Publish the "Don't let it crash" article

1 files changed, 122 insertions, 159 deletions

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     <description>Recent content on Nine Nines</description>
     <generator>Hugo -- gohugo.io</generator>
     <language>en-us</language>
-    <lastBuildDate>Tue, 03 Jan 2017 00:00:00 +0100</lastBuildDate>
+    <lastBuildDate>Sun, 22 Jan 2017 00:00:00 +0100</lastBuildDate>
     <atom:link href="https://ninenines.eu/index.xml" rel="self" type="application/rss+xml" />
     
     <item>
+      <title>Don&#39;t let it crash</title>
+      <link>https://ninenines.eu/articles/dont-let-it-crash/</link>
+      <pubDate>Sun, 22 Jan 2017 00:00:00 +0100</pubDate>
+      
+      <guid>https://ninenines.eu/articles/dont-let-it-crash/</guid>
+      <description>&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;We have a specific mindset when writing Erlang

+programs. We focus on the normal execution of the

+program and don&amp;#8217;t handle most of the errors that may

+occur. We sometimes call this normal execution the

+&lt;em&gt;happy path&lt;/em&gt;.&lt;/p&gt;&lt;/div&gt;

+&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;The general pattern behind writing only for the

+&lt;em&gt;happy path&lt;/em&gt;, letting the VM catch errors (writing

+them to a log for future consumption) and then

+having a supervisor restart the processes that

+failed from a clean state, has a name. We call it

+&lt;em&gt;let it crash&lt;/em&gt;; and it drives many of our design

+decisions.&lt;/p&gt;&lt;/div&gt;

+&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;It&amp;#8217;s a really great way to program and the results

+are fantastic compared to most other programming

+languages. And yet, &lt;em&gt;let it crash&lt;/em&gt; barely convinced

+anyone that they should use Erlang. Why would that

+be?&lt;/p&gt;&lt;/div&gt;

+&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;You may already know that Cowboy is capable of

+handling at least 2 million Websocket connections

+on a single server. This is in large part thanks

+to the capabilities of the VM. Still, 2 million

+is good, much better than most other servers can

+do.&lt;/p&gt;&lt;/div&gt;

+&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;Cowboy is not just a Websocket server; it&amp;#8217;s also

+an HTTP and HTTP/2 server, and it handles many

+related features like long polling or the parsing

+of most request headers.&lt;/p&gt;&lt;/div&gt;

+&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;Can you guess how large the Cowboy codebase is,

+without looking at the source?&lt;/p&gt;&lt;/div&gt;

+&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;Do make sure you have a clear answer in your mind

+before you go check.&lt;/p&gt;&lt;/div&gt;

+&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;Good, you are back. Now what were the results? If

+I am correct, you overestimated the size of Cowboy.

+Cowboy is in fact about five thousand lines of code.

+You probably thought it was at least ten thousand.

+About eighty percent of readers will have

+overestimated the size of Cowboy. And you did only

+because I mentioned it can handle millions of

+Websocket connections.&lt;/p&gt;&lt;/div&gt;

+&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;Numerous studies show this effect. Just mentioning

+the large number already prepared your mind to think

+in that direction. Repeating the number made you

+focus even more on it. Then the question asked for

+a number, which ended up larger than the reality.&lt;/p&gt;&lt;/div&gt;

+&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;The same effect can be applied to negotiation for

+example. You generally want to start by giving your

+offer (and not let the other party initiate) and

+you want to give a really large number first. You

+can also prepare your customer by mentioning an even

+larger number in the previous discussion.&lt;/p&gt;&lt;/div&gt;

+&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;And it&amp;#8217;s not just numbers either. An experiment

+showed that just by looking at an image of clouds,

+customers of a pillow store were buying pillows

+more comfortable (and more expensive) than those

+who didn&amp;#8217;t see that image.&lt;/p&gt;&lt;/div&gt;

+&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;This is the power of associations. It is covered in

+much larger detail in the books

+&lt;a href=&#34;https://www.amazon.com/Influence-Psychology-Persuasion-Robert-Cialdini/dp/006124189X&#34;&gt;Influence&lt;/a&gt;

+and

+&lt;a href=&#34;https://www.amazon.com/Pre-Suasion-Revolutionary-Way-Influence-Persuade/dp/1501109790&#34;&gt;Pre-suasion&lt;/a&gt;.

