Using the Module

    use Smart::Comments;

When loaded it filters the remaining code up to the next:

    no Smart::Comments;

directive, replacing any smart comments with smart code that implements the comments behaviour.

If you're debugging an application you can also invoke it with the module from the command-line:

    perl -MSmart::Comments $application.pl

Of course, this only enables smart comments in the application file itself, not in any modules that the application loads.

You can also specify particular levels of smartness, by including one or more markers as arguments to the "use":

    use Smart::Comments '###', '####';

These arguments tell the module to filter only those comments that start with the same number of "#"'s. So the above "use" statement would "activate" any smart comments of the form:

    ###   Smart...

    ####  Smarter...

but not those of the form:

    ##### Smartest...

This facility is useful for differentiating progress bars (see "Progress Bars"), which should always be active, from debugging comments (see "Debugging via Comments"), which should not:

    #### Debugging here...

    for (@values) {         ### Progress: 0...  100
        do_stuff();
    }

Note that, for simplicity, all smart comments described below will be written with three "#"'s; in all such cases, any number of "#"'s greater than three could be used instead.

Debugging via Comments

"### LABEL : EXPRESSION"

The LABEL is any sequence of characters up to the first colon. The EXPRESSION is any valid Perl expression, including a simple variable. When active, the comment prints the label, followed by the value of the expression. For example:

    ### Expected: 2 * $prediction
    ###      Got: $result
    

prints:

    ### Expected: 42
    ###      Got: 13
    

"### EXPRESSION"

The EXPRESSION is any valid Perl expression, including a simple variable. When active, the comment prints the expression, followed by the value of the expression. For example:

    ### 2 * $prediction
    ### $result
    

prints:

    ### 2 * $prediction: 42
    ### $result: 13
    

"### TEXT..."

The TEXT is any sequence of characters that end in three dots. When active, the comment just prints the text, including the dots. For example:

    ### Acquiring data...

    $data = get_data();

    ### Verifying data...

    verify_data($data);

    ### Assimilating data...

    assimilate_data($data);

    ### Tired now, having a little lie down...

    sleep 900;
    

would print:

    ### Acquiring data...

    ### Verifying data...

    ### Assimilating data...

    ### Tired now, having a little lie down...
    

as each phase commenced. This is particularly useful for tracking down precisely where a bug is occurring. It is also useful in non-debugging situations, especially when batch processing, as a simple progress feedback mechanism.

Within a textual smart comment you can use the special sequence "<now>" (or "<time>" or "<when>") which is replaced with a timestamp. For example:

    ### [<now>] Acquiring data...
    

would produce something like:

    ### [Fri Nov 18 15:11:15 EST 2005] Acquiring data...
    

There are also "spacestamps": "<here>" (or "<loc>" or "<place>" or "<where>"):

    ### Acquiring data at <loc>...
    

to produce something like:

    ### Acquiring data at "demo.pl", line 7...
    

You can also request just the filename ("<file>") or just the line number ("<line>") to get finer control over formatting:

    ### Acquiring data at <file>[<line>]...
    

and produce something like:

    ### Acquiring data at demo.pl[7]...
    

You can, of course, use any combination of stamps in the one comment.

Checks and Assertions via Comments

"### require: BOOLEAN_EXPR"

"### assert: BOOLEAN_EXPR"

"### ensure: BOOLEAN_EXPR"

"### insist: BOOLEAN_EXPR"

These four are synonyms for the same behaviour. The comment evaluates the expression in a boolean context. If the result is true, nothing more is done. If the result is false, the comment throws an exception listing the expression, the fact that it failed, and the values of any variables used in the expression.

For example, given the following assertion:

    ### require: $min < $result && $result < $max
    

if the expression evaluated false, the comment would die with the following message:

    ### $min < $result && $result < $max was not true at demo.pl line 86.
    ###     $min was: 7
    ###     $result was: 1000004
    ###     $max was: 99
    

"### check: BOOLEAN_EXPR"

"### confirm: BOOLEAN_EXPR"

"### verify: BOOLEAN_EXPR"

These three are synonyms for the same behaviour. The comment evaluates the expression in a boolean context. If the result is true, nothing more is done. If the result is false, the comment prints a warning message listing the expression, the fact that it failed, and the values of any variables used in the expression.

The effect is identical to that of the four assertions listed earlier, except that "warn" is used instead of "die".

