magpiebrain

Sam Newman's site, a Consultant at ThoughtWorks

I’m currently working on a personal project by way of learning Clojure – it’s actually a program to match up my itemised phone bill against my list of contacts to help me expense my calls. I find it best to have a real-world problem I need to solve to learn a new programming language. The problem itself is rather dull, but it did give me a chance to consider an issue I’ve hit with many other languages.

One of the core parts of my telephone expense program is the process of normalising phone numbers so I can match them up. What I am trying to do is something long the lines of:

Strip spaces, then add the missing area code, then internationalize it

So in Clojure there are a number of functions I’ve written, each of which take, and return, a string (the program is nowhere near finished, so consider this to be virtually pseudo code) :

(defn #^String normalize [str]
  (internationalize (add-missing-areacode (strip-spaces str))))

In Java, this would look like:

public String normalize(String str) {
  return internationalize(addMissingAreaCode(stripSpaces(str)));
}

The problem is that I, and most of the western world, read from left to right – with both Java and Clojure I’m having to read from right to left to determine what is being done. One system I use frequently has a construct which matches what I’m after – UNIX:

strip-spaces "44 1230 9183" | add-area-code | internationalize

So what other languages support this kind of construct? I suspect I could coax Scala into doing something like this, and it seems that it is right up Python’s alley (Django’s excellent templating system has filters which do exactly that). But if I want to use Clojure, am I stuck with this inside-out programming model? What other JVM-based languages would help me here – Ioke perhaps? It seems right up AINC’s alley, but that syntax makes me want to cry…

Update 11 Jan 2010: Thanks to Matt for pointing me towards Clojure’s ‘->‘ macro. This looks pretty close to what I’m after. So I *think* I should be able to do something along the lines of:

(-> phoneNumber stripSpaces addAreaCode internationalize)

Which is very cool.

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6 Responses to “Data Transformation and Language Syntax”

  1. Ola Bini

    Ioke could definitely do it, although I don’t have an operator for it right now. I’ve considered adding it, though. I would probably not send around raw strings – so if you assume you have something else, you can simply do it like this, where the three methods are defined on PhoneNumberText

    PhoneNumberText from(“44 1230 9183”) stripSpaces addAreaCode internationalize

    F# have a serialization construct with the |> operator. You can also use <| for it. Take a look in Needhams blog for a number of examples.

    Reply
  2. Sam Newman

    Ola: I’d really want to pass around whatever the functions return, rather than limiting myself to a raw string. Which I’d imagine limits me to a language with dynamic typing, or else jumping through lots of hoops getting a sufficiently advanced static typing system to be happy with it.

    Mark: I think comp still works right to left from my reading of the docs.

    Reply
  3. Ben Butler-Cole

    The standard is Haskell is to do right-to-left function composition like this:

    normalize = internationalize . addAreaCode . stripSpaces

    You do soon get used to reading these. However there is also a left-to-right composition operator, like this:

    normalize = stripSpaces >>> addAreaCode >>> internationalize

    Ben

    Reply

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