Why I Program in Lisp (opens in new tab)

> It's not the parens persay, it's the fact that I'm used to reading up to down and left to right. Lisp without something like the clojure macro ->, means that I am reading from right to left, bottom to top - from inside out.

I’m not certain how true that really is. This:

    foo(bar(x), quux(y), z);

looks pretty much identical to:

    (foo (bar x) (quux y) z)

And of course if you want to assign them all to variables:

    int bar_x = bar(x);
    char quux_y = quux(y);
    
    return foo(bar_x, quux_y, z);

is pretty much the same as:

    (let ((bar-x (bar x))
          (quux-y (quux y)))
      (foo bar-x quux-y z))

FWIW, ‘per se’ comes from the Latin for ‘by itself.’

I think it really depends, in Common Lisp for example I don't think that's the case:

  (progn
    (do-something)
    (do-something-else)
    (do-a-third-thing))

The only case where it's a bit different and took some time for me to adjust was that adding bindings adds an indent level.

  (let ((a 12)
        (b 14))
    (do-something a)
    (do-something-else b)
    (setf b (do-third-thing a b)))

It's still mostly top-bottom, left to right. Clojure is quite a bit different, but it's not a property of lisps itself I'd say. I have a hard time coming up with examples usually so I'm open to examples of being wrong here.

It's easy enough to add -> (and related arrow operators) to Common Lisp as macros.

https://github.com/hipeta/arrow-macros

The common complaint that Common Lisp lacks some feature is often addressed by noting how easy it is to add that feature.

Been programming in Lisp for a while. The parents disappear very quickly. One trick to accelerate it is to use a good editor with structural editing (e.g., paredit in Emacs or something similar). All you editing is done on balanced expressions. When you type “(“, the editor automatically inserts “)” with your cursor right in between. If you try to delete a “)”, the editor ignores you until you delete everything inside and the “(“. Basically, you start editing at the expression level, not so much at the character or even line level. You just notice the indentation/shape of the code, but you never spend time counting parentheses or trying to balance anything. Everything is balanced all the time and you just write code.

> reading from right to left, bottom to top - from inside out

I don't understand why you think this. Can you give an example?

I know Lisp since I read the little lister around 1996, and was an XEmacs user until around 2005.

The parenthesis do really disappear, just like the hieroglyphics on C influenced languages, it is a matter of habit.

At least it was for me.

To me any kind of deep nesting is an issue. It goes against the idea of reducing the amount of mental context window needed to understand something.

Plus, if syntax errors can easily take several minutes to fix, because if the syntax is wrong, auto format doesn't work right, and then you have to read a wall of text to find out where the missing close paren should have been.

> Good article. Funnily enough the throw away line "I don't see parentheses anymore". Is my greatest deterrent with lisp. It's not the parens persay, it's the fact that I'm used to reading up to down and left to right.

  Language shapes the way we think, and determines what we can think about.
  - Benjamin Lee Whorf[0]

From the comments in the post:

  Ask a C programmer to write factorial and you will likely
  get something like this (excuse the underbars, they are
  there because blogger doesn't format code in comments):

  int factorial (int x) {
      if (x == 0)
          return 1;
      else
          return x * factorial (x - 1);
  }

  And the Lisp programmer will give you:

  (defun factorial (x)
    (if (zerop x)
        1
        (* x (factorial (- x 1)))))

Let's see how we can get from the LISP version to something akin to the C version.

First, let's "modernize" the LISP version by replacing parentheses with "curly braces" and add some commas and newlines just for fun:

  {
    defun factorial { x },
    {
    if { zerop x },
      1 {
        *,
        x {
          factorial {
            - { x, 1 }
          }
        }
      }
    }
  }

This kinda looks like a JSON object. Let's make it into one and add some assumed labels while we're at it.

  {
    "defun" : {
      "factorial" : { "argument" : "x" },
      "body" : {
        "if" : { "zerop" : "x" },
        "then" : "1",
        "else" : {
          "*" : {
            "lhs" : "x",
            "rhs" : {
              "factorial" : {
                "-" : {
                  "lhs" : "x",
                  "rhs" : "1"
                }
              }
            }
          }
        }
      }
    }
  }

Now, if we replace "defun" with the return type, replace some of the curlies with parentheses, get rid of the labels we added, use infix operator notation, and not worry about it being a valid JSON object, we get:

    int
      factorial ( x )
        {
          if ( zerop ( x )  )
            1
          else
            x * factorial ( x - 1 )
        }

Reformat this a bit, add some C keywords and statement delimiters, and Bob's your uncle.

