Make implicit structures explicit

["Costello, Roger L." <costello@mitre.org>]

Hi Folks,

Suppose that you document the steps to be taken by an aircraft in its landing procedure:

1. Contact control tower

2. Enter glide slope

3. Correct for wind conditions

Suppose those things must be followed in the sequence listed.

The XML could be designed like this:

<aircraft-approach-procedure>
    <transition>Contact control tower</transition>
    <transition>Enter glide slope</transition>
    <transition>Correct for wind conditions</transition>
</aircraft-approach-procedure>

That design relies (implicitly) on the order of the <transition> elements for denoting the sequence of steps to be taken. 

An alternative design is to (explicitly) specify the order. Here is one way to accomplish this:

<aircraft-approach-procedure>
    <transition step="2">Enter glide slope</transition>
    <transition step="3">Correct for wind conditions</transition>
    <transition step="1">Contact control tower</transition>
</aircraft-approach-procedure>

Note that in this design it is not necessary to list the <transition> elements in a particular order since @step explicitly indicates the order.

I vote for the latter as best practice. I invoke this principle as my justification: 

 	Make implicit structures explicit.

What do you think?

/Roger