This is a tiny Lisp interpreter written in Ada. As such, it lacks many of the features of a full Common Lisp, but can provide a simple Lisp-like programming environment.
If you find it useful or interesting, drop me a note at [email protected] and let me know.
Note that this is my first attempt at writing a programming language, so I'm sure that things could have been done better. It has been a learning experience and I'm pleased with how well that it's actually turned out. This is still under active development so most things are subject to change, especially the program internals.
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This is available using alire (alr get bbs_lisp). This will handle all of the dependencies. Alire is available from https://alire.ada.dev.
If you do not with to use alire, you will need to fetch https://github.com/BrentSeidel/BBS-Ada.
I am calling this an interpreter, though the boundaries are a bit fuzzy. The input text is converted into s-expressions that represent the program. The address for the builtin operations are stored in the s-expression and are then directly called when evaluated. It may be considered to be a threaded interpreter (which as nothing to do with programming threads as a method of concurrent programming).
While under initial development, this runs on a host computer (MacOs, in my case), the goal (achieved) is to get it to run on ARM based embedded systems. Many of these little ARM based boards have more computing power than the personal computers that I grew up with and it seemed like a good idea to provide some sort of interpreter that could be used to write simple programs directly on the board. I did briefly toy with the idea of a tiny BASIC interpreter, but quickly abandoned that idea in favor of Lisp.
The idea is that not only can simple programs be written, but it can also be used to develop algorithms for accessing the board's hardware. Once the algorithm development is finished, they can be translated into Ada and compiled.
While the only guarentee is that this contains bugs and is missing features, it is usable. I've been able to write some small programs in it both on the Mac and on the Arduino Due. It is nice to be able to change what the Arduino Due is doing without having to do a whole compile-load cycle.
A set of test cases have been written and code coverage checked. The tests are useful verify that code changes still work and have helped to find a number of bugs in the software. Code coverage has been checked based on the tests and the coverage is just above 90% statement coverage.
It now runs on the Arduino Due. It took a bit of work to remove all dependencies on Ada libraries that aren't available on the Arduino Due. Another feature added was the ability for the host software to add custom lisp commands. Thus, the main Arduino Due program can add custom Lisp commands for accessing the Arduino hardware. An example of this is the Ada-Lisp-Embedded repository at https://github.com/BrentSeidel/Ada-Lisp-Embedded
This Lisp interpreter also builds and runs on Windows 10 as well as a Raspberry PI under Raspberian.
- Integers are the standard Ada integer type.
- Strings are variable length and implemented using a linked list.
- Booleans are either "T" (true) or "NIL" (false).
- Characters are single ASCII characters (Unicode is not supported).
- Lists are linked lists of elements that can be of any datatype (even other lists)
- Basic arithmetic
- Comparisons
- List operations - CAR and CDR
- DOWHILE/DOTIMES
- User defined functions and lambda functions.
- Local variables.
- Hardware access. This is done by allowing the host software to add custom lisp commands. This may also be useful for embedding the lisp interpreter in other applications.
- Peek and Poke functions for accessing memory. This is mainly for use on embedded systems.
- Logical and bitwise logical operations AND, OR, NOT.
There are others, but here are the main missing features. Some of these may eventually be implemented, others will never be implemented. The goal is to have a useful little language, not another port of Common Lisp.
- Macros. This is a long term goal. I would like to implement these, but I need to figure out how first.
- Object oriented features. This will probably never happen.
- Closures. This will probably never happen.
- Packages and similar large program related features. Remember Tiny Lisp.
The following updates to the language are planned. They may not be done in the order shown and other items may be added before some of these.
- Improved error handling and general code cleanup (ongoing).
- Convert the BBS.lisp package to a generic with the data structure sizes as parameters. This will make it easier to resize things for specific targets.
Since this is intended to run on embedded systems without any memory management, the memory pools are pre-allocated and sized arrays from which objects can be allocated. Each object has a reference count and the object it freed when the number of references reaches 0. Note that reference counting is not automatic and has to be done manually.
One advantage that I've discovered about reference counting is that is usually quickly makes one aware that one has done something wrong. Two types of errors are possible - prematurely dereffing an object or not dereffing an object.
The memory manager is probably not thread safe, though this could be an interesting project.
Symbols, cons cells, and strings are allocated from statically defined arrays. This design decision was made to enable the interpreter to run on systems without any sort of dynamic memory management. Basically, I rolled my own. The size of these tables are set when compiling the interpreter and can be changed to suit your use.