Clojure and me » 2009

.toUpperCase

utilities — cgrand, 29 April 2009 @ 20 h 37 min

I recently learnt that when you want to convert the case of a technical identifier (a tagname, a HTTP header etc.) you must not use plain .toUpperCase or .toLowerCase but specify Locale/ENGLISH.

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Counting occurences (solution to the exercise)

unsorted — cgrand, 28 April 2009 @ 18 h 00 min

What does (merge-with + {:a 12} {:b 4} {:a 3 :b 7}) return?: {:b 11, :a 15} when there are several values for a key, these values are merged (two at a time) using the specified function — here they are summed.
Can you count occurrences of each value in a collection using merge-with?: (apply merge-with + (map (fn [x] {x 1}) coll)) or, using for: (apply merge-with + (for [x coll] {x 1})).

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Screenscraping with Enlive

enlive — cgrand, 27 April 2009 @ 19 h 02 min

(select (html-resource (java.net.URL. "http://clojure-log.n01se.net/")) [:#main [:a (attr? :href)]]) returns a seq of link nodes.

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Counting occurences

unsorted — cgrand, @ 17 h 44 min

One asked me how to count occurrences of each value in a collection. I answered (reduce #(assoc %1 %2 (inc (%1 %2 0))) {} coll). Since it can take some time to get accustomed to the functional way of thought, here is how one can work such an expression out:

How to count all occurrences of 42?: (count (filter #{42} coll))
How to express count using reduce?: (defn my-count [coll] (reduce (fn [n _] (inc n)) 0 coll))
How to count all occurrences of 42 using reduce?: (reduce (fn [n _] (inc n)) 0 (filter #{42} coll))
Can you get rid of the filter?: (reduce (fn [n x] (if (= 42 x) (inc n) n)) 0 coll)
I’d like the result to be {42 occurences-count}.: (reduce (fn [m x] (if (= 42 x) (assoc m 42 (inc (m 42))) m)) {42 0} coll)
42 is hardcoded in four places, it’s bad!: (reduce (fn [m x] (if (= 42 x) (assoc m x (inc (m x))) m)) {42 0} coll)
Can’t you replace {42 0} with {}?: No (inc (m x)) would fail because (m x) would return nil.
How does one provide a default value when the key is not found ?: (a-map a-key default-value)
Can’t you replace {42 0} with {}?: (reduce (fn [m x] (if (= 42 x) (assoc m x (inc (m x 0))) m)) {} coll)
I changed my mind I’d like you to count occurrences of each value.: Easy! (reduce (fn [m x] (assoc m x (inc (m x 0)))) {} coll) or, terser, (reduce #(assoc %1 %2 (inc (%1 %2 0))) {} coll)

Exercise:

What does (merge-with + {:a 12} {:b 4} {:a 3 :b 7}) return?
Can you count occurrences of each value in a collection using merge-with?

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Vicious bug

mistakes — cgrand, 22 April 2009 @ 11 h 44 min

What’s wrong with this code (do (defmacro foobar [x] (doto x println)) (foobar (+ 1 1)))?: Nothing.
Run it again.: The first time, foobar is treated as a function (its argument has been evaluated) because foobar is unknown before the whole top-level expression is compiled.
The second time, the var named foobar preexists and is a macro, (foobar (+ 1 1)) is expanded.

This behaviour bit me while using with-test to test macros. It’s the kind of bug that goes unnoticed while developing incrementally and evaluating in the same REPL, it was waiting for me to start a fresh REPL.

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Sanitizing HTML with Enlive

enlive — cgrand, @ 11 h 35 min

net.cgrand.enlive-html=> (sniptest "&lt;div id=user-data>" 
  [:#user-data] (html-content "code injection&lt;script>alert('boo')&lt;/script>") 
  [:#user-data (but #{:p :br :a :strong :em})] nil)
"&lt;html>&lt;body>&lt;div id=\"user-data\">code injection&lt;/div>&lt;/body>&lt;/html>"

You also need to remove most attributes but it’s just a demo of something that was impossible with the old Enlive.

By the way, the old Enlive is no more. Long live the new Enlive!

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Make it work, make it right, make it fast

enlive — cgrand, 18 April 2009 @ 9 h 33 min

Make it work, make it right, make it fast.

Make it work: sort of done: the old Enlive,
Make it right: work in progress, the new Enlive,
Make it fast: once the “right” branch is merged into master.

See the README file to know what’s new and here for an example.

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Mapping every second item

utilities — cgrand, 17 April 2009 @ 10 h 58 min

I wanted to apply a function to every second item in a coll. I was considering writing something using interleave, take-nth and map or a combination of mapcat and partition when I thought of this:

(map #(%1 %2) (cycle [f identity]) coll)

I really love clojure’s map parallel processing. (I should ask if every? and some could be allowed to take several colls.)

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