Scalaz Show

In this post we’ll look at Scalaz Show. The only purpose of this trait is to provide a String representation for its subtypes.

A Simple Example

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@ import scalaz._
import scalaz._
@ import Scalaz._
import Scalaz._
@ 3.show
res2: Cord = Cord(1 [3])
@ 3.println
3

As always, there are implicits defined for Int, Float, etc. Calling println returns a String whereas calling show returns a Cord. Going over the source code for Cord:

A Cord is a purely functional data structure for efficiently storing and manipulating Strings that are potentially very long.

Why Use Show?

A reasonable question to ask is why use Show when we have a toString which produces a String representation of objects. The answer is that toSring doesn’t always produce a useful representation. 

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@ println(new Thread())
Thread[Thread-185,5,main]

Showing a Thread

So, let’s create a Show for Thread. 

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@ implicit object ThreadShowable extends Show[Thread] {
    override def shows(t: Thread): String = s"Thread Id=${t.getId} name=${t.getName}"
  }
defined object ThreadShowable
@ val t = new Thread()
t: Thread = Thread[Thread-322,5,main]
// Scala way
@ println(t)
Thread[Thread-322,5,main]
// Scalaz way
@ t.println
Thread Id=352 name=Thread-322

Conclusion

Show is probably not that interesting and probably exists because there is Show class in Haskell[1]:

The instances of class Show are those types that can be converted to character strings (typically for I/O)