Multi-Stage Programming in Lisp

I have been reading up on MSP recently, and trying to grok the examples in Walid Taha's Gentle Introduction. When all you have is a Lispy hammer, every problem calls for a program with parentheses, so the next order of business is to find Lisp equivalents of Bracket, Escape and Run.

'Run' is trivial; to run a Lisp program, we evaluate expressions; so 'eval' it is.

'Escape' is for dropping in actual values into placeholders, so a comma seems appropriate. However, a comma has to occur within the context of a backquote, so it seems we're stuck. But wait, an Escape has to occur within the context of a Bracket, so maybe we can use backquotes for Brackets? Further analysis, in the form of seeing how well the two words rhyme, proves that backquotes are indeed the equivalents of Brackets in Lisp.

I went through a slightly more involved process in arriving at the above mapping, but this version of the story is a lot more fun.

Without further ado, here is the famous MSP power example in Lisp:

CL-USER> (defun mypower (x n)
           (if (eq n 0)
               `1
               `(* ,x ,(mypower x (- n 1)))))
MYPOWER

CL-USER> (mypower 10 3)
(* 10 (* 10 (* 10 1)))

CL-USER> (eval (mypower 10 3))
1000
CL-USER>

Yeah, we're missing the well-typed and well-formed guarantees, but Lisp's 'code-is-data' proves its worth once again. And we're not even using macros.

If we want to bring more structure to the staged expressions, we can express them as, say, closures. That's probably a topic for a future post.

I still have a lingering doubt whether we have accomplished truly multi-stage programming, seeing how trivial it has been to implement.

On a related note, what's common between the power and the logger functions? They are the only use cases that provide credibility to their respective patron saint programming paradigms, viz. MSP and aspect oriented programming. Just kidding.

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With all the recent changes at the helm, can we expect less of full front-page real estate ads promising free gold coins?

October 16, 2013

Deccan Chronicle has an irritating daily feature called 'Word Spy' where they (on second thought, they're not smart enough to do this; it's probably syndicated) tweak regular words slightly, in a way nobody else would, and create supposedly charming and quirky new words that make you -- in theory -- smile and nod your head intelligently.

Example from a recent edition: 'Trialogue'. Before looking at the answer, let's try to figure it out ourselves: hmm, sounds like 'dialogue' which is a conversation between two people, so could this be a conversation among three people? Bingo! You win a free subscription to DC for a year! What? You don't have a to-be-potty-trained pet? Maybe you have a cat litter to fill up? OK then.

To see how easy it is to make up new (crappy) words from regular ones, here are three of my own inventions:

1. Polisex: 1. Using sex as a weapon in politics 2. The dynamics of physical relations between people
2. Furvasive: Condition of too much fur on a dog's back
3. Introduckory: Evasive behaviour of someone who wants to avoid meeting new people.

I did say they're crappy words.They have to be, since I took about 1.2 seconds to make up each.

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Kudos to John Michael Greer for being the first person from the native English-speaking world to use the word 'crores' in a normal way (at least to us Indians).

 

October 9, 2013

From the shedding-crocodile-tears department:

“What is the need to privatise the profitable Chennai airport? After investing over Rs. 2,000 crore to modernise the airport, why should a private party run the airport? After Delhi and Mumbai airports were privatised, passengers are being charged an exorbitant User Development Fee,” said L. George, regional secretary of the AAEU.

Yeah, they care for passengers that much. Why not be honest and admit that they fear a threat to their livelihoods because of the possible loss of jobs?