lab 26 - greenspun's rule

By

NAME

lab 26 - greenspun's rule

DESCRIPTION

Let me state up front, I do not know much about lisp. I've always been aware of it but never saw it's benefits so never used it. However, recently I've begun to read more about it, particularly the essays by Paul Graham, and his book On Lisp (which I'm still reading, on and off). I went a bit further and read Steele and Sussman's paper The Art of the Interpreter. Also, as a symptom of my Mathematica envy I became curious about symbolic programming languages, computer algebra systems, and how these are based on lisp dialects. I started playing with Maxima and yacas and also Mockmma

And so I think I finally got what lisp was about. (I still have much more reading to do. It's funny how a large and rich subculture within computer science suddenly opens up to you when you read about its core language. I have to ask myself, How could I not have noticed this for so long? The same surprise occurred to me when reading about smalltalk. It all makes me feel quite stupid and parochial.)

As an exercise in understanding lisp better I turned Inferno shell into a lisp dialect. I wrote a builtin module that provides an eval command and the seven primitive operators: quote, atom, car, cdr, eq, cons and cond.

I followed approximately the code in Paul Graham's paper The Roots of Lisp, and the interpreters described in the Steele and Sussman paper. 

     % load ./lisp0.dis
     % eval {atom `a}
     t
     % eval {atom `{a b c}}
     nil
     % eval {car `{a b c}}
     a

Functions can be defined by assigning to a variable in shell. Scope within eval is dynamic. 

     % subst={{x y z} 
      {cond {{atom z} 
          {cond {{eq z y} x} 
            {`t z}}}
       {`t {cons {subst x y {car z}}
              {subst x y {cdr z}}}}}}

     % eval {subst `m `b `{a b { a b c} d} }
     {a m {a m c} d}

The backquote is used as an abbreviation for the quote primitive.

In the builtin I hijack the parse tree and read or transform it for each primitive operator. I also use the shell Context as my symbol table for function parameters, definitions and variable values.

CONCLUSION

The attached file is the first of a few versions I'll provide as I add more features as described in Steele and Sussman, in particular lexical binding.

I wonder whether there is more value to having a dialect of lisp within the shell or whether a proper lisp implementation like scheme should be linked into the inferno VM or written in limbo. The shell already has a good interface to the whole system and mixing programming paradigms may have advantages. The problem is we may realize Greenspun's rule by being ad hoc, informally specified, bug ridden and slow.

FILES

lisp0.b

Comments

Anonymous said…
A nice, usable, native (compiling or otherwise easily run) functional language is one of the things that both plan9 and inferno are sorely missing.
6:47 PM
Anonymous said…
Another good book for Lisp Programming is Peter Seibel's Practical Common Lisp.
3:00 PM
Post a Comment

Popular posts from this blog

lab 110 - inferno archive edition

By
I've been occupied recently with archiving my digital media. I've been copying home videos on DV tapes to hard-disk, ripping audio CD's to WAV files, gathering photo collections, and trying to copy documents from Iomega disks, floppies, and my dusty old Acorn RiscPC. The plan is to have a copy of this data to give to each of my children. My Dad recently scanned and sent me all his photographs of me and my siblings growing up; he also included pictures of himself and my Mother when they met in Africa. With technology today each generation can build a digital library of family history to hand on to the next generation. In the past a family album may have been passed on to only one person. The accumulation of digital data still presents problems. It requires discipline to store files that are open and not locked into devices or proprietary formats. With digital preservation in mind I've tried to use file formats recommended for long term archiving. WAV files for audio, D
Read more

lab 107 - midiplay

By
NAME lab 107 - midiplay NOTES Midiplay plays back MIDI files. It uses the synthesizer I described in lab 62 and the MIDI module from lab 73. The command takes only one argument, the path to the midi file. I've included one in the lab to demonstrate. Bind the audio device before using it. % bind -a '#A' /dev % midiplay invent15.mid The synthesizer has 16 note polyphony. It uses three oscillators, one at the pitch, one at half pitch, one at double pitch. There is also a filter, two delays and a vibrato. The sample rate is 8000hz and there is one mono channel (MIDI channel events are ignored). It performs well enough to work on my laptop without JIT turned on. All the synthesizer parameters can only be tweaked from inside the code at the moment. FILES lab - 107
Read more

The Cartesian Theater of AI

By
Image
The Cartesian Theater was an intuition pump proposed by philosopher Daniel Dennett to describe a false theory of consciousness. Inside the mind is a stage watched by a humunculus that observes what passes on the stage, hears the sounds, and controls the rest of the mind by responding to the action of what the smaller version of the self sees. Of course, what makes the smaller self conscious? It is an infinite regression. Our understanding of mind is that there is no such theater. There is no one place in the mind that is the locality of consciousness. I want to offer another thought experiment based on the concept of a Cartesian Theater. Imagine a theater containing a screen with a camera pointing at it, and a speaker with a microphone directed toward the speaker. The camera is pointing at the screen. The mic is pointing at the speaker. However, instead of there being an infinite loop due to a direct connection between the camera and screen, insert an AI in the loop. The imag
Read more