I know a lot of people are going to poo poo on this as "not a lisp" but I think it makes lisps more approachable for a python programmer who is not used to them.
I used experimented with Hy a few years ago and I felt that it made it easier to write recursive functions (even if python doesn't have tail call optimization). Being able to bind multiple values to multiple variables was nice, and I believe I could get multi-line lambdas in Hy as well. And it wouldn't be great but if someone new Python by not Hy, there was always hy2py to convert your Hy program back to Python. It was also nice to be able to write some sexps but still have the entire python ecosystem.
Doing dataframe manipulation through parens with pandas made me laugh, and while it wasn't always clear how to translate my python code into Hy It was definitely waay easier then learning an entirely new language.
I eventually moved onto Racket after I ran into some issues with Hy being "not a lisp" but Hy certainly made Racket easier for me to grok after playing around with it
Side comment: Python doesn't have TCO, but Hy does have the loop/recur macros in its contrib library. They give you a version of tail recursion that looks a bit odd at first, but quite nice once you get used to it.
This seems a bit like the story with Clojure vis-a-vis Java, to me. Yes, Clojure does a lot to clean up Java's verbosity, flaky implementation of generics, etc. But I don't think that's actually because Clojure is a Lisp; I think it's mostly because Clojure is a dynamic language. For example, Clojure largely resolves Java's generics situation by simply not needing them in the first place.
That worked for Java, where a lot of people are stuck on the JVM for legacy reasons, but yearning to at least be able to work in a more ergonomic language. Go lives in a different social environment, though. Nobody's stuck on Go because they've got 25 years' worth of massive legacy Go monoliths that they can't just chuck out the door. They're on Go because Go is what they want. And I'm pretty sure, given that they're invested in Go, that a dynamic language with a heavy focus on metaprogramming is approximately the opposite of what they want.
I spent some time with Hy about a year back, and that is approximately the impression I brought away. In order to maintain good interop with Python, they had to make a lot of compromises about how things work in Hy, relative to what a Lisper might expect. The most striking example I can think of offhand is that `let` had to get banished from the standard library: https://github.com/hylang/hy/issues/844
That said, Hy is still a nice language, and very well thought out. It's just that billing it as a lisp dialect for Python (as the project's website does) might lead to some false expectations. That GH thread I linked above is a great example of this. There's a lot of good, careful thought going into the design of the language. But it also has this sentence in the opening comment: "Hy is not Clojure, nor Common Lisp, but homoiconic Python." If you're interested in a Python variant with good macro system, this is it.
(Also, I'm not sure it necessarily disqualifies Hy from being a lisp. There are too many different conflicting opinions about the definition of 'lisp' for me to want to say either way, and, while a lot of good and insightful things have been said on the subject, I've personally always found that particular argument to be rather boring.)
Cool, I thought it was dead (like the fictional character called, coincidently, "Snake"). I see that active development has restarted 6 months ago, seemingly. Kudos to everyone involved, specially @Kodiologist who seems the main contributor over the recent period.
Is it possible to losslessly translate from Hy or Python to the Abstract Syntax Tree and back? That is, is there way to have a code viewer that, instead of highlighting syntax, changes it to another syntax on the fly?
Could each member of a development team look at the same piece of code but the first sees Lisp syntax, the second sees Python, a third sees a C-style syntax, and a fourth has a mishmash of their own design?
Obviously there are language differences that go beyond syntax, but the learning curve of reading unfamiliar syntax often seems to be a sticking point for people considering/learning a new language. For example, someone who has experience in curly-brace languages may initially have trouble adapting to the use of indentation in Python or parentheses in Lisp.
Lisp and Python have different semantics, even between Python and Hy: an obvious one is that Lisp's `if` and all other statements are expressions. Coming up with a syntax is not the hard part.
