a tiny k interpreter for educational purposes by arthur whitney

the contents of this repository assume a readership who have read and understood k&r at some point in their careers. any experience with using interpreters of vector languages is beneficial but not necessary. any experience with implementing vector language interpreters would definitely help a lot.

in early january 2024, a group of motivated individuals asked arthur whitney to demonstrate the fundamentals of the design, organization and style of his method of writing c in the most succinct and approachable way possible.

about a day later, arthur offered a tiny interpreter of a toy vector language, implemented in about 25 lines of c with a small header file. his code is published in this repository under /ref.

a few days later, the regents of kparc were invited to share their impressions on this codebase, and optionally provide some additional commentary.

such commentary is offered in form of two files a.[ch] in the root of this repository, which serve a number of purposes:

1. both files can be seen as two chapters of a small "essay", header file a.h being the first chapter, a.c source file being the second. not only the text seeks to explain the code in reasonable detail, which is strictly line by line, but also takes a few liberties and diversions in a bid to elucidate the "bigger picture" behind this tiny piece of code.

2. for the ease of comprehension, the enclosed narrative has been made completely linear, and hopefully makes for an easy, entertaining and useful read. to make this possible, the original code had seen some very minor restructuring and regrouping; however, no refactoring, no changes to logic or naming conventions have taken place, except for a handful cosmetic enhancements which have been discussed with atw and are seen as beneficial to the cause. the net amount of non-cosmetic changes made to ref/a.c should be in range of 20-25 keystrokes. a number of less trivial one-line functions (e.g. e()) are presented in "exploded view" to simplify discussion of their control flow and role of individual components.

   although the c code of k/simple is formatted to fit on mobile phone screens in portrait mode, for a more comfortable reading experience we recommend using professional equipment.

3. with help of the included makefile, kparc's "essay" can be built to any available architecture, e.g. arm, riscv, wasm32 or extensa. default make target assumes presence of a recent gcc, clang or tinyc compiler. there are also two additional handy build targets: make mm, which removes kparc remarks from the code except for the most essential markup, while the make m target removes it completely and restores the formatting to resemble the original as close as possible for the ease of reference. these build targets produce files a.min.c and a.m.c respectively, build them, and perform a bitwise diff to ensure all three resulting binaries are identical.

$ cd && git clone [email protected]:kparc/ksimple.git
$ cd ksimple && make
$ ./a
k/simple (c) 2024 atw/kpc
 2 2
4
 x:!9
 y:2 !9
 x-y
-2 -2 -2 -2 -2 -2 -2 -2 -2
 z:x,y
 #z
18
 x !3
Add:length
 x@1,5,7
1 5 7
 ^C
$ vim k.c

[WIP]

the authors hope that this material enables and inspires further experimentation on reader's own, which is a very rewarding and fun pastime. for example, one might consider the following toy problems of various degrees of difficulty:

simple:

• implement a few more simple verbs, e.g. dyadic f*x.
• modify repl to support the exit command (traditionally, a double backslash \\)
• add workspace size monitor (i.e. memory allocation) to e.g. prompt or via extra command (\w)

hurt me plenty:

• inspect verb implementations provided by atw for edge cases, and add checks (which are intentionally omitted)
• extend maximum vector length to MAX_UINT
• find and fix at least three segfaults

ultraviolence:

• change base type from 8bit integer to signed long
• fix tokenizer to support integer numerals greater than 9 and less than 0
• fix tokenizer to support efficient direct vector input (e.g. 42 57 1010 instead of 1,2,3,4)
• implement memory management by refcounting

nightmare:

• implement a simple parser (e.g. to support quoted strings and parens)
• implement nested vectors, and verb flip
• make vector arithmetic penetrating
• implement adverbs scan and over

progress takes sacrifice:

• implement floating point type
• implement functions and local scope
• implement a test suite for your k

//:~