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HELP PRMISHAP                                        Steven Hardy, March 1982

    prmishap(<string>, <list of items>)

This is a user-assignable procedure variable with default value *SYSPRMISHAP.
The procedure it contains is used by *MISHAP to print a mishap message: it
should be able to take the arguments that SYSPRMISHAP takes (see HELP
*SYSPRMISHAP).

By altering PRMISHAP, locally or globally, users can vary the format of
error messages. In addition, by using procedures like *CHAIN, *EXITTO,
*EXITFROM and *CHAINTO, PRMISHAP can be redefined to return to a higher
level procedure, thus avoiding the call of *INTERRUPT.

-- AN EXAMPLE ---------------------------------------------------------------

Here is an example of the use of a locally redefined version of PRMISHAP.
The procedure ISELEMENT takes an item and a list and returns true if the item
is in the list, otherwise false. If the second argument is not a list, it
should produce an error. The program uses SUB_ELEMENT to search down the list,
comparing the item with successive *HD's. SUB_ELEMENT does not check that it is
actually given a non-empty list, relying on HD to do that. HD calls the error
procedure if it is given anything but a non-empty list. If the 'culprit' is
the empty list, the error is trapped by the re-defined PRMISHAP, which chains
back out of ISELEMENT, leaving FALSE on the stack to be the result of
ISELEMENT. Otherwise an error message is produced.

    define iselement(item,list);
        ;;; re-define the error procedure locally
        define prmishap(string,list);
            if issubstring('NON-EMPTY LIST NEEDED',1,string) and list = [[]]
            then
                false;
                exitfrom(iselement)
            else sysprmishap(string,list)
            endif;
        enddefine;

        sub_element(item,list)      ;;; this does the real work
    enddefine;

    define sub_element(item,list);
        if hd(list) = item then true        ;;; HD could cause an error
        else sub_element(item,tl(list))
        endif
    enddefine;

    trace iselement,sub_element;

    iselement(2, [1 2 3])=>
    >iselement 2 [1 2 3]
    !>sub_element 2 [1 2 3]
    !!>sub_element 2 [2 3]
    !!<sub_element <true>
    !<sub_element <true>
    <iselement <true>
    ** <true>

    iselement(4, [1 2 3]) =>
    >iselement 4 [1 2 3]
    !>sub_element 4 [1 2 3]
    !!>sub_element 4 [2 3]
    !!!>sub_element 4 [3]
    !!!!>sub_element 4 []
    <iselement <false>
    ** <false>

    iselement(4, 44) =>
    >iselement 4 44
    !>sub_element 4 44

    ;;; MISHAP - LIST NEEDED
    ;;; INVOLVING:  44
    ;;; DOING    :  prmishap mishap sub_element iselement compile

NOTE: The test 'if hd(list) = item' could not have been done in the reverse
order. It is important that HD is called before ITEM is put on the stack for
the equality test, so that if the error procedure causes EXITFROM to be called
then the value of ITEM is not left on the stack.

See HELP
    *MISHAP      - POP-11 error procedures
    *SYSPRMISHAP - prints error messages
    *INTERRUPT   - On POP-11 interrupt procedures
    *SETPOP      - Resets the POP-11 system
    *POPSYSCALL  - controls inclusion in mishap messages of systems procedures