A couple gotchas that basically all Lisps-on-other-languages have to keep in mind are: variables, since Lisps usually don't have the same scopes as anything imperative (with the ‘let’ blocks); and function names and possibly functions-as-variables, since Lisps are liberal with both. As a rule, you'd have to implement your own variable scopes and name mangling for where the semantics don't match. Plus macros are an obvious addition—IIRC Hy still has them in separate files (though I might confuse that with Fennel). And if you wish to have keywords as a separate type, that's usually entirely on you—afaik Clojure went full-in here, others not so much.
> reading unfamiliar syntax often seems to be a sticking point for people considering/learning a new language
Precisely the opposite of my and many other people's experience. Remembering whether blocks are delimited with braces, indentation or e.g. ‘then/end’ is a no-brainer. Learning the subtle differences in semantics, the type system, the library, package system and build environments—that's the bitch.
Consider that there are dozens of even kinda-popular Lisps, despite them not having much syntax in the first place. They don't just differ in the function names.
You seem to be imagining a ‘universal-ish intermediate language’, so to say, that would permit people with different experience to quickly adapt to an environment of a different language. Not possible, because a) semantics is where the difference is, you'd need a full-blown language translator, which still can be lossy or incomplete (e.g. no types in your standard Lisp); b) one would still have to deal with all the non-language parts of the environment, like the library, packages, the build system.
> You seem to be imagining a ‘universal-ish intermediate language’
To clarify, I do not mean that each developer would be using a different language -- they would all be writing "Python", they'd just using a superficially distinct syntax that each finds most pleasant/familiar/ergonomic.
> Remembering whether blocks are delimited with braces, indentation or e.g. ‘then/end’ is a no-brainer.
I tend to find the same, at least after an initial learning phase. In fact, I sometimes find distinct languages with similar syntax confusing since I subconsciously expect the same behavior and packages too.
Having said that I've taught and TA'd Scheme, Java, Jess, and block languages like Scratch & Alice many years ago, and in those cases a significant number of students found unfamiliar syntax to be a major hurdle, especially if it was one of their first two programming languages.
Also, judging by the volume of online complaints about Python's semantically meaningful indentation, Ada's array indexing, and Lisp's parentheses these kinds of conventions do seem like a concern even for experienced developers.
> one would still have to deal with all the non-language parts of the environment, like the library, packages, the build system.
Right, that would be the point, to be able to use a language's library/packages/build system without necessarily using its original syntax.
Well, doing this on top of a single language is more feasible, however the Python-Lisp pair is still not the best choice due to Python's strict semantics of separation between statements and expressions: when you generate an AST from Lisp, you'd have some sorta verbose shims in place of Lisp's expression-ic statements—precisely ones that Hy now creates. And vice versa, a coder's normal Python might not map cleanly to Hy.
If you're free to invent the languages in the first place, it would be kinda trivial to make several with the same semantics but different superficial syntax—basically just substitute one bunch of characters for another, no need to go via AST even. And these languages can then be transpiled to a single language of your choice, be that Python or JVM bytecode—if the semantics are made to be close enough.
Hissp takes a different approach than Hy. Where Hy has to use shims to pretend statements are expressions, Hissp just targets the expression subset in the first place. (Actually a somewhat smaller subset than that if you're not injecting any raw Python: literals, lambdas, identifiers, and calls.)
If you do not use the contributed “let” macro, then auto generated Python code from Hy source code looks fine. If you look at the GitHub repo for the Hy book I wrote, you will see a Makefile target for generating Python code from the Hy examples: https://github.com/mark-watson/hy-lisp-python
EDIT: not a Makefile target, I used a shell script create_python_source_from_hy.sh
Somebody should try the Mulisp approach. There was simple set of rules to transform Algol-like syntax to Lisp syntax.
You would not need to learn Lisp of any kind, you just add some parenthesis to a Python line and it becomes linked list data for a macro definition. And with one-to-one mapping, we can transform the macro output to back to regular Python line.
One thing I remember from trying it about 2 years ago, was that its syntax conventions etc was similar to clojure but different in some aspects, made it a bit of a chore for someone used to Clojure to keep track of the differences while using Hy.