+I highly recommend reading those and applying what

+you learn to your daily life. I&amp;#8217;m definitely not

+a professional psychologist so take this post with

+a grain of salt.&lt;/p&gt;&lt;/div&gt;

+&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;When selling Erlang, whether we are selling it to

+a customer or trying to convince a developer friend

+to start using it, we often talk about how Erlang

+&lt;em&gt;lets you sleep at night&lt;/em&gt;, that it is auto healing

+and always gets fantastic uptimes.&lt;/p&gt;&lt;/div&gt;

+&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;And then we talk about &lt;em&gt;let it crash&lt;/em&gt;.&lt;/p&gt;&lt;/div&gt;

+&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;And we describe what it means.&lt;/p&gt;&lt;/div&gt;

+&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;We might as well just say that Erlang crashes a lot

+and then take the door. It would have the same effect.

+It doesn&amp;#8217;t even stop at programs crashing. You know

+what else crashes? Cars, planes, trains. Often with

+disastrous consequences. Is that really the message

+we want to convey?&lt;/p&gt;&lt;/div&gt;

+&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;They even &lt;a href=&#34;https://img.youtube.com/vi/oEUBW2lCkIk/0.jpg&#34;&gt;printed it on a t-shirt&lt;/a&gt;!

+Keep calm and let it crash. It&amp;#8217;s the kind of t-shirt

+you probably shouldn&amp;#8217;t wear in an airport, and for good

+reasons. A few people did, then realized what they were

+wearing and were not too smug about it.&lt;/p&gt;&lt;/div&gt;

+&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;And yet this is how we sell Erlang.&lt;/p&gt;&lt;/div&gt;

+&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;A better way would be to focus on the positives, of

+course, but also to make sure that those positives

+are phrased in a way that prevents bad associations

+to be formed in people&amp;#8217;s minds.&lt;/p&gt;&lt;/div&gt;

+&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;Instead of &lt;em&gt;let it crash&lt;/em&gt;, you can say that Erlang

+has &lt;em&gt;auto healing mechanisms&lt;/em&gt;. Healing is a good

+thing and accurately describes what happens in the

+system.&lt;/p&gt;&lt;/div&gt;

+&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;Should you need to go into more details, you will

+probably want to avoid &lt;em&gt;recover from crashes&lt;/em&gt; and

+instead say &lt;em&gt;recover from exceptions&lt;/em&gt;. Exceptions

+are a pretty neutral word and, should you explain

+what you mean by that, you can talk about exceptions

+that occur for reasons unrelated to Erlang, like

+hardware failure or network instability.&lt;/p&gt;&lt;/div&gt;

+&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;The trick is to always use positive words and

+phrases to describe Erlang, and to use external

+factors to explain how Erlang deals with failures.

+Never mention the failures internal to Erlang

+systems unless you are asked specifically, in

+which case you can say that the auto healing

+applies to all exceptions.&lt;/p&gt;&lt;/div&gt;

+&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;The &lt;em&gt;let it crash&lt;/em&gt; philosophy is great when

+learning Erlang or when writing fault-tolerant

+systems. But it&amp;#8217;s not going to convince anyone

+to use it unless they were already looking for

+it.&lt;/p&gt;&lt;/div&gt;

+&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;Do you like this post? Tell me on Twitter. I might

+make more.&lt;/p&gt;&lt;/div&gt;

+</description>
+    </item>
+    
+    <item>
       <title>Cowboy 2.0 pre-release 4</title>
       <link>https://ninenines.eu/articles/cowboy-2.0.0-pre.4/</link>
       <pubDate>Tue, 03 Jan 2017 00:00:00 +0100</pubDate>
@@ -1132,163 +1253,5 @@ project and make sure it doesn&amp;#8217;t happen again.&lt;/p&gt;&lt;/div&gt;
 </description>
     </item>
     
-    <item>
-      <title>Cowboy 2.0 and query strings</title>
-      <link>https://ninenines.eu/articles/cowboy2-qs/</link>
-      <pubDate>Wed, 20 Aug 2014 00:00:00 +0100</pubDate>
-      
-      <guid>https://ninenines.eu/articles/cowboy2-qs/</guid>
-      <description>&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;Now that Cowboy 1.0 is out, I can spend some of my time thinking

-about Cowboy 2.0 that will be released soon after Erlang/OTP 18.0.

