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Date:2010-02-05T22:58:35.000000Z git-svn-id: https://svn.eiffel.com/eiffel-org/trunk@439 abb3cda0-5349-4a8f-a601-0c33ac3a8c38
222 lines
14 KiB
Plaintext
222 lines
14 KiB
Plaintext
[[Property:title|ET: Instructions (beta)]]
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[[Property:link_title|ET: Instructions]]
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[[Property:weight|-6]]
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[[Property:uuid|628bf3db-728f-0b3c-bdbb-fe52deaae5b7]]
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==Instructions==
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Eiffel has a remarkably small set of instructions. The basic computational instructions have been seen: creation, assignment, assignment attempt, procedure call, retry. They are complemented by control structures: conditional, multi-branch, loop, as well as debug and check.
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===Conditional===
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A conditional instruction has the form
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<code>
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if ... then
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...
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elseif ... then
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...
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else
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...
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end
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</code>
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The <code>elseif</code> ... <code>then</code> ... part (of which there may be more than one) and the <code>else</code> ... part are optional. After <code>if</code> and <code>elseif</code> comes a boolean expression; after <code>then</code> and <code>else</code> come zero or more instructions.
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===Multi-branch===
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A multi-branch instruction has the form
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<code>
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inspect
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exp
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when v1 then
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inst
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when v2 then
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inst2
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...
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else
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inst0
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end
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</code>
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where the <code>else inst0</code> part is optional, <code>exp</code> is a character or integer expression, <code>v1</code>, <code>v1</code>, ... are constant values of the same type as <code>exp</code>, all different, and <code>inst0</code>, <code>inst1</code>, <code>inst2</code>, ... are sequences of zero or more instructions.
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The effect of such a multi-branch instruction, if the value of <code>exp</code> is one of the <code>vi</code>, is to execute the corresponding <code>insti</code>. If none of the <code>vi</code> matches, the instruction executes <code>inst0</code>, unless there is no <code>else</code> part, in which case it triggers an exception.
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{{note|Raising an exception is the proper behavior, since the absence of an <code>else</code> indicates that the author asserts that one of the values will match. If you want an instruction that does nothing in this case, rather than cause an exception, use an <code>else</code> part with an empty <code>inst0</code>. In contrast, <code>if c then</code> <code>inst</code> <code>end</code> with no <code>else</code> part does nothing in the absence of an <code>else</code> part, since in this case there is no implied claim that <code>c</code> must hold. }}
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===Loop===
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The loop construct provides a flexible framework for iterative computation. Its flexibility lies in how the complete form can be tailored and simplified for certain purposes by omitting optional parts. We will explore the entire mechanism, but let's approach things a little at a time.
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First let's take a look at two examples. These examples both use a loop to visit and print the content of each node of a linked list of character strings. So, the list in question might be declared like this:
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<code>
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my_list: LINKED_LIST [STRING]
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</code>
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Now for the two loop examples:
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<code>
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from
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my_list.start
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until
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my_list.off
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loop
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print (my_list.item)
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my_list.forth
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end
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</code>
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''Loop example 1.''
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and:
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<code>
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across my_list as ic loop print (ic.item) end
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</code>
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''Loop example 2.''
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At first observation, it may not appear that both of these examples are using the same language construct. But, indeed, they are simply two different forms of a single language construct, as you will see.
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Of course, there is no requirement that ''Loop example 1'' occupy multiple lines, and ''Loop example 2'' occupy only one line. ''Loop example 1'' could have been written like this:
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<code>
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from my_list.start until my_list.off loop print (my_list.item) my_list.forth end
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</code>
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just as ''Loop example 2'' could have been written to take multiple lines. It comes down to a matter of what can be read easily as a single line.
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In fact, these two examples illustrate the two basic usage forms of the loop construct in Eiffel. The two basic forms can be differentiated by the parts of the construct with which they begin.
