New chapter: no booleans on parameters

content/books/things-i-learnt/_index.md

@@ -25,3 +25,4 @@ template = "section-contentless.html"
 	* Writing code
 		* [Be Ready To Throw Your Code Away](throw-away)
 		* [Future Thinking Is Future Trashing](future-trashing)
+		* [Don't Use Booleans As Parameters](boolean-parameters)

content/books/things-i-learnt/boolean-parameters/index.md

@@ -0,0 +1,32 @@
++++
+title = "Things I Learnt The Hard Way - Don't Use Booleans As Parameters"
+date = 2019-06-23
+
+[taxonomies]
+tags = ["en-au", "books", "things i learnt", "booleans", "functions", "parameters"]
++++
+
+When you're designing a function, you may be tempted to add a flag. Don't do
+this.
+
+<!-- more -->
+
+Here, let me show you an example: Suppose you have a messaging system and you
+have a function that returns all the messages to an user, called
+`getUserMessages`. But there is a case where you need to return a summary of
+each message (say, the first paragraph) or the full message. So you add a
+flag/Boolean parameter called `retrieveFullMessage`.
+
+Again, don't do that.
+
+'Cause anyone reading your code will see `getUserMessage(userId, true)` and
+wonder what the heck that `true` means.
+
+You can either rename the function to `getUserMessageSummaries` and have
+another `getUserMessagesFull` or something around those lines, but each
+function just call the original `getUserMessage` with true or false -- but the
+interface to the outside of your class/module will still be clear.
+
+But _don't_ add flags/Boolean parameters to your functions.
+
+{{ chapters(prev_chapter_link="/books/things-i-learnt/future-trashing", prev_chapter_title="Future Thinking is Future Trashing") }}

content/books/things-i-learnt/future-trashing/index.md

@@ -23,4 +23,4 @@ solutions and _then_ you'll find your "solve everything". This pattern is the
 _abstraction_ you're looking for and _then_ you'll be able to solve it in a
 simple way.
 
-{{ chapters(prev_chapter_link="/books/things-i-learnt/throw-away", prev_chapter_title="Be Ready To Throw Your Code Away") }}
+{{ chapters(prev_chapter_link="/books/things-i-learnt/throw-away", prev_chapter_title="Be Ready To Throw Your Code Away", next_chapter_link="/books/things-i-learnt/boolean-parameters", next_chapter_title="Don't Use Booleans As Parameters") }}