-This entry discusses the proposed changes to query string handling

-in Cowboy.&lt;/p&gt;&lt;/div&gt;

-&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;Cowboy 2.0 will respond to user wishes by simplifying the interface

-of the &lt;code&gt;cowboy_req&lt;/code&gt; module. Users want two things: less

-juggling with the Req variable, and more maps. Maps is the only

-dynamic key/value data structure in Erlang that we can match directly

-to extract values, allowing users to greatly simplify their code as

-they don&amp;#8217;t need to call functions to do everything anymore.&lt;/p&gt;&lt;/div&gt;

-&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;Query strings are a good candidate for maps. It&amp;#8217;s a list of

-key/values, so it&amp;#8217;s pretty obvious we can win a lot by using maps.

-However query strings have one difference with maps: they can have

-duplicate keys.&lt;/p&gt;&lt;/div&gt;

-&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;How are we expected to handle duplicate keys? There&amp;#8217;s no standard

-behavior. It&amp;#8217;s up to applications. And looking at what is done in

-the wild, there&amp;#8217;s no de facto standard either. While some ignore

-duplicate keys (keeping the first or the last they find), others

-require duplicate keys to end with &lt;code&gt;[]&lt;/code&gt; to automatically

-put the values in a list, or even worse, languages like PHP even

-allow you to do things like &lt;code&gt;key[something][other]&lt;/code&gt; and

-create a deep structure for it. Finally some allow any key to have

-duplicates and just gives you lists of key/values.&lt;/p&gt;&lt;/div&gt;

-&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;Cowboy so far had functions to retrieve query string values one

-value at a time, and if there were duplicates it would return the

-first it finds. It also has a function returning the entire list

-with all duplicates, allowing you to filter it to get all of them,

-and another function that returns the raw query string.&lt;/p&gt;&lt;/div&gt;

-&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;What are duplicates used for? Not that many things actually.&lt;/p&gt;&lt;/div&gt;

-&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;One use of duplicate keys is with HTML forms. It is common practice

-to give all related checkboxes the same name so you get a list of

-what&amp;#8217;s been checked. When nothing is checked, nothing is sent at all,

-the key is not in the list.&lt;/p&gt;&lt;/div&gt;

-&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;Another use of duplicate keys is when generating forms. A good

-example of that would be a form that allows uploading any number

-of files. When you add a file, client-side code adds another field

-to the form. Repeat up to a certain limit.&lt;/p&gt;&lt;/div&gt;

-&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;And that&amp;#8217;s about it. Of note is that HTML radio elements share

-the same name too, but only one key/value is sent, so they are not

-relevant here.&lt;/p&gt;&lt;/div&gt;

-&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;Normally this would be the part where I tell you how we solve

-this elegantly. But I had doubts. Why? Because there&amp;#8217;s no good

-solutions to solving only this particular problem.&lt;/p&gt;&lt;/div&gt;

-&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;I then stopped thinking about duplicate keys for a minute and

-started to think about the larger problem.&lt;/p&gt;&lt;/div&gt;

-&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;Query strings are input data. They take a particular form,

-and may be sent as part of the URI or as part of the request

-body. We have other kinds of input data. We have headers and

-cookies and the request body in various forms. We also have

-path segments in URIs.&lt;/p&gt;&lt;/div&gt;

-&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;What do you do with input data? Well you use it to do

-something. But there is one thing that you almost always do

-(and if you don&amp;#8217;t, you really should): you validate it and

-you map it into Erlang terms.&lt;/p&gt;&lt;/div&gt;

-&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;Cowboy left the user take care of validation and conversion

-into Erlang terms so far. Rather, it left the user take care

-of it everywhere except one place. Guess where? That&amp;#8217;s right,

-bindings.&lt;/p&gt;&lt;/div&gt;

-&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;If you define routes with bindings then you have the option

-to provide constraints. Constraints can be used to do two things:

-validate the data and convert it in a more appropriate term. For

-example if you use the &lt;code&gt;int&lt;/code&gt; constraint, Cowboy will

-make sure the binding is an integer, and will replace the value

-with the integer representation so that you can use it directly.

