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On indentation as a reflection of
logical structure

It is common in many kinds of writing to use varying amounts of indentation to give a visual representation of some underlying structure for what is being written.  This is particularly true in many of the specialized languages that are used to program computers, even when the language processor does not require (or even recognize) indentation as having any effect on the interpretation of the language.  Arranging the pieces of a computer program according to a systematic method of indentation helps the programmer to visualize the logical relationships between those pieces.  Such a habit is so useful that there are even programming tools that can assist a programmer in this effort.  So let's take a look at how this might work in practice.

As an example, we might write in English, "If a certain condition is true, then do one thing; otherwise do something else."  (Of course in reality we would use a lot more words to describe just what the condition was, what the "one thing" was, and what the "something else" was.  But for the purposes of this example we don't need to do that.)  Now if we want to use indentation to visualize that logical statement, we could do it this way:

     If a certain condition is true, then
          do one thing;
     otherwise
          do something else.

That's pretty straightforward, and there really isn't any other way to do it.  While it is of course possible to vary the amount of indentation, consistency of indentation is essential to maintain the clarity of the logical structure.

In reality, things don't happen in isolation; some preparation is needed beforehand, and some finishing work afterward.  Furthermore, now that we're using indentation, we don't really need to follow the capitalization and punctuation rules that apply to writing plain text.  So perhaps it might look like this:

     do some preparation
     if a certain condition is true then
          do one thing
     otherwise
          do something else
     finish with some cleanup work
We should also mention that the "otherwise" clause is always optional, so it's possible to have just this:
     do some preparation
     if a certain condition is true then
          do something
     finish with some cleanup work

Either way, this demonstrates one of the characteristics of indentation methods, namely that all the things that share a common level of indentation happen sequentially.  In the longer example above, that means first (#1) do some preparation, next (#2) do either one thing or another thing, and finally (#3) finish.

Actually, we've also just demonstrated a common characteristic of many programming languages - each line is a separate statement.  So if the two logical branches involved more extensive work, we might have this:

     do some preparation
     if a certain condition is true then
          do one thing
          do a second thing
     otherwise
          do another thing
          do yet another thing
     finish with some cleanup work
This illustrates that within the mainline sequence set out above, each of the logical alternatives is a subordinate sequence, and only one of those subordinate sequences will be performed.

But while that makes logical sense to you, dear reader, it wouldn't make sense in a programming language unless that language explicitly recognized indentation as our eyes do.  I know of only one such language (JOVIAL); the vast majority of programming languages require either keywords or special punctuation to delineate the logical structure of a program.

In a keyword language (such as FORTRAN), the same assertion of logical alternatives might look something like this:

     IF condition THEN statement_1 ELSE statement_2
and if you needed to do more complicated work, then a statement could be replaced with a block of statements like this:
     IF condition THEN BEGIN
     	statement_1
     	statement_2
     	END
     ELSE statement_3
(If you don't like the indentation on that END statement, just take a deep breath and keep going - we'll get back to that in a moment.)

In a punctuated language (such as Pascal or Javascript), the same logical assertion might look something like this:

     IF (condition) {statement1} ELSE {statement2}
and if you needed to do more complicated work, then multiple statements could be placed within the curly brackets like this:
     IF (condition) {statement_1;statement_2} ELSE {statement_3}
It's when we consider how to convert such logical assertions into indented displays that we encounter the dichotomy that prompted the writing of this essay.  It was hinted at in the parenthesized sentence just above, and it has to do with how far the terminating keyword or punctuation of a block of statements should be indented.  There are two possibilities, as follows:
     IF (condition) {
          statement_1;statement_2
          }
     ELSE {
          statement_3
          }
or
     IF (condition) {
          statement_1;statement_2
     }
     ELSE {
          statement_3
     }
Not surprisingly, there are two schools of thought about this in the world of programmers, one holding that the first form is the only one that makes sense, and the other making the same claim about the second form.  Let's restate these possibilities in a keyword language and add some context:
     preceding statements
     IF condition THEN BEGIN
          statement_1
          statement_2
          END
     ELSE statement_3
     following statements
or
     preceding statements
     IF condition THEN BEGIN
          statement_1
          statement_2
     END
     ELSE statement_3
     following statements
For those who favor the second form, perhaps they would argue that the END isn't part of the conditional block, but rather part of the frame for the executable statements that make up the block.

But to my mind, the un-indented END in the second form is just getting in the way, pretending to be an executable statement following the IF while obscuring the presence of the optional ELSE clause.  After all, the END isn't needed by the human reader; it's needed by the computer language processor, which doesn't care about indentation.  For the human reader, the indentation pattern itself carries all the necessary information about the logical structure, and it's important to see immediately that the optional ELSE clause is indeed present.  Therefore it's better to present the END as being the last line of block which it terminates.

Similar logic applies to the question of how to indent the closure of a repetition block:

     preliminary work
     loop under some control
          work inside the loop
          end marker for the loop
     next thing after the loop
not this:
     preliminary work
     loop under some control
          work inside the loop
     end marker for the loop
     next thing after the loop

So now you know my opinion on the subject.  If you disagree, please use the email link at the bottom of this page to tell me your reason(s).

By the way, this is NOT a trivial matter, because it significantly affects the readability, and therefore the maintainability, of computer programs and Webpages.


This page was created on 2015/02/17 and last revised on 2015/05/04.

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