Ch 6 + 0.5 of Ch 7 of Brave Clojure

Jul 10, 2020

Pascal’s Triangles
Chapter 6
- Interning a var
- Project Organization
Chapter 7
Takeaways

Pascal’s Triangles

In mathematics, Pascal’s triangle is a triangular array of the binomial coefficients.

For example -

    [1]
   [1 1]
  [1 2 1]
 [1 3 3 1]
[1 4 6 4 1]

The 4Clojure problem asks you to create a function which returns the nth row of Pascal’s Triangle. I tried solving it before with map and reduce but it didn’t work out.

Today, I decided to give the problem another shot. And lo and behold, a solution came to me in about 5 minutes.

(defn pascalize
  ([n] (pascalize (dec n) '(1)))
  ([n p]
   (if (zero? n)
     p
     (recur (dec n) (conj (map #(apply + %)  (partition-all 2 1 p)) 1)))))

Dead simple. I think this was the first time I instinctively reached for a loop recur construct to solve a problem.

Chapter 6

map and inc are symbols

When you give clojure a symbol like map, it finds the corresponding var in the current namespace, gets a shelf address, and retrieves an object from that shelf for you.

if you want to use the symbol itself, quote it

'inc
=> inc

'(map inc [1 2 3])
=> (map inc [1 2 3])

(def great-books ["Sherlock Holmes" "Alice in Wonderland"])
=> #'clojure-noob.ch6/great-books

great-books
=> ["Sherlock Holmes" "Alice in Wonderland"]

Interning a var

Updates the current ns map with the association between great-books and the var
Stores the vector on a free shelf spot
Returns the address of the shelf on the var Which is #'clojure-noob.ch6/great-books
use (ns-interns ns) to see the namespace’s map

(get (ns-interns *ns*) 'great-books)
#'clojure-noob.ch6/great-books -> reader form of a var

;; use #' to grab ahold of the var corresponding to the symbol

(deref #'clojure-noob.ch6/great-books)
;; => ["Sherlock Holmes" "Alice in Wonderland"]

(def great-books ["The power of bees" "Journey to Upstairs"])

great-books
;; => ["The power of bees" "Journey to Upstairs"]

create-ns -> take a symbol, creats a namepsace with that name if it doesnt exist already and returns the namespace

(create-ns 'cheese.taxonomy)
=> #namespace[cheese.taxonomy]

You can use the returned namespace as an argument in a function call

(ns-name (create-ns 'cheese.taxonomy))
=> cheese.taxonomy

in-ns -> creates a namespace and switches to it
use the fully-qualified name to refer to vars and fns from a different ns
refer -> fine grained control over how to refer objects in other namespaces calling refer with a ns symbol lets you refer to the corresponding namespace’s symbols without having to use fully-qualified symbols
filters -> :only, :exclude, :rename
alias lets you call functions in other namespaces with a shorter name (or more semantically relevant)

Project Organization

source code is in src/
Dashes in namespace names correspond to underscores in the filesystem
To use code from a different file, we have to require it
require takes a symbol designating a namespace and ensures that the namespace exists and is ready to be used.
require lets you alias a namespace when you require it using :as or :alias
(require '[the-divine-cheese-code.visualization.svg :as svg])
equivalent to require and then alias

ns macro allows you to incorporate require, use, in-ns alias and refer ns refers the clojure.core namespace by default

(ns the-divine-cheese-code.core
  (:refer-clojure :exclude [println]))

Chapter 7

Macros allow you to transform arbitrary expressions into valid Clojure

Clojure Evaluation model -

Reads textual source code first -> produces Clojure data structures
Clojure data structures -> evaluated

Homoiconic -> reason about the code as a set of data structures that you can manipulate programmatically.

Lisps evaluate native data structures
Clojure still evaluates tree structures, but the trees are structured using Clojure liss and the nodes are Clojure values
Clojure reader converts code (+ 1 (* 6 7)) into a nested list.
Clojure evaluator takes that data as input and produces a result. (also compiles to the JVM bytecode)

Takeaways

Coming back to problems after a while is good and can yield productive results. I also need to revisit old functions and what I’ve learned previously to keep the knowledge fresh in my mind.

Today’s tally -

1 4Clojure problem
1.5 Brave and True Clojure chapters

Overthunk