-In this particular case it not only routes the URI, but also

-validates and converts the bindings directly.&lt;/p&gt;&lt;/div&gt;

-&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;This is very relevant in the case of our duplicate keys,

-because if we have a list with duplicates of a key, chances

-are we want to convert that into a list of Erlang terms, and

-also make sure that all the elements in this list are expected.&lt;/p&gt;&lt;/div&gt;

-&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;The answer to this particular problem is simple. We need a

-function that will parse the query string and apply constraints.

-But this is not all, there is one other problem to be solved.&lt;/p&gt;&lt;/div&gt;

-&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;The other problem is that for the user some keys are mandatory

-and some are optional. Optional keys include the ones that

-correspond to HTML checkboxes: if the key for one or more

-checkbox is missing from the query string, we still want to

-have an empty list in our map so we can easily match. Matching

-maps is great, but not so much when values might be missing,

-so we have to normalize this data a little.&lt;/p&gt;&lt;/div&gt;

-&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;This problem is solved by allowing a default value. If the

-key is missing and a default exists, set it. If no default

-exists, then the key was mandatory and we want to crash.&lt;/p&gt;&lt;/div&gt;

-&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;I therefore make a proposal for changing the query string

-interface to three functions.&lt;/p&gt;&lt;/div&gt;

-&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;The first function already exists, it is &lt;code&gt;cowboy_req:qs(Req)&lt;/code&gt;

-and it returns only the query string binary. No more Req returned.&lt;/p&gt;&lt;/div&gt;

-&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;The second function is a renaming of &lt;code&gt;cowboy_req:qs_vals(Req)&lt;/code&gt;

-to something more explicit: &lt;code&gt;cowboy_req:parse_qs(Req)&lt;/code&gt;.

-The new name implies that a parsing operation is done. It was implicit

-and cached before. It will be explicit and not cached anymore now.

-Again, no more Req returned.&lt;/p&gt;&lt;/div&gt;

-&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;The third function is the one I mentioned above. I think

-the interface &lt;code&gt;cowboy_req:match_qs(Req, Fields)&lt;/code&gt; is

-most appropriate. It returns a normalized map that is the same

-regardless of optional fields being provided with the request,

-allowing for easy matching. It crashes if something went wrong.

-Still no Req returned.&lt;/p&gt;&lt;/div&gt;

-&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;I feel that this three function interface provides everything

-one would need to comfortably write applications. You can get

-low level and get the query string directly; you can get a list

-of key/value binaries without any additional processing and do it

-on your own; or you can get a processed map that contains Erlang

-terms ready to be used.&lt;/p&gt;&lt;/div&gt;

-&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;I strongly believe that by democratizing the constraints to

-more than just bindings, but also to query string, cookies and

-other key/values in Cowboy, we can allow the developer to quickly

-and easily go from HTTP request to Erlang function calls. The

-constraints are reusable functions that can serve as guards

-against unwanted data, providing convenience in the process.&lt;/p&gt;&lt;/div&gt;

-&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;Your handlers will not look like an endless series of calls

-to get and convert the input data, they will instead be just

-one call at the beginning followed by the actual application

-logic, thanks to constraints and maps.&lt;/p&gt;&lt;/div&gt;

-&lt;div class=&#34;listingblock&#34;&gt;

-&lt;div class=&#34;content&#34;&gt;&lt;!-- Generator: GNU source-highlight 3.1.8

-by Lorenzo Bettini

-http://www.lorenzobettini.it

-http://www.gnu.org/software/src-highlite --&gt;

-&lt;pre&gt;&lt;tt&gt;&lt;span style=&#34;font-weight: bold&#34;&gt;&lt;span style=&#34;color: #000000&#34;&gt;handle&lt;/span&gt;&lt;/span&gt;(&lt;span style=&#34;color: #009900&#34;&gt;Req&lt;/span&gt;, &lt;span style=&#34;color: #009900&#34;&gt;State&lt;/span&gt;) &lt;span style=&#34;color: #990000&#34;&gt;-&amp;gt;&lt;/span&gt;