0 - https://www.goodreads.com/quotes/573737-language-shapes-the-...

This is the most elementary hurdle a lisp programmer will face. You do indeed become adjusted to it quite quickly. I wouldn’t let this deter you from exploring something like Clojure more deeply.

per se

The idea of pure functional programming is that you can really go quite far if you think of your program as a pure function f(input) -> outputs with a messy impure thing that calls f and does the necessary I/O before/after that.

Batch programs are easy to fit in this model generally. A compiler is pretty clearly a pure function f(program source code) -> list of instructions, with just a very thin layer to read/write the input/output to files.

Web servers can often fit this model well too: a web server is an f(request, database snapshot) -> (response, database update). Making that work well is going to be gnarly in the impure side of things, but it's going to be quite doable for a lot of basic CRUD servers--probably every web server I've ever written (which is a lot of tiny stuff, to be fair) could be done purely functional without much issue.

Display also can be made work: it's f(input event, state) -> (display frame, new state). Building the display frame here is something like an immediate mode GUI, where instead of mutating the state of widgets, you're building the entire widget tree from scratch each time.

In many cases, the limitations of purely functional isn't that somebody somewhere has to do I/O, but rather the impracticality of faking immutability if the state is too complicated.

Think of it like other features:

* Encapsulation? What's the point of having it if's perfectly sealed off from the world? Just dead-code eliminate it.

* Private? It's not really private if I can Get() to it. I want access to that variable, so why hide it from myself? Private adds nothing because I can just choose not to use that variable.

* Const? A constant variable is an oxymoron. All the programs I write change variables. If I want a variable to remain the same, I just wont update it.

Of course I don't believe in any of the framings above, but it's how arguments against FP typically sound.

Anyway, the above features are small potatoes compared to the big hammer that is functional purity: you (and the compiler) will know and agree upon whether the same input will yield the same output.

Where am I using it right now?

I'm doing some record linkage - matching old transactions with new transactions, where some details may have shifted. I say "shifted", but what really happened was that upstream decided to mutate its data in-place. If they'd had an FPer on the team, they would not have mutated shared state, and I wouldn't even need to do this work. But I digress.

Now I'm trying out Dijkstra's algorithm, to most efficiently match pairs of transactions. It's a search algorithm, which tries out different alternatives, so it can never mutate things in-place - mutating inside one alternative will silently break another alternative. I'm in C#, and was pleasantly surprised that ImmutableList etc actually exist. But I wish I didn't have to be so vigilant. I really miss Haskell doing that part of my carefulness for me.

Not even the most fanatical functional programming zealots would claim that programs can be 100% functional. By definition, a program requires inputs and outputs, otherwise there is literally no reason to run it.

Functional programming simply says: separate the IO from the computation.

> Pretty much anything I've written over the last 30 years, the main purpose was to do I/O, it doesn't matter whether it's disk, network, or display.

Every useful program ever written takes inputs and produces outputs. The interesting part is what you actually do in the middle to transforms inputs -> outputs. And that can be entirely functional.

It's always hard to parse if people mean functional programming when bringing up Lisp. Common Lisp certainly is anything but a functional language. Sure, you have first order functions, but you in a way have that in pretty much all programming languages (including C!).

But most functions in Common Lisp do mutate things, there is an extensive OO system and the most hideous macros like LOOP.

I certainly never felt constrained writing Common Lisp.

That said, there are pretty effective patterns for dealing with IO that allow you to stay in a mostly functional / compositional flow (dare I say monads? but that sounds way more clever than it is in practice).

Any useful program has side-effects. IMHO the point is to isolate the part of the code that has the side-effects as much as possible, and keep the rest purely functionsl. That makes it easier to debug, test, and create good abstractions. Long term it is a very good approach.

> Pretty much anything I've written over the last 30 years, the main purpose was to do I/O, it doesn't matter whether it's disk, network, or display.

Erlang is a strictly (?) a functional language, and the reason why it was invented was to do network-y stuff in the telco space. So I'm not sure why I/O and functional programming would be opposed to each other like you imply.

This is discussing Common Lisp which is not even a mostly-functional language, and far from purely functional.

I wrote a recursive descent parser in Lisp for a YAML replacement language[1]. It wasn't difficult. Lisp makes it easy to write I/O, but also easy to separate logic from I/O. This made it easy for me to write unit tests without mocking.

