Add cookies

_config.yml

@@ -17,7 +17,7 @@ title: Main
 description: > # this means to ignore newlines until "baseurl:"
   Programmer and programming related content.
 baseurl: "" # the subpath of your site, e.g. /blog
-url: "http://pgebal.github.io" # the base hostname & protocol for your site, e.g. http://example.com
+url: "http://pawelgebal.com"
 twitter_username: pawelwgebal
 github_username: pgebal
 disqus:

_posts/2016-12-04-covariant_contravariant.markdown

@@ -0,0 +1,60 @@
+---
+layout: post
+title:  "A short one: Covariant type in a contravariant position"
+date:   2016-12-04 12:00:00 +0100
+categories: scala update
+tags: scala functional-programming
+excerpt: It will never happen to you after reading this.
+---
+There is an exercise in the great [Functional Programming in Scala][functional-programming-in-scala-affiliate-link] book, that requires you to
+implement **Option** trait and it's **getOrElse** method.
+My first try on it was:
+{: .text-justify}
+{% highlight scala %}
+sealed trait Option[+A] {
+
+  def getOrElse(default: => A): A = this match {
+    case Some(a) => a
+    case None => default
+  }
+}
+
+case class Some[+A](value: A) extends Option[A]
+case object None extends Option[Nothing]
+{% endhighlight %}
+
+IntelliJ Idea was quick to tell me that it's not a right solution because:
+{% highlight scala %}
+Covariant type A in contravariant position in type A of value default.
+{% endhighlight %}
+
+I looked to the book and noticed that they suggest to use the following definition for **getOrElse**:
+{% highlight scala %}
+def getOrElse[B >: A](default: => B): B = this match {
+  case Some(a) => a
+  case None => default
+}
+{% endhighlight %}
+
+At first I didn't know why the solution required to use type parameter **B** that is a supertype of **A**.
+It turned out to be quite simple.
+Notice that **A** is a covariant type parameter for **Option**.
+That means for every **A** that is a subtype of **B**, **Option[A]** is a subtype of **Option[B]**.
+So whenever we use expression of type **Option[A]**, we have to be able to substitute it with expression of type **Option[B]**.
+Study this:
+{: .text-justify}
+
+{% highlight scala %}
+class B
+class A extends B
+
+val b: Option[B] = None
+val a: Option[A] = None
+
+val default: B = b.getOrElse(new B)
+{% endhighlight %}
+Note that in the last line we could use **a** instead of **b** and the code would stay valid. That's thanks to <nobr><b>[B >: A]</b></nobr> bound in **getOrElse** method definition. If it wasn't for that **a.getOrElse** would not accept an argument of type **B**. Keep this example in mind and embrace [Liskov substitution principle][liskov-substitution-principle] when implementing your own classes with type parameters.
+{: .text-justify}
+
+[functional-programming-in-scala-affiliate-link]: https://www.amazon.com/gp/product/1617290653/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=1617290653&linkCode=as2&tag=pawelgebal-20&linkId=f3ec949cadbc0ff936a5aae1dcc51c0a
+[liskov-substitution-principle]:https://en.wikipedia.org/wiki/Liskov_substitution_principle

_site/about/index.html

@@ -41,6 +41,10 @@
 
 
 
+          <a class="page-link" href="/cookies/">Cookies</a>
+
+
+
 
 
 

_site/feed.xml

@@ -1,11 +1,11 @@
-<?xml version="1.0" encoding="utf-8"?><?xml-stylesheet type="text/xml" href="http://localhost:4000/feed.xslt.xml"?><feed xmlns="http://www.w3.org/2005/Atom"><generator uri="http://jekyllrb.com" version="3.3.1">Jekyll</generator><link href="http://localhost:4000/feed.xml" rel="self" type="application/atom+xml" /><link href="http://localhost:4000/" rel="alternate" type="text/html" /><updated>2016-11-27T13:03:30+01:00</updated><id>http://localhost:4000//</id><title type="html">Main</title><subtitle>Programmer and programming related content.
+<?xml version="1.0" encoding="utf-8"?><?xml-stylesheet type="text/xml" href="http://localhost:4000/feed.xslt.xml"?><feed xmlns="http://www.w3.org/2005/Atom"><generator uri="http://jekyllrb.com" version="3.3.1">Jekyll</generator><link href="http://localhost:4000/feed.xml" rel="self" type="application/atom+xml" /><link href="http://localhost:4000/" rel="alternate" type="text/html" /><updated>2016-12-02T22:07:19+01:00</updated><id>http://localhost:4000//</id><title type="html">Main</title><subtitle>Programmer and programming related content.
 </subtitle><entry><title type="html">Type class pattern</title><link href="http://localhost:4000/scala/update/2016/11/27/type-classes.html" rel="alternate" type="text/html" title="Type class pattern" /><published>2016-11-27T11:00:00+01:00</published><updated>2016-11-27T11:00:00+01:00</updated><id>http://localhost:4000/scala/update/2016/11/27/type-classes</id><content type="html" xml:base="http://localhost:4000/scala/update/2016/11/27/type-classes.html">&lt;p&gt;To get the most out of this article I encourage you to put the pieces of code you find here into a scala worksheet and play with it.&lt;/p&gt;
 
