Write chapter 4 summary

README.md

@@ -20,16 +20,14 @@ I haven't completely solved the following exercises.
   does not work for joined queries.
 - Exercise 4.79. I did not attempt to solve this exercise because of not
   finishing the previous one, and it is the last one in the chapter.
-- Exercise 5.52. I have implemented the basic structure of a Scheme to C
-  translator, but I only finished a basic proof-of-concept. Most of the
-  additional work would be similar to 5.51.
+- Exercise 5.52. I have implemented the Scheme to C interpreter and it works
+  for simple programs like computing factorials or Fibonacci numbers.
 
 I had a great time working through this book. I feel like my mental capabilities
 improved throughout the process, and finishing all the exercises gives me an
 incredible feeling of accomplishment. I have written short summaries for each of
 the chapters below.
 
-
 # Chapter 1
 
 The first chapter of SICP starts by explaining the Scheme syntax. The first
@@ -190,3 +188,68 @@ example, in embedded development.
 
 # Chapter 4
 
+Chapter 4 introduces the concept of a metacircular evaluator. That is a Scheme
+program that interprets Scheme programs. This approach enables us to use the
+native interpreter while gradually replacing procedures with our
+implementations.
+
+In particular, we can read the Scheme code with the `read` procedure without
+worrying about lexing or parsing. Calling read on a Scheme program returns a
+data representation of the program. In other words, we can think of a program as
+an abstraction for a machine.
+
+Throughout the first section, we finish a first version of the metacircular
+evaluator, including implementing the environment model via frames. We then
+optimize the evaluator by adding an analyzer layer that optimizes a program for
+faster interpretation by avoiding repetitive interpretation of static data.
+
+In the next section, we change the metacircular evaluator to lazy evaluation. In
+one of the book's first exercises, we learned that Scheme is an
+applicative-order language, meaning that Scheme evaluates all arguments before a
+procedure call. In contrast, lazy evaluation delays the evaluation until a
+primitive procedure requires the actual value. By making this change, we turn
+Scheme into a normal-order language.
+
+Moving forward, we learn about even more exciting models for computation. In the
+next section, we implement a non-deterministic version of Scheme via the `amb`
+operator. This operator allows us to assign multiple values to a variable. The
+amb evaluator then determines valid values based on constraints given in the
+program. In the most straightforward implementation, the amb evaluator searches
+over all possible ways. That means the programmer must add conditions to
+optimize the program.
+
+Here is a simple example that shows how the amb evaluator works:
+
+```scheme
+]=> (list (amb 1 2 3) (amb 'a 'b))
+(1 a) (1 b) (2 a) (2 b) (3 a) (3 b)
+```
+
+In the following section, we implement an interpreter for logic-based programs.
+The book introduces a database of employees in a company, and we can then query
+that database based on specific constraints. For example, to see who Ben
+Bitdiddle supervises, we could use the following query.
+
+```
+(eval-query '(supervisor ?x (bitdiddle ben)))
+```
+
+I have found it fascinating to learn about these alternative models for
+computation. The chapter has helped me better understand database query
+languages like SQL and the Prolog programming language that I dabbled with a
+couple of years ago. It's fascinating to see how we can add entirely different
+programming concepts to Scheme.
+
+
+# Chapter 5
+
+
+
+# Resources
+
+- [SICP book](https://mitpress.mit.edu/sites/default/files/sicp/full-text/book/book-Z-H-4.html#%_toc_start)
+- [SICP solutions](http://community.schemewiki.org/?SICP-Solutions)
+- [SICP code from book](https://mitpress.mit.edu/sites/default/files/sicp/code/index.html)
+- [Scheme specification](https://schemers.org/Documents/Standards/R5RS/r5rs.pdf)
+- [Scheme parser spec](https://amirkamil.github.io/project-scheme-parser/scheme-parser-spec.pdf)
+