Morc: Mock Arc Programming Language as Scheme Extension

Morc: Mock Arc Programming Language as Scheme Extension
*******************************************************

Version 0.1, 2008-08-31, `http://www.neilvandyke.org/morc/'

by <neil@neilvandyke.org>

     Copyright (C) 2008 Neil Van Dyke.  This program is Free Software;
     you can redistribute it and/or modify it under the terms of the
     GNU Lesser General Public License as published by the Free
     Software Foundation; either version 3 of the License, or (at your
     option) any later version.  This program is distributed in the
     hope that it will be useful, but without any warranty; without
     even the implied warranty of merchantability or fitness for a
     particular purpose.  See <http://www.gnu.org/copyleft/lesser.html>
     for details.  For other licenses and consulting, contact the
     author.

Introduction
************

Note: This is an incomplete implementation that I do not plan to pursue
further.  It is being released only for purposes of publicly archiving
the work thus far, in case someone in the future might benefit from it.
The current state of this is the result of two weekends of work, and
then it was set aside due to other demands on my time.  This work was
done with PLT 360, and looks like minor tweaks would be required to get
it to work with PLT 4.  As I recall, I was last working on a namespace
issue with PLT `defmacro'.

   At the end of 2008-01, Paul Graham and Robert Morris made the initial
release of the Arc programming language
(http://en.wikipedia.org/wiki/Arc_(programming_language)).  There was
no reference documentation, and as I read through the tutorial text
file, I noticed a striking resemblance of Arc to Scheme - Scheme, with
lots of syntactic sugar, some Common Lisp-isms, some clever
conveniences and shorthands that were inconsistent with Scheme.  I
strongly suspected that Scheme was used as a starting point for Arc.
The similarities of Arc to Scheme inspired me to implement Arc as a set
of Scheme macros and using a few extra features of PLT Scheme.  It was
in the course of implementing much of Arc that I noticed the
siginificant differences.  I decided to call my implementation Morc, as
in "mock Arc," as in an imitation Arc.

   The Morc implementation is cleanroom, based only on the Arc
tutorial, what little tidbits I'd noticed in public information, and my
own knowledge of various programming languages and theory.

   Because Morc worked by expanding Arc code to PLT Scheme code, the PLT
compilers could be used, which, incidentally, I suspected would make
Morc much faster than the Arc reference implementation.  That is not a
criticism of Arc, as the authors stated at the time that the
implementation was to be more of an executable informal specification
than efficient.

   I was planning on having the API reference for Morc double as a
commentary on how Arc concepts relate to those in Scheme and other
languages.  Two perceptions I noted at the time were "Arc seems to
value terseness, whereas Scheme values purity," and the even more
inciting "You can do Arc in [PLT] Scheme, but not Scheme in Arc."

   File `test-morc.arc' is the beginning of a test suite for Arc,
derived from the original Arc tutorial.

   Here are some instructions I wrote while developing this under PLT
360, which might or might not be correct.  "To use, install the Morc
`.plt' file.  (If you don't know how to install a `.plt' file, see
`http://download.plt-scheme.org/doc/dotplt.html'.)  Start DrScheme.
Select Choose Language... from the File menu, and choose Morc.  Morc
can also be invoked as `mred -Z -z -M morc', such as from within Quack
(http://www.neilvandyke.org/quack/).

   Please note that I've not touched any of the documentation, other
than to add this Introduction for the release.  "!!!" is my notation
meaning that the documentation there needs work before release (since
three exclamation marks together should never occur under any
circumstance), and is usually followed by cryptic notes.  Some of those
notes allude to points I was intending to make in the annotated Morc
reference documentation.

   Good night, and good luck.

nil and Booleans
****************

!!! "Some implementations provide variables `nil' and `t' whose values
in the initial environment are `#f' and `#t' respectively." [R3RS sec.
6.1]

> t
     Mostly same as Scheme, except it's spelled `t' instead of `#t',
     and in Scheme it is not a variable.  (Scheme's choice has the
     advantage of not using a single-letter symbol, which conceivably
     might come in handy when single letter symbols are used to
     represent letters in symbolic programming.)

> nil
     In Arc, this is both false and the null list value, whereas Scheme
     has distinguished the two for a long time.  Morc secretly uses
     Scheme null list in its representation of pairs, but exposes the
     null list only as `nil' through Arc.  Scheme's `#f' is not a
     variable.

> (no val)
     Same as Scheme, except Arc boolean values are used instead of
     Scheme's.

Reader and Quoting
******************

The Arc reader is the same as R5RS Scheme's.  Morc uses the MzScheme
reader, but with some features disabled.

> (: +{ proc })
     !!! not-arc

Types
*****

> (type x)
     !!! scheme has predicate procedures.  you can ask "number?" or
     "integer?"

     !!! though not shown in arc example, we return nil if we don't
     know type.

     !!! why spell out "string" but not abbreviate "number" as "num"?