-    #{&lt;span style=&#34;color: #FF6600&#34;&gt;name&lt;/span&gt;&lt;span style=&#34;color: #990000&#34;&gt;:=&lt;/span&gt;&lt;span style=&#34;color: #009900&#34;&gt;Name&lt;/span&gt;, &lt;span style=&#34;color: #FF6600&#34;&gt;email&lt;/span&gt;&lt;span style=&#34;color: #990000&#34;&gt;:=&lt;/span&gt;&lt;span style=&#34;color: #009900&#34;&gt;Email&lt;/span&gt;, &lt;span style=&#34;color: #FF6600&#34;&gt;choices&lt;/span&gt;&lt;span style=&#34;color: #990000&#34;&gt;:=&lt;/span&gt;&lt;span style=&#34;color: #009900&#34;&gt;ChoicesList&lt;/span&gt;, &lt;span style=&#34;color: #FF6600&#34;&gt;remember_me&lt;/span&gt;&lt;span style=&#34;color: #990000&#34;&gt;:=&lt;/span&gt;&lt;span style=&#34;color: #009900&#34;&gt;RememberMe&lt;/span&gt;} &lt;span style=&#34;color: #990000&#34;&gt;=&lt;/span&gt;

-        &lt;span style=&#34;font-weight: bold&#34;&gt;&lt;span style=&#34;color: #000000&#34;&gt;cowboy_req:match_qs&lt;/span&gt;&lt;/span&gt;(&lt;span style=&#34;color: #009900&#34;&gt;Req&lt;/span&gt;, [

-            &lt;span style=&#34;font-weight: bold&#34;&gt;&lt;span style=&#34;color: #000080&#34;&gt;name&lt;/span&gt;&lt;/span&gt;, {&lt;span style=&#34;color: #FF6600&#34;&gt;email&lt;/span&gt;, &lt;span style=&#34;color: #FF6600&#34;&gt;email&lt;/span&gt;},

-            {&lt;span style=&#34;color: #FF6600&#34;&gt;choices&lt;/span&gt;, &lt;span style=&#34;font-weight: bold&#34;&gt;&lt;span style=&#34;color: #0000FF&#34;&gt;fun&lt;/span&gt;&lt;/span&gt; &lt;span style=&#34;font-weight: bold&#34;&gt;&lt;span style=&#34;color: #000000&#34;&gt;check_choices&lt;/span&gt;&lt;/span&gt;&lt;span style=&#34;color: #990000&#34;&gt;/&lt;/span&gt;&lt;span style=&#34;color: #993399&#34;&gt;1&lt;/span&gt;, []},

-            {&lt;span style=&#34;color: #FF6600&#34;&gt;remember_me&lt;/span&gt;, &lt;span style=&#34;color: #FF6600&#34;&gt;boolean&lt;/span&gt;, &lt;span style=&#34;color: #000080&#34;&gt;false&lt;/span&gt;}]),

-    &lt;span style=&#34;font-weight: bold&#34;&gt;&lt;span style=&#34;color: #000000&#34;&gt;save_choices&lt;/span&gt;&lt;/span&gt;(&lt;span style=&#34;color: #009900&#34;&gt;Name&lt;/span&gt;, &lt;span style=&#34;color: #009900&#34;&gt;Email&lt;/span&gt;, &lt;span style=&#34;color: #009900&#34;&gt;ChoicesList&lt;/span&gt;),

-    &lt;span style=&#34;font-weight: bold&#34;&gt;&lt;span style=&#34;color: #0000FF&#34;&gt;if&lt;/span&gt;&lt;/span&gt; &lt;span style=&#34;color: #009900&#34;&gt;RememberMe&lt;/span&gt; &lt;span style=&#34;color: #990000&#34;&gt;-&amp;gt;&lt;/span&gt; &lt;span style=&#34;font-weight: bold&#34;&gt;&lt;span style=&#34;color: #000000&#34;&gt;create_account&lt;/span&gt;&lt;/span&gt;(&lt;span style=&#34;color: #009900&#34;&gt;Name&lt;/span&gt;, &lt;span style=&#34;color: #009900&#34;&gt;Email&lt;/span&gt;); &lt;span style=&#34;color: #000080&#34;&gt;true&lt;/span&gt; &lt;span style=&#34;color: #990000&#34;&gt;-&amp;gt;&lt;/span&gt; &lt;span style=&#34;color: #FF6600&#34;&gt;ok&lt;/span&gt; &lt;span style=&#34;font-weight: bold&#34;&gt;&lt;span style=&#34;color: #0000FF&#34;&gt;end&lt;/span&gt;&lt;/span&gt;,

