From 29b9359b1dfbfd3ff7a78456f14f1ed9d9967ca6 Mon Sep 17 00:00:00 2001
From: WJH <hou32hou@gmail.com>
Date: Sat, 18 May 2019 20:28:48 +0800
Subject: [PATCH] Explain purpose of Kleisli Categories from the programmer's
 perspective

---
 src/content/1.4/kleisli-categories.tex | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/src/content/1.4/kleisli-categories.tex b/src/content/1.4/kleisli-categories.tex
index ed539f31..ceba1ece 100644
--- a/src/content/1.4/kleisli-categories.tex
+++ b/src/content/1.4/kleisli-categories.tex
@@ -400,6 +400,23 @@ \section{Kleisli Categories}
 programs that in imperative languages are traditionally implemented
 using side effects.
 
+But, what is the real purpose of this concept from the perspective of a 
+programmer? Improved code maintainability. Recall that the \code{process}
+function can actually be written without using the fish operator:
+
+\begin{snip}{hs}
+process :: String -> Writer [String]
+process s = 
+    let (s' , log)  = upCase s   in
+    let (s'', log') = toWords s' in
+    (s'', log ++ log')
+\end{snip}
+
+However, this piece of code is not only harder to comprehend, it is also
+difficult to modify (imagine that you want to add new operations or 
+change the sequence of exisitng operations). That is why we need to leverage 
+the power of Kleisli Categories along with the fish operator.  
+
 \section{Challenge}
 
 A function that is not defined for all possible values of its argument
@@ -443,4 +460,4 @@ \section{Challenge}
   Compose the functions \code{safe\_root} and \code{safe\_reciprocal} to implement
   \code{safe\_root\_reciprocal} that calculates \code{sqrt(1/x)}
   whenever possible.
-\end{enumerate}
\ No newline at end of file
+\end{enumerate}