I also wrote a toy resource scheduler at an HTTP endpoint in Haskell[2]. Writing I/O in Haskell was a learning curve but was ultimately fine. Keeping logic separate from I/O was the easy thing to do.

1: https://github.com/djha-skin/nrdl

2: https://github.com/djha-skin/lighthouse

It's about minimizing and isolating state and side effects, not eliminating them completely

Functional core, imperative shell is a common pattern. Keep the side effects on the outside. Instead of doing side effects directly, just return a data structure that can be used to enact the side effect

As others have said, a pure program is a useless program. The only place stuff like that has in this world is as a proof assistant.

What I will add is look up how the GHC runtime works, and the STGM. You may find it extremely interesting. I didn't "get" functional programming until I found out about how exotic efficient execution of functional programs ends up being.

"Purely Functional Programming", I guess mostly Haskell/Purescript.

So this only really mean:

Purely Functional Programming by default.

In most programming languages you can write

"hello " + readLine()

And this would intermix pure function (string concatenation) and impure effect (asking the user to write some text). And this would work perfectly.

By doing so, the order of evaluation becomes essential.

With a pure functional programming (by default).

you must explicitely separate the part of your program doing I/O and the part of your program doing only pure computation. And this is enforced using a type system focusing on I/O. Thus the difference between Haskell default `IO` and OCamL that does not need it for example.

in Haskell you are forced by the type system to write something like:

    do 
      name <- getLine
      let s = "Hello " <> name <> "!"
      putStrLn s

you cannot mix the `getLine` directly in the middle of the concatenation operation.

But while this is a very different style of programming, I/O are just more explicit, and they "cost" more, because writing code with I/O is not as elegant, and easy to manipulate than pure code. Thus it naturally induce a way of coding that try to really makes you conscious about the part of your program that need IO and the part that you could do with only pure function.

In practice, ... yep, you endup working in a "Specific to your application domain" Monad that looks a lot like the IO Monad, but will most often contains IO.

Another option is to use a free monad for your entire program that makes you able to write in your own domain language and control its evaluation (either using IO or another system that simulates IO but is not really IO, typically for testing purpose).

OpenSCAD is such a good school of functional programming. There is no "time" or flow of execution. Or variables, scopes and environments. You are not constructing a program, but a static model which has desired properties in space and time.

The point of functional, "sans I/O" style is to separate the definition of I/O from the rest of your logic. You're still doing I/O, but what sorts of I/O you're doing has a clear self-contained definition within your program. https://sans-io.readthedocs.io/how-to-sans-io.html

There is no reason you can't use side effects in pure functional programming. You just need to provide the appropriate description of the side effect to avoid caching and force a particular evaluation order. If you have linear types, you do it by passing around opaque tokens. I'm not entirely sure how IO works in Haskell, but I think the implementation is similar. Even C compilers use a system like that internally.

The boundary between the program and the rest of the system allows I/O of course. What FP does is "virtualize" I/O by representing it as data (thus it can be passed around). Then at some point these changes get "committed" to the outside. Representing I/O separately from how it is carried out allows a lot of things to be done, such as cancelling (ctrl+z) operations.

Everyone writes real programs that have side effects. Functional programming is no different. But the side effects happen in specific ways or places, rather than all over the place.

Most of the code in most programs is not the part that is doing the I/O. It's doing stuff on a set of values to transform them. It gets values from somewhere, does stuff using those values, and then outputs some values. The complicated part is not the transfer of the final byte sequence to whatever I/O interface they go to, the core behavior of the program is the stuff that happens before that.

There are ways to handle side effects with pure functions only (it’s kind of cheating, because the actual side effects are performed by the non-pure runtime/framework that’s abstracted away, while the pure user code just defines when to perform them and how to respond to them). It’s possible, but it gets very awkward very fast. I wouldn’t use FP for any part of the code that deals with IO.

I had the same question until I understood one key pattern of pure functional programming. Not sure it has a name but here goes.

There is world, and there is a model of the world - your program. The point of the program, and all functions, is to interact with the model. This part, data structures and all, is pure.

The world interacts with the model through an IO layer, as in haskell.

Purity is just an enforcement of this separation.

> Whenever I hear someone talking about purely functional programming, no side effects, I wonder what kind of programs they are writing

Where have you ever heard anyone talk about side-effect free programs, outside of academic exercises? The linked post certainly isn't about 100% side-effect/state free code.

