It's Python with a Lissp.
Hissp is a modular Lisp implementation that compiles to a functional subset of Python—Syntactic macro metaprogramming with full access to the Python ecosystem.
Table of Contents
Python is already a really nice language, so why do we need Hissp?
The answer is metaprogramming: code that writes code. When you can shape the language itself to fit your problem domain, the incomprehensible becomes obvious.
Python really is a great language to work with.
"Executable pseudocode" is not far off.
But it is too complex to be good at metaprogramming.
The use of exec() is frowned upon.
It's easy enough to understand, but hard to get right.
Python Abstract Syntax Tree (AST)
manipulation is a somewhat more reliable technique,
but not for the faint of heart.
Python AST is not simple, because Python isn't.
Hissp is a streamlined skin on Python: a simplified AST that you can program in directly. Hissp code is made of specially formatted tuples—easier to manipulate than Python AST, but still more reliable than text manipulation. In Hissp, code is just another kind of data.
Lisp is renowned as the "programmable programming language", extensible through its powerful macro system which hooks into the compiler itself. Macros are Lisp's secret weapon. And Hissp brings their power to Python.
Adding features that historically required a new version of the Python language,
like with statements, are almost as easy as writing a new function in Lisp.
Hissp serves as a modular component for other projects. The language and its implementation are meant to be small and comprehensible by a single individual.
The Hissp compiler should include what it needs to achieve its goals, but no more. Bloat is not allowed. A goal of Hissp is to be as small as reasonably possible, but no smaller. We're not code golfing here; readability still counts. But this project has limited scope. Hissp's powerful macro system means that additions to the compiler are rarely needed. Feature creep belongs in external libraries, not in the compiler proper.
Hissp compiles to an unpythonic functional subset of Python. This subset has a direct and easy-to-understand correspondence to the Hissp code, which makes it straightforward to debug, once you understand Hissp. But it is definitely not meant to be idiomatic Python. That would require a much more complex compiler, because idiomatic Python is not simple.
Hissp's basic macros are meant to be just enough to bootstrap native unit tests and demonstrate the macro system. They may suffice for small embedded Lissp projects, but you will probably want a more comprehensive macro suite for general use. Hebigo has macro equivalents of most Python statements. The Hebigo project includes an alternative indentation-based Hissp reader, but the macros are written in readerless mode and are also compatible with Lissp.
Why base a Lisp on Python when there are already lots of other Lisps?
Python has a rich selection of libraries for a variety of domains and Hissp can mostly use them as easily as the standard library. This gives Hissp a massive advantage over other Lisps with less selection. If you don't care to work with the Python ecosystem, perhaps Hissp is not the Lisp for you.
Note that the Hissp compiler is written in Python 3.8. (Supporting older versions is not a goal, because that would complicate the compiler. This may limit the available libraries.) But because the compiler's target functional Python subset is so small, the compiled output can usually run on Python 3.5 without too much difficulty. Watch out for positional-only arguments (new to 3.8) and changes to the standard library. Running on versions even older than 3.5 is not recommended, but may likewise be possible (even for Python 2) if you carefully avoid using newer Python features. (Keyword-only arguments, for example.)
Python code can also import and use packages written in Hissp, because they compile to Python.
One of Python's best features. Any errors that prevent compilation should be easy to find.
A language is not very usable without tools. Hissp's basic reader syntax (Lissp) should work with Emacs.
This is part of Hissp's commitment to modularity.
One can, of course, write Hissp code that depends on any Python library. But the compiler does not depend on emitting calls out to any special Hissp helper functions to work. You do not need Hissp installed to run the final compiled Python output, only Python itself.
Hissp includes some very basic Lisp macros to get you started. Their expansions have no external requirements either.
Libraries built on Hissp need not have this limitation.
A Lisp tradition, and Hissp is no exception. Even though it's a compiled language, Hissp has an interactive shell like Python does. The REPL displays the compiled Python and evaluates it. Printed values use the normal Python reprs. (Translating those to back to Lissp is not a goal.)
Not all Lisps support this, but Clojure is a notable exception. Functions are generally preferable to macros when functions can do the job. They're more reusable and composable. Therefore it makes sense for macros to delegate to functions where possible. But such a macro should work in the same module. This requires incremental compilation and evaluation of forms, like the REPL.
The Hissp language is made of tuples (and values), not text. The basic reader included with the project just implements a convenient way to write them. It's possible to write Hissp in "readerless mode" by writing these tuples in Python.
Batteries are not included because Python already has them.
Hissp's standard library is Python's.
There are only two special forms: quote and lambda.
Hissp does include a few basic macros and reader macros,
just enough to write native unit tests,
but you are not obligated to use them when writing Hissp.
It's possible for an external project to provide an alternative reader with different syntax, as long as the output is Hissp code (tuples). One example of this is Hebigo, which has a more Python-like indentation-based syntax.
Because Hissp produces standalone output, it's not locked into any one Lisp paradigm. It could work with a Clojure-like, Scheme-like, or Common-Lisp-like, etc., reader, function, and macro libraries.
It is a goal of the project to support a more Clojure-like reader and a complete function/macro library. But while this informs the design of the compiler, it will be an external project in another repository.
Hissp is a language written as Python data and compiled to Python code:
>>> from hissp.compiler import readerless
>>> readerless(
... ('lambda',('name',),
... ('print',('quote','Hello'),'name',),)
... )
"(lambda name:\n print(\n 'Hello',\n name))"
>>> print(_)
(lambda name:
print(
'Hello',
name))
>>> eval(_)('World')
Hello WorldHissp's utility as a metaprogramming language is the ease of manipulating simple data structures representing executable code.
Hissp can be written directly in Python using the "readerless mode" demonstrated above, or it can be read in from a lightweight textual language called Lissp that represents these data structures.
>>> from hissp.reader import Lissp
>>> next(Lissp().reads("""
... (lambda (name)
... (print 'Hello name))
... """))
('lambda', ('name',), ('print', ('quote', 'Hello'), 'name'))As you can see, this results in exactly the same Python data structure as the previous example, and can by compiled to executable Python code the same way.
Hissp comes with a basic REPL (read-eval-print-loop, or interactive shell) which compiles Hissp (read from Lissp) to Python and passes it to the Python REPL for execution.
Lissp can also be read from .lissp files.
The reader and compiler are both extensible with macros.
See the tutorial in the
for more details.