-    {&lt;span style=&#34;color: #FF6600&#34;&gt;ok&lt;/span&gt;, &lt;span style=&#34;color: #009900&#34;&gt;Req&lt;/span&gt;, &lt;span style=&#34;color: #009900&#34;&gt;State&lt;/span&gt;}&lt;span style=&#34;color: #990000&#34;&gt;.&lt;/span&gt;

-

-&lt;span style=&#34;font-weight: bold&#34;&gt;&lt;span style=&#34;color: #000000&#34;&gt;check_choices&lt;/span&gt;&lt;/span&gt;(&lt;span style=&#34;color: #990000&#34;&gt;&amp;lt;&amp;lt;&lt;/span&gt;&lt;span style=&#34;color: #FF0000&#34;&gt;&#34;blue&#34;&lt;/span&gt;&lt;span style=&#34;color: #990000&#34;&gt;&amp;gt;&amp;gt;&lt;/span&gt;) &lt;span style=&#34;color: #990000&#34;&gt;-&amp;gt;&lt;/span&gt; {&lt;span style=&#34;color: #000080&#34;&gt;true&lt;/span&gt;, &lt;span style=&#34;color: #FF6600&#34;&gt;blue&lt;/span&gt;};

-&lt;span style=&#34;font-weight: bold&#34;&gt;&lt;span style=&#34;color: #000000&#34;&gt;check_choices&lt;/span&gt;&lt;/span&gt;(&lt;span style=&#34;color: #990000&#34;&gt;&amp;lt;&amp;lt;&lt;/span&gt;&lt;span style=&#34;color: #FF0000&#34;&gt;&#34;red&#34;&lt;/span&gt;&lt;span style=&#34;color: #990000&#34;&gt;&amp;gt;&amp;gt;&lt;/span&gt;) &lt;span style=&#34;color: #990000&#34;&gt;-&amp;gt;&lt;/span&gt; {&lt;span style=&#34;color: #000080&#34;&gt;true&lt;/span&gt;, &lt;span style=&#34;color: #FF6600&#34;&gt;red&lt;/span&gt;};

-&lt;span style=&#34;font-weight: bold&#34;&gt;&lt;span style=&#34;color: #000000&#34;&gt;check_choices&lt;/span&gt;&lt;/span&gt;(&lt;span style=&#34;color: #990000&#34;&gt;_&lt;/span&gt;) &lt;span style=&#34;color: #990000&#34;&gt;-&amp;gt;&lt;/span&gt; &lt;span style=&#34;color: #000080&#34;&gt;false&lt;/span&gt;;&lt;/tt&gt;&lt;/pre&gt;&lt;/div&gt;&lt;/div&gt;

-&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;(Don&amp;#8217;t look too closely at the structure yet.)&lt;/p&gt;&lt;/div&gt;

-&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;As you can see in the above snippet, it becomes really easy

-to go from query string to values. You can also use the map

-directly as it is guaranteed to only contain the keys you

-specified, any extra key is not returned.&lt;/p&gt;&lt;/div&gt;

-&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;This would I believe be a huge step up as we can now

-focus on writing applications instead of translating HTTP

-calls. Cowboy can now take care of it.&lt;/p&gt;&lt;/div&gt;

-&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;And to conclude, this also solves our duplicate keys

-dilemma, as they now automatically become a list of binaries,

-and this list is then checked against constraints that

-will fail if they were not expecting a list. And in the

-example above, it even converts the values to atoms for

-easier manipulation.&lt;/p&gt;&lt;/div&gt;

-&lt;div class=&#34;paragraph&#34;&gt;&lt;p&gt;As usual, feedback is more than welcome, and I apologize

-for the rocky structure of this post as it contains all the

-thoughts that went into this rather than just the conclusion.&lt;/p&gt;&lt;/div&gt;

-</description>
-    </item>
-    
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