYRIGHT (c) 2002 by eXept Software AG

              All Rights Reserved

 This software is furnished under a license and may be used

 only in accordance with the terms of that license and with the

 inclusion of the above copyright notice.   This software may not

 be provided or otherwise made available to, or used by, any

 other person.  No title to or ownership of the software is

 hereby transferred.

"

"{ Package: 'stx:libbasic' }"

Notification subclass:#Query

	instanceVariableNames:''

	classVariableNames:''

	poolDictionaries:''

	category:'Kernel-Exceptions'

!

!Query class methodsFor:'documentation'!

copyright

"

 COPYRIGHT (c) 2002 by eXept Software AG

              All Rights Reserved

 This software is furnished under a license and may be used

 only in accordance with the terms of that license and with the

 inclusion of the above copyright notice.   This software may not

 be provided or otherwise made available to, or used by, any

 other person.  No title to or ownership of the software is

 hereby transferred.

"

!

documentation

"

    Query is an abstract superclass for queries.

    A query is an exception which by default proceeds if unhandled and returns

    a default value.

    As a class based reimplementation, it replaces and obsoletes the old 

    QuerySignal instance based mechanism.

    Note:

        Query does not add/refine any functionality from its superclass.

        It exists for the more descriptive class name only.

    [author:]

        Claus Gittinger

    [see also:]

        Warning Signal QuerySignal

"

!

examples

"

                                                                        [exBegin]

    Query answer:'hello'

    do:[

        Transcript showCR:(Query query)

    ]

                                                                        [exEnd]

  an up-query from a deeply nested operation to a higher level:

  the example below demonstrates that a Query is not an Error

  (i.e. the Error-handler does not interfere with Queries)

                                                                        [exBegin]

    |zero|

    zero := 0.

    Query handle:[:ex |

        Transcript showCR:'query'.

        ex proceedWith:true

    ] do:[

        'nesting'.

        [

            [

                Error handle:[:ex |

                    Transcript showCR:'some error: ' , ex errorString.

                    ex proceed

                ] do:[

                    [

                        1 // zero.  'an error which is caught in the handler'.

                        (Query query) == true ifTrue:[

                            Transcript showCR:'query says: ok'.

                        ] ifFalse:[

                            Transcript showCR:'query says: no'

                        ]

                    ] value

                ]

            ] value

        ] value

    ]

                                                                        [exEnd]

  for lazy typists, a more compact interface for query-answerign

  is also provided (which is also easier to read):

                                                                        [exBegin]

    Query answer:true do:[

        'nesting'.

        [

            [

                (Query query) == true ifTrue:[

                    Transcript showCR:'query says: ok'.

                ] ifFalse:[

                    Transcript showCR:'query says: no'

                ]

            ] value

        ] value

    ]

                                                                        [exEnd]

  an up-query from a deeply nested operation, for which there

  is no handler:

  (notice, this would not work with normal signals, which would raise

   another unhandled exception-exception;

   also notice the == check #raise's return value being true,

   instead of a simple ifTrue; this handles a nil-value from

   the unhandled query)

                                                                        [exBegin]

    |zero|

    zero := 0.

    [

        'nesting'.

        [

            [

                Error handle:[:ex |

                    Transcript showCR:'some error: ' , ex errorString.

                    ex proceed

                ] do:[

                    [

                        1 // zero.  'an error which is caught in the handler'.

                        (Query raise) == true ifTrue:[

                            Transcript showCR:'query says: ok'.

                        ] ifFalse:[

                            Transcript showCR:'query says: no'

                        ]

                    ] value

                ]

            ] value

        ] value

    ] value

                                                                         [exEnd]

  counter-example, just to show that things would not work this way

  with regular signals:

                                                                        [exBegin]

    |signal|

    signal := Signal new.

    'nesting deeply'.

    [

        [

            [

                [

                    [

                        (signal raise) == true ifTrue:[

                            Transcript showCR:'query says: ok'.

                        ] ifFalse:[

                            Transcript showCR:'query says: no'

                        ]

                    ] value

                ] value

            ] value

        ] value

    ] value

                                                                         [exEnd]

   except, by handling the unhandled exception

   (but we think, that querySignals are easier to use and

    better document the intent):

                                                                        [exBegin]

    |signal|

    signal := Signal new.

    'nesting deeply'.

    [

        [

            [

                [

                    [

                        Signal noHandlerSignal handle:[:ex |

                            ex proceedWith:nil

                        ] do:[

                            (signal raise) == true ifTrue:[

                                Transcript showCR:'query says: ok'.

                            ] ifFalse:[

                                Transcript showCR:'query says: no'

                            ]

                        ]

                    ] value

                ] value

            ] value

        ] value

    ] value

                                                                         [exEnd]

"

! !

!Query class methodsFor:'initialization'!

initialize

    NotifierString := 'Query'.

    "

     self initialize

    "

! !

Query initialize!