Errata

Clutch project's URL has changed.

https://github.com/clojure-clutch/clutch

Old URL in current safari version redirects to that.

Note from the Author or Editor:
Correct.

Change

https://github.com/ashafa/clutch

to

https://github.com/clojure-clutch/clutch

Executing the code in a REPL works if not modified.
(letfn [(odd? [n]
(even? (dec n)))
(even? [n]
(or (zero? n)
(odd? (dec n))))]
(odd? 2))
(odd? 11))
;= true

If you change the last line to execute something false, it produces a StackOverflowError.
(odd? 2)
(odd? 0)
(even? 1)
All produce the StackOverflowError.

I was analyzing the code to see if I could figure out how it works when it reaches a false condition. I couldn't. So I decided to attempt to execute it to learn. It failed.

Thanks.

Note from the Author or Editor:
Good catch, sorry for the error!

Change on page 37, replace this code listing:

(letfn [(odd? [n]
(even? (dec n)))
(even? [n]
(or (zero? n)
(odd? (dec n))))]
(odd? 11))
;= true

with this one:

(letfn [(odd? [n]
(if (zero? n)
false
(even? (dec n))))
(even? [n]
(or (zero? n)
(odd? (dec n))))]
(odd? 11))
;= true

Equivalent expression for condition checking in if special form is wrong. It doesn't work for bolean false (it evaluates to true).

Should be (and (not (nil? x)) (not (false? x)))

Note from the Author or Editor:
PDF page 42, last line, change

... which is equivalent to (or (not (nil? x)) (true? x)) ...

to :

... which is equivalent to (and (not (nil? x)) (not (false? x))) ...

(= five (StatefulInteger. 42))
;= true

should read:

(= six (StatefulInteger. 42))
;= true

my full interaction:

$ javac StatefulInteger.java
$ mv StatefulInteger.class ../classes/
user> (import 'StatefulInteger)
StatefulInteger
user> (def five (StatefulInteger. 5))
#'user/five
user> (def six (StatefulInteger. 6))
#'user/six
user> (.intValue five)
5
user> (= five six)
false
user> (.setInt five 6)
nil
user> (= five six)
true
user> (defn print-number
[n]
(println (.intValue n))
(.setInt n 42))
#'user/print-number
user> (print-number six)
6
nil
user> (= five six)
false
user> (= five (StatefulInteger. 42))
false
user> (= six (StatefulInteger. 42))
true

Note from the Author or Editor:
Thanks for catching that!

Change on page 56:

`(= five (StatefulInteger. 42))`

to

`(= six (StatefulInteger. 42))`

(reduce max [0 -3 10 48])
;= 10

should be

(reduce max [0 -3 10 48])
;= 48

Great book! Thanks for writing it.

On my machine:

((comparator compare-magnitude) 10 10000)
;= -1
((comparator compare-magnitude) 100 10)
;= -1
((comparator compare-magnitude) 10 75)
;= -1

not what is in the book.

AFAICS compare-magnitude is not much of a predicate - it is never logically false - no?

Thanks again.

Note from the Author or Editor:
PDF page 108, change this function:

(defn compare-magnitude
[a b]
(- (magnitude a) (magnitude b)))

to:

(defn compare-magnitude
[a b]
(neg? (- (magnitude a) (magnitude b))))

swapped the order of parameter of type set:

you call :

(neighbours %) #{2 3} #{3 } )

but the set are swapped : the correct order of set are , maybe

(neighbours %) #{3 } #{2 3} )

Note from the Author or Editor:
Good catch, Thanks!

Change on page 145:

#{2 3} #{3}

to:

#{3} #{2 3}

The definition of hex-grid is missing two closing parentheses. The last line should be:
:when w] #{v w}))))

Note from the Author or Editor:
Bizarre?thanks for catching that print error!

Change:

Add two closing parentheses to the end of the `hex-grid` function.

The definition of hex-outer-walls is missing two closing parentheses. The last line should be:
:when (not (vertices w))] #{v (vec w)}))))

Note from the Author or Editor:
Bizarre?thanks for catching that print error.

Change:

Add two closing parentheses to the end of the `hex-outer-walls` function.

The function enforce-max-health only accepts one parameter, but in the character function 2 values are being passed to it, so you get an arity error if you try to use the code as written.

The easiest fix is to change the parameters in enforce-max-health from [{:keys [name health]}] to just [name health].

Also, it isn't clear in that function that the health being passed to it is the max-health value when the function is being created as a validator, it just happens to be that max-health is set to the health value at the same time. It does state in point 1 that it returns a function that acceps a char's "potential" new state, but still, It might be clearer to declare the variable in enforce-max-health as "max-health" instead of health.

Note from the Author or Editor:
Yup, we caught this as well when preparing the sample code repository (i.e. https://github.com/clojurebook/ClojureProgramming/blob/0913e2edc38b38f2da80d41aa2b28d3861a69947/ch04-concurrency-game/src/com/clojurebook/concurrency/game_validators.clj#L29).

Change on page 190:

[{:keys [name health]}]

[name health]

You're right that the parameter should probably be called `max-health`?we'll see about that in the second edition, etc. Thanks!

On the line
(println "Error!" (.getMessage) @e)))

the .getMessage method is missing the parameter. Should be:
(println "Error!" (.getMessage e) @e)))

Note from the Author or Editor:
In the code listing on page 380, change:

(println "Error!" (.getMessage) @e)))

to

(println "Error!" (.getMessage e) @e)))

Getting the declared methods for the round function uses (round foo), but should be (class round).

So this:

(seq (.getDeclaredMethods (round foo)))

should be:

(seq (.getDeclaredMethods (class round)))

Note from the Author or Editor:
Thank you for the report!

Change:

s/round foo/class round/g

The argument for attrs function should be attr-map instead of attrs. Accordingly, the line
(->> attrs
should be replaced with
(->> attr-map

Note from the Author or Editor:
In the code listing on page 486 (in the middle of the page), change:

[attrs] to [attr-map]

AND

->> attrs

to

->> attr-map

Code reads:
((PUT ["/:id" :id #"\d+"]
[id url]
(list "You requested that " url " be assigned id " id))
{:uri "/590" :params {:url "http://clojurebook.com"} :request-method :put})
;= {:status 200, :headers {"Content-Type" "text/html"},
;= :body "You requested that http://clojurebook.com be assigned id 590"}

You don't get the result in the last line from the function
(list "You requested that " url " be assigned id " id))

You can get it from
(str "You requested that " url " be assigned id " id))

Note from the Author or Editor:
In the code listing in the middle of page 541, change:

(list

to

(str

The plugin slot of defproject:
plugins [[lein-ring "0.6.2"]]
is missin the colon. Should be:
:plugins [[lein-ring "0.6.2"]]

Note from the Author or Editor:
In Example 17-4, change:

plugins

to

:plugins