vemod.net

def state = 'confused

Thu, 27 Jan 2011 22:22:40 +0100

I think method definitions in Scala can be a bit confusing. Here’s a REPL session with some comments and questions:

scala> def A() = 'A
A: ()Symbol

scala> def B = () => 'B
B: () => Symbol

scala> def C = 'C
C: Symbol

So far nothing strange, but note the the type of C; is it really a function? We’ll see later, so let’s move on:

scala> A()
res: Symbol = 'A

scala> B()
res: Symbol = 'B

Note that A and B can be called in the same way, even though B was defined in terms of an anonymous function.

scala> C()
<console>:7: error: C of type Symbol does not take parameters
       C()
        ^

Well, seems like we’ve discovered that C isn’t really a function and we could have defined it with val instead… Or? No, it’s still a function: the function body wasn’t evaluated until we referred to the function. Look here:

scala> def t = currentTime 
t: Long

scala> t
res: Long = 1296163141271

scala> t
res: Long = 1296163141949

Clever and useful, but not completely obvious.

Back to A, B and C. Let’s try grabbing a function reference Scheme/Haskell/Python style:

scala> A
res: Symbol = 'A

scala> B
res: () => Symbol = <function0>

scala> C
res: Symbol = 'C

B worked fine and C was expected because of what we previously discovered, but what about A? It surprised me that it evaluated to 'A instead of a function that returns 'A. What would the result be if the method took arguments?

scala> def id[T](x: T) = x
id: [T](x: T)T

scala> id
<console>:7: error: missing arguments for method id in object $iw;
follow this method with `_' if you want to treat it as a partially applied function
       id
       ^

Different result – yes – but not valid as is. It is noteworthy that it is possible to referr to id like this:

scala> Some(123).map(id)
res: Option[Int] = Some(123)

It seems like the expression is context-sensitive. :/

What I’ve learnt from this that you can’t consider functions and methods as equals in Scala (like in Python); they live in two different worlds and you need to bridge them explicitly sometimes.

To be continued…

Using Racket to Configure Apache

Sun, 10 Apr 2011 10:52:30 +0200

Tags: Racket and Lisp

I have a confession to make: I really hate writing boilerplate code.

I use Apache to serve lots of websites, have many lines of configuration code and have often longed for a way to capture all the patterns and reduce code duplication. I thought about using a macro language like m4 or the C preprocessor or even Ruby but neither alternative was particulary attractive. Then it hit me that I could describe the Apache configuration with plain S-expressions and use the full power of Racket to manipulate these and output normal configuration files. What makes this a compelling alternative is of course that S-expressions allow one to express tree-structured data very succinctly, like here:

(group (VirtualHost *) 
       (ServerName www.defsoftware.se) 
       (RedirectPermanent / http://defsoftware.se/))

Racket—being a Lisp equipped with many powerful built-in functions—makes a full translator for these kind of trees simple to write.

Update: I got an e-mail from Eli Barzilay (of Racket fame) with some valuable feedback on how to improve my code by using functions in the scribble/text and racket/list libraries that are included with Racket. I’ve updated my code and put the old code at the bottom¹ for reference. Eli also wrote about using the cool at-exp reader to mix text and code.

(require scribble/text racket/list)

(define (apache-config . complete-config)
  (define (format-apache-config config)
    (match config
      ((list 'group (list group ...) directives ...)
       (list "<"  (add-between group " ") ">" "\n" 
             "  " (add-newlines (map format-apache-config directives)) "\n" 
             "</" (first group) ">"))
      ((list (list _ ...) ...)
       (add-newlines (map format-apache-config config)))
      ((list _ ...)
       (add-between config " "))))
  (output (format-apache-config complete-config)) (newline))

The output function takes care of the indentation in a pretty clever way.

Now—equipped with apache-config—we can start building functions that work with Apache S-expressions:

(define (host-redirect from-hostname to-hostname)
  `(group (VirtualHost *)
          (ServerName ,from-hostname)
          (RedirectPermanent / ,(string-append "http://" to-hostname "/"))))

(apache-config
  (host-redirect "defsoftware.com" "defsoftware.se")
  (host-redirect "defsoftware.net" "defsoftware.se"))

Output:

<VirtualHost *>
  ServerName defsoftware.com
  RedirectPermanent / http://defsoftware.se/
</VirtualHost>
<VirtualHost *>
  ServerName defsoftware.net
  RedirectPermanent / http://defsoftware.se/
</VirtualHost>

Now I’m happy.

¹ Click here to see the old code.

(define (apache-config . config)
  (let self ((config-fragment config) (indent 0))
    (define (line-with-indent . args)
      (display (make-string (* 2 indent) #\ ))
      (apply printf args)
      (newline))

    (define (join xs (sep " "))
      (string-join (map (curry format "~a") xs) sep))

    (match config-fragment
      ((list)
       (void))
      ((list (list nested ...) more ...)
       (self nested indent)
       (self more   indent))
      ((list 'group (list group ...) directives ...)
       (line-with-indent "<~a>" (join group))
       (self directives (add1 indent))
       (line-with-indent "</~a>" (first group)))
      ((list (or (? string?) (? symbol?)) ...)
       (line-with-indent (join config-fragment))))))

Objective-R

Fri, 27 May 2011 11:38:40 +0200

Tags: Racket and Lisp

Objective-C-like syntax for method calls in Racket

Liquorice

Thu, 22 Sep 2011 19:24:17 +0200

I’m a big fan of liquorice. Here are my top dealers:

1. Amarelli

Their spezzatina/spezzata (pastilles of pure liquorice) can’t be beaten. The variants flavoured with anise are great as well.

2. Lakrids by Johan Bülow

If you like soft liquorice this is for you. Great flavours: salmiak, habanero chili and more. The product names are stupid. Don’t miss the raw liquorice drops; they are almost as good as Amarelli’s.