Progress Bars

    foreach my VAR ( LIST ) {       ### Progressing...   done

    for my VAR ( LIST ) {           ### Progressing...   done

    foreach ( LIST ) {              ### Progressing...   done

    for ( LIST ) {                  ### Progressing...   done

    while (CONDITION) {             ### Progressing...   done

    until (CONDITION) {             ### Progressing...   done

    for (INIT; CONDITION; INCR) {   ### Progressing...   done

In each case, the module animates the comment, causing the dots to extend from the left text, reaching the right text on the last iteration. For "open ended" loops (like "while" and C-style "for" loops), the dots will never reach the right text and their progress slows down as the number of iterations increases.

For example, a smart comment like:

    for (@candidates) {       ### Evaluating...     done

would be animated is the following sequence (which would appear sequentially on a single line, rather than on consecutive lines):

    Evaluating                          done

    Evaluating......                    done

    Evaluating.............             done

    Evaluating...................       done

    Evaluating..........................done

The module animates the first sequence of three identical characters in the comment, provided those characters are followed by a gap of at least two whitespace characters. So you can specify different types of progress bars. For example:

    for (@candidates) {       ### Evaluating:::     done

or:

    for (@candidates) {       ### Evaluating===     done

or:

    for (@candidates) {       ### Evaluating|||     done

If the characters to be animated are immediately followed by other non-whitespace characters before the gap, then those other non-whitespace characters are used as an "arrow head" or "leader" and are pushed right by the growing progress bar. For example:

    for (@candidates) {       ### Evaluating===|    done

would animate like so:

    Evaluating|                         done

    Evaluating=====|                    done

    Evaluating============|             done

    Evaluating==================|       done

    Evaluating==========================done

If a percentage character ("%") appears anywhere in the comment, it is replaced by the percentage completion. For example:

    for (@candidates) {       ### Evaluating [===|    ] % done

animates like so:

    Evaluating [|                ]   0% done

    Evaluating [===|             ]  25% done

    Evaluating [========|        ]  50% done

    Evaluating [============|    ]  75% done

    Evaluating [=================] 100% done

If the "%" is in the "arrow head" it moves with the progress bar. For example:

    for (@candidates) {       ### Evaluating |===[%]    |

would be animated like so:

    Evaluating |[0%]                       |

    Evaluating |=[25%]                     |

    Evaluating |========[50%]              |

    Evaluating |===============[75%]       |

    Evaluating |===========================|

For "open-ended" loops, the percentage completion is unknown, so the module replaces each "%" with the current iteration count. For example:

    while ($next ne $target) {       ### Evaluating |===[%]    |

would animate like so:

    Evaluating |[0]                        |

    Evaluating |=[2]                       |

    Evaluating |==[3]                      |

    Evaluating |===[5]                     |

    Evaluating |====[7]                    |

    Evaluating |=====[8]                   |

    Evaluating |======[11]                 |

Note that the non-sequential numbering in the above example is a result of the "hurry up and slow down" algorithm that prevents open-ended loops from ever reaching the right-hand side.

As a special case, if the progress bar is drawn as two pairs of identical brackets:

    for (@candidates) {       ### Evaluating: [][]

    for (@candidates) {       ### Evaluating: {}{}

    for (@candidates) {       ### Evaluating: ()()

    for (@candidates) {       ### Evaluating: <><>

Then the bar grows by repeating bracket pairs:

    Evaluating: [

    Evaluating: []

    Evaluating: [][

    Evaluating: [][]

    Evaluating: [][][

etc.

Finally, progress bars don't have to have an animated component. They can just report the loop's progress numerically:

    for (@candidates) {       ### Evaluating (% done)

which would animate (all of the same line):

    Evaluating (0% done)

    Evaluating (25% done)

    Evaluating (50% done)

    Evaluating (75% done)

    Evaluating (100% done)

Time-Remaining Estimates

Normally this estimate is not shown, unless the estimate becomes large enough to warrant informing the user. Specifically, the estimate will be shown if, after five seconds, the time remaining exceeds ten seconds. In other words, a time-remaining estimate is shown if the module detects a "for" loop that is likely to take more than 15 seconds in total. For example:

    for (@seven_samurai) {      ### Fighting: [|||    ]
        fight();
        sleep 5;
    }

would be animated like so:

    Fighting: [                           ]

    Fighting: [||||                       ]

    Fighting: [|||||||||                  ]  (about 20 seconds remaining)

    Fighting: [||||||||||||||             ]  (about 20 seconds remaining)

    Fighting: [||||||||||||||||||         ]  (about 10 seconds remaining)

    Fighting: [|||||||||||||||||||||||    ]  (less than 10 seconds remaining)

    Fighting: [|||||||||||||||||||||||||||]

The precision of the reported time-remaining estimate is deliberately vague, mainly to prevent it being annoyingly wrong.