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/*
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Crown Copyright (c) 1997
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This TenDRA(r) Computer Program is subject to Copyright
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owned by the United Kingdom Secretary of State for Defence
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acting through the Defence Evaluation and Research Agency
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(DERA). It is made available to Recipients with a
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royalty-free licence for its use, reproduction, transfer
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to other parties and amendment for any purpose not excluding
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product development provided that any such use et cetera
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shall be deemed to be acceptance of the following conditions:-
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(1) Its Recipients shall ensure that this Notice is
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reproduced upon any copies or amended versions of it;
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(2) Any amended version of it shall be clearly marked to
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show both the nature of and the organisation responsible
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for the relevant amendment or amendments;
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(3) Its onward transfer from a recipient to another
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party shall be deemed to be that party's acceptance of
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these conditions;
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(4) DERA gives no warranty or assurance as to its
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quality or suitability for any purpose and DERA accepts
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no liability whatsoever in relation to any use to which
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it may be put.
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*/
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/* This module gives possible definitions for Throw for single process
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error handling. In addition it gives a definition of the token TRY where:
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TRY(body, ev, handler) delivers the result of body if no exceptions occur;
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delivers the result of handler if the first exception in body has
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error_code ev; otherwise Throws ev to an outer context.
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*/
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Var handler_place:pointer(code_alignment);
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/* The label of the highest exception handler */
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Var handler_env: pointer(unite_alignments(locals_alignment,
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callers_alignment(true)));
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/* the environment of the highest exception handler */
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Var except_val: Int;
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/* The thrown exception value - just take it be an int */
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Tokdef Throw = [err_val:NAT]EXP
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{
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except_val = + err_val(Int);
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long_jump( * handler_env, * handler_place)
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};
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Tokdef TRY = [body:EXP, err_val:NAT, handler:EXP]EXP
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Let ohp = * handler_place
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Let ohe = * handler_env
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/* stack old exception handler */
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Labelled {
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handler_place = make_local_lv(place);
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handler_env = current_env;
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Let x = body
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{ handler_place = ohp; handler_env = ohe;
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/* reinstate old exception handler on successful body */
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x
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}
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| :place:
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?{ handler_place = ohp; handler_env = ohe;
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/* reinstate old exception handler on exceptional body */
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?(* except_val == + err_val(Int));
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/* is it the right exception? ... */
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handler
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/* ... then do handler */
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| long_jump(ohp, ohe)
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/* otherwise re-throw the exception */
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}
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};
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Keep ( handler_place, handler_env, Throw, TRY, except_val )
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