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The form shown in ''Loop example 1'' begins with an ''Initialization'' part ( <code>from my_list.start</code> ), which starts with the keyword <code>from</code>. Let's call this form the '''traditional''' form. So, the type of loop you see in ''Loop example 1'' has been the traditional mechanism for accomplishing iterative computation, including iterating across data structures. However, extensions to Eiffel's loop construct have provided a more concise way of expressing traversing "iterable" structures.
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This is the form shown in ''Loop example 2''. It begins with an ''Iteration'' part ( <code>across my_list as c</code> ), which starts with the keyword <code>across</code>. We'll call this form the '''iteration''' form.
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====A closer look at the ''traditional'' form====
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What is happening in ''Loop example 1''? Let's dissect it and see.
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First there is the ''initialization'' part:
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<code>
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from
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my_list.start
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</code>
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''Initialization part.''
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The first thing to occur in the execution of the traditional loop is the execution of any instructions in the initialization part (it is permissible for the initialization part to be empty of instructions, but the keyword <code>from</code> must be present to distinguish the traditional loop form). In our example, the feature <code>start</code> is applied to <code>my_list</code> which will attempt to set the list cursor to the first element in <code>my_list</code>.
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The ''Exit condition'' part:
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<code>
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until
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my_list.off
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</code>
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''Exit condition part.''
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The exit condition part of the loop construct defines the conditions under which the loop body (explained below) should no longer be executed. In our example, the loop will no long execute if the cursor is "off", that is, there is no current item. So, if the list is empty, the loop body will not execute at all.
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The ''<code>loop</code> body'' part:
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<code>
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loop
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print (my_list.item)
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my_list.forth
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</code>
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''<code>loop</code> body part.''
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The loop body part contains the sequence of instructions to be executed during each iteration. In the example, that includes printing the current list item and then advancing the cursor. At some point, the cursor will pass the last item in the list, causing the exit condition to become true and stop the loop's execution. So, at the risk of stating the obvious, the key to loops that always complete is to ensure that there is something in the loop body that is guaranteed always to cause the exit condition eventually to become true. Loop correctness will discussed in more detail later.
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And finally, there's the ''End'' part:
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<code>
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end
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</code>
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''End part.''
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====A closer look at the ''iteration'' form====
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Now let's examine the iteration form used in ''Loop example 2.''
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The example begins with an iteration part:
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<code>
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across my_list as ic
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</code>
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''Iteration part.''
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The iteration form is special in the sense that it is designed to work with objects which are ''iterable'', usually data structures. The iteration form always targets a particular object (usually a data structure) based on a class that inherits, either directly or indirectly from the library class <code>ITERABLE</code>. The iteration part specifies such a target for the iteration, in the case of our example, the target is <code>my_list</code>.
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The "<code>as ic</code>" indicates that a local iteration cursor object referenced by the name <code>ic</code>, and available only for the scope of the iteration, will be created to effect the iteration. The element of <code>my_list</code> which is currently referenced by the cursor <code>ic</code> is accessed through <code>ic.item</code> as you see in the loop body:
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<code>
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loop print (ic.item)
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</code>
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''<code>loop</code> body part''.
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And lastly, of course, the iteration form includes an end part ... at the end.
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Notice that the loop body does not contain the call to the structure's <code>forth</code> feature, as our example in traditional form did. Neither do you see the call to <code>start</code> nor the check of <code>off</code> in the exit condition. The iteration form abstracts these for you, relieving you of their burden ... while eliminating some opportunities for error.
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Notice also that the call "<code>print (ic.item)"</code>" accesses the current item as "<code>ic.item"</code>" versus "<code>my_list.item"</code>" in the traditional form. This is because in the iteration form, access to the current item is through the cursor variable, "<code>ic</code>" in the case of ''Loop example 2''.