 &lt;p&gt;Let’s have a class that we want to serialize to JSON:&lt;/p&gt;
 
 &lt;figure class=&quot;highlight&quot;&gt;&lt;pre&gt;&lt;code class=&quot;language-scala&quot; data-lang=&quot;scala&quot;&gt;&lt;span class=&quot;k&quot;&gt;case&lt;/span&gt; &lt;span class=&quot;k&quot;&gt;class&lt;/span&gt; &lt;span class=&quot;nc&quot;&gt;Color&lt;/span&gt;&lt;span class=&quot;o&quot;&gt;(&lt;/span&gt;&lt;span class=&quot;n&quot;&gt;name&lt;/span&gt;&lt;span class=&quot;k&quot;&gt;:&lt;/span&gt; &lt;span class=&quot;kt&quot;&gt;String&lt;/span&gt;&lt;span class=&quot;o&quot;&gt;,&lt;/span&gt; &lt;span class=&quot;n&quot;&gt;red&lt;/span&gt;&lt;span class=&quot;k&quot;&gt;:&lt;/span&gt; &lt;span class=&quot;kt&quot;&gt;Int&lt;/span&gt;&lt;span class=&quot;o&quot;&gt;,&lt;/span&gt; &lt;span class=&quot;n&quot;&gt;green&lt;/span&gt;&lt;span class=&quot;k&quot;&gt;:&lt;/span&gt; &lt;span class=&quot;kt&quot;&gt;Int&lt;/span&gt;&lt;span class=&quot;o&quot;&gt;,&lt;/span&gt; &lt;span class=&quot;n&quot;&gt;blue&lt;/span&gt;&lt;span class=&quot;k&quot;&gt;:&lt;/span&gt; &lt;span class=&quot;kt&quot;&gt;Int&lt;/span&gt;&lt;span class=&quot;o&quot;&gt;)&lt;/span&gt;&lt;/code&gt;&lt;/pre&gt;&lt;/figure&gt;
 
-&lt;p&gt;If we would go Java style, we might have come up with this:&lt;/p&gt;
+&lt;p&gt;If we went Java style, we might come up with this:&lt;/p&gt;
 
 &lt;figure class=&quot;highlight&quot;&gt;&lt;pre&gt;&lt;code class=&quot;language-scala&quot; data-lang=&quot;scala&quot;&gt;&lt;span class=&quot;k&quot;&gt;type&lt;/span&gt; &lt;span class=&quot;kt&quot;&gt;Json&lt;/span&gt; &lt;span class=&quot;o&quot;&gt;=&lt;/span&gt; &lt;span class=&quot;nc&quot;&gt;String&lt;/span&gt;
 
@@ -47,7 +47,7 @@ How about sticking to the Single Responsibility Principle and pushing serializat
 &lt;span class=&quot;k&quot;&gt;val&lt;/span&gt; &lt;span class=&quot;n&quot;&gt;json&lt;/span&gt;&lt;span class=&quot;k&quot;&gt;:&lt;/span&gt; &lt;span class=&quot;kt&quot;&gt;Json&lt;/span&gt; &lt;span class=&quot;o&quot;&gt;=&lt;/span&gt; &lt;span class=&quot;nc&quot;&gt;ColorJsonSerializer&lt;/span&gt;&lt;span class=&quot;o&quot;&gt;.&lt;/span&gt;&lt;span class=&quot;n&quot;&gt;toJson&lt;/span&gt;&lt;span class=&quot;o&quot;&gt;(&lt;/span&gt;&lt;span class=&quot;n&quot;&gt;yellow&lt;/span&gt;&lt;span class=&quot;o&quot;&gt;)&lt;/span&gt;&lt;/code&gt;&lt;/pre&gt;&lt;/figure&gt;
 
 &lt;p class=&quot;text-justify&quot;&gt;to get a JSON representation of yellow.
-That’s better then the previous solution, but we can pimp it using some implicit magic.
+That’s better than the previous solution, but we can pimp it using some implicit magic.
 First let’s make &lt;strong&gt;JsonSerializer.toJson&lt;/strong&gt; method look like it’s a method of &lt;strong&gt;Color&lt;/strong&gt; class.
 To achieve that, let’s add an implicit wrapper over &lt;strong&gt;Color&lt;/strong&gt;.&lt;/p&gt;
 

_site/index.html

@@ -41,6 +41,10 @@
 
 
 
+          <a class="page-link" href="/cookies/">Cookies</a>
+
+
+
 
 
 

_site/scala/update/2016/11/20/method-function-eta.html

@@ -41,6 +41,10 @@
 
 
 
+          <a class="page-link" href="/cookies/">Cookies</a>
+
+
+
 
 
 

cookies.md

@@ -0,0 +1,8 @@
+---
+layout: page
+title: Cookies
+permalink: /cookies/
+---
+This blog uses cookies to track site usage by Google Analytics.
+
+By browsing this site, you consent to the use of cookies.