### On Lisp -> Clojure (Chapter 4)

; As always, I will post when the code is “complete”, but my progress can be followed on Github. Also, this post is executable, just copy and paste into a Clojure REPL.

Posts in this series: ch. 2, ch. 2 redux, ch. 3, ch. 4, ch. 5

; pg. 42

;; PG defines a general find function `(find2)`

that given a function and a list, returns both the first matched value and the value returned from the matching function. The most direct translation to Clojure is:

;

```
(defn findr [f sq]
(when (seq sq)
(let [value (f (first sq))]
(if (nil? value)
(recur f (rest sq))
[(first sq) value]))))
(defn evenr [elem]
(if (even? elem)
"is even"))
(findr evenr '(1 13 3 4))
(findr evenr '(1 13 3 5))
(findr evenr nil)
;
```

; pg. 45

;; L@@k: Why rseq not lazy.

;; Clojure really shines with a simple problem like this because it will work on lists, vectors, sorted maps, and strings.

;

```
(defn last1 [sq]
(last s))
;
```

;; Test for a sequence of one element ;

```
(defn single [sq]
(and (seq? sq) (not (rest sq))))
;
```

;; `(append1)`

seems too complicated. It needs some work, but for now it works.

;

```
(defn append1 [sq elem]
(if (seq elem)
(lazy-cons sq elem)
(concat sq (vector elem))))
;
```

;; It’s generally frowned on to produce side-effects in Clojure along the lines of what PG’s `(conc1)`

function does, but if you were so inclined it would be done as:

;

```
(defn conc1 [sq elem]
(dosync
(if (seq? elem)
(ref-set sq (lazy-cons @sq elem))
(ref-set sq (concat @sq (vector elem))))))
(def x (ref '(4 5 6)))
@x
(conc1 x 4)
@x
;
```

;

```
(defn makelist [elem]
(lazy-cons elem '()))
;
```

; pg. 47

;

```
(defn longer [x y]
(let [cmp (fn [x y]
(and (seq? x)
(or (nil? y)
(recur (rest x) (rest y)))))]
(if (and (seq? x) (seq? y))
(cmp x y)
(> (count x) (count y)))))
;
```

;; The list-comprehension macro supplied by Clojure provides a powerful way to filter a sequence based on a `:when`

or `:while`

. Therefore an implementation of the filter function is trivial.

;

```
(defn filtr [f lst]
(for [x lst :when (f x)]
x))
(filtr even? '(1 2 3 4))
(filtr even? (range 100000))
(filtr seq? '(1 2 3 (4 5) 6 [7] 8 (9) "10"))
;
```

;; `(group)`

could stand to be (a lot) more elegant

;

```
(defn group [source n]
(if (zero? n) (pr "error: zero length"))
(let [t (int (/ (count source) n))
remainder (drop (* n t) source)]
(lazy-cat
(for [i (range t)]
(take n (nthrest source (* i n))))
(if (nil? remainder)
'()
(list remainder)))))
(group '(1 2 3 4 5 6 7) 4)
(group '(1 2 3 4 5 6 7) 400)
;
```

; pg. 49

;

```
(defn flatten [x]
(if (seq? x)
(apply concat (map flatten x))
(list x)))
;
```

;; Clojure provides a powerful library for walking and editing trees functionally using a structure called a zipper. The library is fairly comprehensive, and its power is apparent in a few minutes playtime. There are a couple of missing features (outlined below), but overall it makes something like PG’s prune function a piece of cake.

;; One feature missing from the zip library is the ability to create a zip structure from any given type of seq-able data structure. That is, before you build a zipper you have to know the form of the data so that you can call one of `(seq-zip)`

, `(vector-zip)`

, or `(xml-zip)`

. This is a minor point overall, but making the prune more generic requires some work up front.

;

```
(defn zip-util [root]
(if (seq? root)
(zip/seq-zip root)
(zip/vector-zip root)))
;
```

;; The prune function itself is taken alomst verbatim from the clojure zip examples except for a couple minor changes, one of which is to allow a filter function to decide the pruning as in “On Lisp” plus a call to `(zip-util)`

to handle different types of structures. Another useful feature missing from the zip library is a predicate that can be used to check if a structure is already a zipper. As it stands trying to zip a zipper throws an exception and there was no clear way (that I could find) to perform such a check.

;

```
(defn prune [f tree]
(loop [loc (zip-util tree)]
(if (zip/end? loc)
(zip/root loc)
(recur
(zip/next
(if (f (zip/node loc))
(zip/remove loc)
loc))))))
;
```

;; PG’s `(prune)`

function works on nodes only, but I thought it might be better to work on branches also. Of course, this breaks the ability to just send in something like `(even?)`

, but I think that is a minor tradeoff.