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Concerning cursors, both ways of using the loop construct to traverse a structure employ a cursor. In the traditional usage, the cursor is internal to the structure object. In the case of the example, that would be the instance of <code>LINKED_LIST [STRING]</code> called <code>my_list</code>. Applying the feature <code>item</code> to <code>my_list</code> retrieves the list element currently referenced by the cursor. In the iteration version of traversal, the variable <code>ic</code> holds the iteration cursor, external to the list object. So, you apply <code>ic.item</code> to get the current list element. The advantage to the external cursor is that multiple traversals of the structure can occur simultaneously without interfering with one another. This is possible in the traditional usage, but only by saving and restoring the structure's cursor.
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====The ''iteration'' form as a boolean expression====
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In ''Loop example 2'', the loop behaves as an instruction. But it is possible to have the iteration loop form behave as a boolean expression. This is helpful in cases in which you might want to ask a question that can only be answered by traversing all or part of a structure.
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To get this effect, you use the iteration form with one of two alternative body notations, the ''<code>all</code> body'' or the ''<code>some</code> body'' in place of the ''<code>loop</code> body''.
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===Debug===
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An occasionally useful instruction is <code>debug</code> <code>(</code>''Debug_key'', ... <code>)</code> ''instructions'' <code>end</code> where ''instructions'' is a sequence of zero or more instructions and the part in parentheses is optional, containing if present one or more strings, called debug keys. The EiffelStudio compiler lets you specify the corresponding <code>debug</code> compilation option: <code>yes</code>, <code>no</code>, or an explicit debug key. The ''instructions'' will be executed if and only if the corresponding option is on. The obvious use is for instructions that should be part of the system but executed only in some circumstances, for example to provide extra debugging information.
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===Check===
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The final instruction is connected with Design by Contract™. The instruction
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<code>
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check
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Assertion
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end
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</code>where Assertion is a sequence of zero or more assertions, will have no effect unless assertion monitoring is turned on at the <code>Check</code> level or higher. If so it will evaluate all the assertions listed, having no further effect if they are all satisfied; if any one of them does not hold, the instruction will trigger an exception.
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This instruction serves to state properties that are expected to be satisfied at some stages of the computation -- other than the specific stages, such as routine entry and exit, already covered by the other assertion mechanisms such as preconditions, postconditions and invariants. A recommended use of <code>check</code> involves calling a routine with a precondition, where the call, for good reason, does not explicitly test for the precondition. Consider a routine of the form
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<code>
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r (ref: SOME_REFERENCE_TYPE)
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require
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not_void: ref /= Void
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do
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ref.some_feature
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...
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end
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</code>
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Because of the call to <code>some_feature</code>, the routine will only work if its precondition is satisfied on entry. To guarantee this precondition, the caller may protect it by the corresponding test, as in
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<code>
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if x /= Void then
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a.r (x)
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end
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</code>
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but this is not the only possible scheme; for example if an <code>create x</code> appears shortly before the call we know <code>x</code> is not void and do not need the protection. It is a good idea in such cases to use a <code>check</code> instruction to document this property, if only to make sure that a reader of the code will realize that the omission of an explicit test (justified or not) was not a mistake. This is particularly appropriate if the justification for not testing the precondition is less obvious. For example <code>x</code> could have been obtained, somewhere else in the algorithm, as <code>clone (y)</code> for some <code>y</code> that you know is not void. You should document this knowledge by writing the call as
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<code>
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check
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x_not_void: x /= Void
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-- Because x was obtained as a clone of y,
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-- and y is not void because [etc.]
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end
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a.r (x)
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</code>
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{{recommended|An extra indentation of the <code>check</code> part to separate it from the algorithm proper; and inclusion of a comment listing the rationale behind the developer's decision not to check explicitly for the precondition. }}
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In production mode with assertion monitoring turned off, this instruction will have no effect. But it will be precious for a maintainer of the software who is trying to figure out what it does, and in the process to reconstruct the original developer's reasoning. (The maintainer might of course be the same person as the developer, six months later.) And if the rationale is wrong somewhere, turning assertion checking on will immediately uncover the bug.
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