Group:
repetitive control
Topic:
defining a process
Topic:
exception handling by termination
Topic:
generalized repetition
Topic:
goto statement
Topic:
return from procedure
Topic:
program blocks for control
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Summary
Program control often applies to statement blocks where entrance is controlled and exit is at block's end. Explicit termination reduces the need for internal gotos. The standard format is: exit/leave/quit identifying-name-for-block value-for-block where each component is optional. Loop structures provide the most commonly exited blocks, but all control structures may find exit statements useful. Bochmann (QuoteRef: bochGV7_1973) describes an interesting extension where each unit includes a case statement for how the unit was terminated. The mode of termination can be seen as an exception which is explicitly signaled and handled. This structure becomes a universal control structure by combining repetition with selection and sequence. (cbb 5/80)
While 'exit' statements add conceptual power, they can be hidden inside a block. Other debates are allowing named exits or multi-level exits. (cbb 1/93)
Subtopic: process exit 
| Quote: in NIL, when a process terminates, it sends an obituary message to its owner on a specified port [»stroRE5_1985]
| | Quote: Unix 'exit' terminates a process, destroys its image, closes its files, and generally obliterates it [»ritcDM7_1978a]
| | Quote: if an upcall fails, need to recover the calling task; the simple solution is to fail; i.e., tasks are expendable [»clarDD12_1995]
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Subtopic: loop break or continue
| Quote: C's break statement terminates the immediately enclosing while, for, do, or switch [»ritcDM7_1978c]
| | Quote: C's continue statement initiates the next iteration of the immediately enclosing loop [»ritcDM7_1978c]
| | Quote: loop exits can allow specialized terminations indicated by a label [»bochGV7_1973, OK]
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Subtopic: labeled exit statement
| QuoteRef: wulfWA10_1971 ;;49 feels that leave labeled-unit better than leaveloop etc.
| | QuoteRef: bochGV7_1973 ;; Wulf on Bliss: LEAVE name-of-program-section-to-leave
| | QuoteRef: zahnCT_1974 ;;174 until no_more_data do ... if eof then no_more_data ... this does the event exiting
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Subtopic: exit statement
| Quote: in paper solutions with looping constructs, 75% had only a terminating condition [»paneJF2_2001]
| | Quote: in Kosaraju's control structure hierarchy, 'exit' statements added power to D and D' structures
| | QuoteRef: wegnE11_1973 ;;704 "It is sufficient to allow an action to complete the execution of any action in which it is contained.
| | Quote: an exit statement is like a 'signal' except that it raises an condition that is handled in the current procedure activation [»liskBH11_1979]
| | QuoteRef: wegbB2_1974 ;;42 conditional exits in EL1 by boolean-expression => value-of-exit
| | QuoteRef: wileDS11_1973 ;;36 else exit sequence (exs) terminates sequence when argument is empty otherwise its value is its argument (do while)
| | QuoteRef: wileDS11_1973 ;;37 then exit sequence (txs) terminates sequence and includes argument in result when arg is non-empty. otherwise returns nil (do until).
| | QuoteRef: wulfWA12_1971 ;;786 can exit any unit and return the value of that unit e.g., exitblock exp. or exitloop exp or return exp. of functions
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Subtopic: tail recursion
| Quote: recall(p,x) is for simplifying program design; like recursion [»thimH2_1980]
| | Quote: recall(p,x) is equivalent to leave(p,p(x)); it replaces the most recent activation with a new one [»thimH2_1980]
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Subtopic: exit syntax
| Quote: if a second exit is added to a loop, Programmers_Assistant embeds the loop in a conditional and notifies the programmer of the additional level [»wateRC1_1982]
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Subtopic: problems with exit/break/etc.
| Quote: the need for high level control structures such as 'exit' statements is not proven [»ledgHF11_1975]
| Quote: internal 'exit' statements in a control structure can be hidden in the code yet are very important
[»ledgHF11_1975]
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Related Topics
Group: repetitive control (7 topics, 117 quotes)
Topic: defining a process (23 items)
Topic: exception handling by termination (16 items)
Topic: generalized repetition (16 items)
Topic: goto statement (25 items)
Topic: return from procedure (9 items)
Topic: program blocks for control
(20 items)
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