;

```
(defn node-filter [x]
(if (zip/branch? x)
nil
(even? x)))
(def s '(1 2 (3 (4 5) 6) 7 8 (9)))
(def v [1 2 [3 [4 5] 6] 7 8 [9]])
(prune node-filter s)
(prune node-filter v)
;
```

; pg. 50

;; PG’s `(before)`

is alomst a direct translation, except for one major difference: Clojure does not have default argument values. Therefore, one must always pass in a test function to get the same effect. I decided to drop the test function *for now* to simplify the function.

;

```
(defn before? [x y sq]
(and sq
(let [elem (first sq)]
(if (= y elem)
nil
(if (= x elem)
sq
(recur x y (rest sq)))))))
(before? 'b 'c '(1 2 a b c))
(before? 1 'c '(1 2 a b c))
(before? 'a 1 '(1 2 a b c))
;; This is from CS-101
(defn member [x sq]
(if (seq sq)
(if (= x (first sq))
sq
(recur x (rest sq)))))
(defn after? [x y sq]
(let [elem (before? y x sq)]
(and elem
(member x elem))))
(after? 'b 'c '(1 2 a b c))
(after? 'c 'a '(1 2 a b c))
(defn duplicate? [obj sq]
(member obj (rest (member obj sq))))
(duplicate? 'a '(1 2 a b c a d))
;; Needs work
(defn split-if [f sq]
[(for [x sq :when (f x)] x) (for [x sq :when (not (f x))] x)])
(split-if #(= % 5) '(1 2 3 4 5 6 7 8 9 10))
(split-if #(< % 5) '(1 2 3 4 5 6 7 8 9 10))
(split-if #(> % 500) '(1 2 3 4 5 6 7 8 9 10))
(split-if #(if (< % 7) true false) '(1 2 3 4 5 6 7 8 9 10))
;
```

;; Something to note: Clojure provides shorhand notation for lambda's containing a single call of #(). This notation allows for the passing of multiple arguments each accessed via the % operator (e.g. %1 %2 etc...)

; pg. 52

;; Continuing with my abuse of the list-comprehension (only until it becomes second nature... any day now), the most function is pretty trivial.

;

```
(defn most [f sq]
(let [wins (ref (first sq))
mx (ref (f @wins))]
(doall
(for [x (rest sq) :when (> (f x) @mx)]
[(dosync (ref-set wins x) (ref-set mx (f x)))]))
(list @wins @mx)))
(most count '((1 2 3) (1 2)))
(most count '((1 2 3) (1 2 3 4)))
;
```

;; It's important to note that the `(for)`

had to be wrapped in a `(doall)`

because the former generates a lazy sequence and will only supply the first argument, therefore you're always going to see the first item in the sequence as the answer without the latter. It took me a while to track this down. :(

;

```
(defn best [f sq]
(let [wins (ref (first sq))]
(last
(for [x (rest sq) :when (f x @wins)]
(dosync (ref-set wins x))))))
(best > '(1 2 3 4 5))
(best > '(1 2 7 4 5))
(best > nil)
;
```

;; But of course, when Rich Hickey looked at my `(best)`

function he gave me a virtual pat on the head and hit me with:

;

```
(defn best [f xs] (reduce #(if (f %1 %2) %1 %2) xs))
;
```

;; I remain humbled.

;; So of course, that means that `(most)`

can more elegantly be written as:

;

```
(defn most [f xs] (reduce #(if (> (f %1) (f %2)) (list %1 (count %1)) (list %2 (count %2))) xs))
;
```

;; So PG goes on with this vein for a few more pages creating more utlity functions, some of which could be useful later on. PG's main goal for Chapter 4 is to propone the virtues of Bottom-up programming, which I buy, but since *my* main goal is to learn Clojure and work out the long-dormant FP muscles, I only chose a handful of functions that furthered my goal.

;; Stay tuned for Chapter 5.

;-m

## 5 Comments, Comment or Ping

David Siegel

Shouldn’t (most) be:

(defn most [f xs] (let [x (reduce #(if (> (f %1) (f %2)) %1 %2) xs)] [x (f x)]))

Dec 14th, 2008

Kunjan

Hi, I am learning clojure. And your posts are very very helpful. Could you tell me why you use lazy to add to the list. I have seen a lot of lazy-cons and lazy-cat. What is the advantage of these?

Thanks.

Jul 22nd, 2009

fogus

@Kunjan,

One thing to be aware of with these posts — they are hopelessly out of date. My use of

`lazy-cons`

is actually no longer legal Clojure code. At the time of this writing it was the way to cons an element onto the head of a sequence in a lazy way. By lazy I mean that the elements of the sequence are not calculated until accessed. Clojure is nowmostlylazy and therefore it makes`lazy-cons`

unnecessary. The more appropriate way to do this now is to use`lazy-seq`

like so:Hopefully, that clarifies matters.

-m

Jul 23rd, 2009

Nevena

flattenfunction won’t work. clojure.core 1.2 has the right one.Dec 3rd, 2010

efraim

Shouldn’t

`seq?`

be`sequential?`

in the definition of`flatten`

, since testing vectors for`seq?`

returns false ?Nov 23rd, 2017

## Reply to “On Lisp -> Clojure (Chapter 4)”