> (isa x typ)
     !!! achtung.  arc tutorial defines num and int types, and says
     `isa' is

          (def isa (x y) (is (type x) y))

     which means that:

          (isa 42 'int) => t
          (isa 42 'num) => nil

     !!! scheme integers answer both to integer? and number?

          (integer? 42) => #t
          (number?  42) => #t

> (coerce x typ ?{ extra })

Numbers
*******

> (even number)
> (odd number)
     Same as Scheme `even?' and `odd?', respectively, except they
     return Arc boolean values.

> (< num1 +{ num })
> (> num1 +{ num })
> (<= num1 +{ num })
> (>= num1 +{ num })
     Same as in Scheme, except they return Arc boolean values.

     !!! While not required for Arc tutorial, Morc extends these to
     also work on strings.

> (expt x y)
> (sqrt x)
     Same as in Scheme.

> (++ var)
> (- var)

Strings
*******

> (string *{ arg })
     !!! we convert lists and pairs together

Lists
*****

> (cons car cdr)
> (car pair)
> (cdr pair)
> (list *{ arg })
     Same as in Scheme, except that the Arc `nil' object is used
     instead of the Scheme `()' null list.

> (cadr pair)
     !!! why have this when arc has car:cdr composition?  the old lisp
     ca*d*r procedures could be seen a kludge around the lack of
     syntactic sugar.

          (cadr x) == (car:cdr x)

> (nthcdr n lst)
> (firstn n lst)

> (tuples lst ?{ size })
     !!! Scheme has none.  Morc's implementation is in theory twice as
     efficient as the one in the Arc0 tutorial, and is implemented
     using named-`let' and multiple-value returns.  (example of why
     powerful fundamentals important to good algorithms)

     !!! first need fundamentals, rather than composing out of fancy
     primitives that might specify the functional behavior but aren't
     very good algorithmically.  example is "tuples"

> (push val list-var)
> (pop list-var)

Association Lists
*****************

> (alref alist key)
     !!! what about not-found value or not-found thunk?  in morc, we've
     currently defined it to yield nil if key not found.  in scheme,
     you'd use assq, assv, and assoc.


Hash Tables
***********

> (table)

> (listtab lst)

> (obj *{ key val })

> (keys ht)
> (vals ht)
     !!! plt-scheme has hash-table-map, which is better

> (maptable proc ht)
     !!! Why call it "map" if it doesn't function analogously to `map'?
     And why not return the result of mapping via PROC?  "There is a
     function called maptable for hash tables that is like map for
     lists, except that it returns the original table instead of a new
     one."

Equality and Identity
*********************

> (is obj1 obj2)
      -- Procedure: iso obj1 obj2
          !!!

> (in key *{ item })
     !!!

Closures
********

> (fn ( *{ arg } ) *{ body })
     Same as Scheme `lambda', without the apparent ability [!!!] to
     collect the rest of a list of values into one argument.

     !!! optional args is like opt-lambda, just with extraneous "o"

Application
***********

!!! scheme list sexy syntax is either a special form or a procedure
application.  arc permits...

> (apply func args)
     !!! does special dispatch.  unclear whether Arc requires this.

Binding and Assignment
**********************

> (def !!!)

> (let !!!)
     !!! we'll make this a letrec ... (does that help self-recursive
     lambda?)

> (with !!!)
     !!! we'll make this a letrec ..., but maybe it should be a let*

> (= !!!)

Conditionals
************

> (do !!!)

> (and !!!)
> (or !!!)

> (if !!!)

> (when !!!)

   !!! mention i defined when and unless for mzscheme, and now no
longer use them.

> (case !!!)

Iteration
*********

> (each !!!)

> (for !!!)

> (while expr *{ body })

> (repeat num +{ body })

List Processing
***************

> (map func +{ lst })

> (keep pred list-or-string)
> (rem pred list-or-string)

   !!! note that arc doesn't say whether the equality comparison is
"is" or "iso".

   !!! "keep" with equality is a little odd

> (all !!!)
> (some !!!)
     !!!

> (pos !!!)
     !!! unclear when would use this, since you don't have direct
     access to list elements by index.  makes more sense for string
     functions, especially when there is efficient direct access to
     string elements (not necessarily the case with non-ASCII strings).

> (trues !!!)

Input and Output
****************

> (pr !!!)
> (prn !!!)

> (tostring +{ body })

Misc.
*****

> (len x)

> (sort less-than lst)
     !!! use sort function included with plt-scheme, albeit with
     arguments reversed.

> (compare less-than convert)

Macros
******

> (mac name args +{ body })

> (uniq)
     Morc implements this by simply calling MzScheme's (non-Scheme) Lisp
     `gensym'.

> (w/uniq VARS)

Non-Arc Utilities
*****************

> (.expand expr)
> (.expand1 expr)

> .null

Kludge
******

Namespace
*********

Tests
*****

Some tests are in the file `test-morc.arc', which is included in the
Morc installation package.

History
*******

Version 0.1 -- 2008-08-31
     First release, and only intended release.