Usually, people talk about minimizing side-effects as much as possible, but since we build programs to do something, sometimes connected to the real world, it's basically impossible to build a program that is both useful and 100% side-effect free, as you wouldn't be able to print anything to the screen, or communicate with other programs.

And minimizing side-effects (and minimizing state overall) have a real impact on how easy it is to reason about the program. Being really carefully about where you mutate things, leads to most of the code being very explicit about what it's doing, and code only affects data that is close to where the code itself is, compared to intertwined state mutation, where things everywhere in the codebase can affect state anywhere.

The pragmatic approach is to see that FP's key point is statelessness and use that in your code (written in more mainstream languages) when appropriate.

I never believed any FP evangelist ever since I realized I can't even write quicksort with it *.

(* Yes, you can technically write it procedurally like a good C programmer, sure.)

Though these are minor complaints, there is a couple things I'd like to change about a Lisp language.

One is its the implicit function calls. For example, you'll usually see calls like this: `(+ 1 2)` which translates to 1 + 2, but I would find it more clear if it was `(+(1,2))` where you have a certain explicitness to it.

It doesn't stop me from using Lisp languages (Racket is fun, and I been investigating Clojure) but it took way too long for the implicit function stuff to grok in my brain.

My other complain is how the character `'` can have overloaded meaning, though I'm not entirely sure if this is implementation dependent or not

> No types barking

No thanks

Looking for a nice, solid, well-documented library to do something is difficult for most stuff. There are some real gems out there, but usually you end up having to roll your own thing. And Lisp generally encourages rolling your own thing.

People smart enough to read and write it are rare.

The only software I use that I know runs a lisp is Hacker News.

JavaScript and Python have adopted almost every feature that differentiated Lisp from other languages. So in comparison Lisp is just more academic, esoteric, and advanced.

It's pretty rare because when it was being originally designed the main intended use case was for bragging about the cool unique niche programming language you use on your blog posts. While it's a very good language for being able to recursively get itself onto the front page, it's not so good at conformist normie use cases.

In my case, my boss won't let me.

Common Lisp can compete with Python no problem, that's what matters to me. You get:

- truly interactive development (never wait for something to restart, resume bugs from any stack frame after you fixed them),

- self-contained binaries (easy deployment, my web app with all the dependencies, HTML and CSS is ±35MB)

- useful compile-time warnings and errors, a keystroke away, for Haskell levels see Coalton (so better than Python),

- fast programs compiled to machine code,

- no GIL

- connect to, inspect or update running programs (Slime/Swank),

- good debugging tools (interactive debugger, trace, stepper, watcher (on some impls)…)

- stable language and libraries (although the implementations improve),

- CLOS and MOP,

- etc

- good editor support: Emacs, Vim, Atom/Pulsar (SLIMA), VScode (ALIVE), Jetbrains (SLT), Jupyter kernel, Lem, and more: https://lispcookbook.github.io/cl-cookbook/editor-support.ht...

What we might not get:

- advanced refactoring tools -also because we need them less, thanks to the REPL and language features (macros, multiple return values…).

---

For a lisp machine of yesterday running on Ubuntu or the browser: https://interlisp.org/

Of course you're right. I've written non-trivial programs in Scheme, Emacs is a good tool for it, but certainly don't know of an environment that matches the Lisp machines.

IDEs provide such environments for the most common languages but major IDEs offer meager functionality for Lisp/Scheme (and other "obscure" languages). With a concerted effort it's possible an IDE could be configured to do more for Lisp. Thing is the amount of effort required is quite large. Since AFAIK no one has taken up the challenge, we can only conclude it's not worth the time and energy to go there.

The workflow I've used for Scheme programming is pretty simple. I can keep as many Emacs windows ("frames") open as necessary with different views of one or several modules/libraries, a browser for documentation, terminals with REPL/compiler, etc. Sort of a deconstructed IDE. Likely it does take a bit more cognitive effort to work this way, but it gets the job done.

Watch Rich Hickey's early Clojure videos and be blown away.

Lispworks has a free editition with lots of examples. Look into PAIP from Peter Norvig.

Paul Graham's On Lisp is also a powerful argument to try the language, even if some of the stuff it presents is totally bonkers. :-D

https://www.paulgraham.com/onlisp.html

Next time you see a HN post on a lisp-centric topic, click into the comments. I'll bet you a nickel that they'll be happier than most. Instead of phrases like "dumpster fire" they're using words like "joyful".

That's why I keep rekindling my learn-lisp effort. It feels like I'm just scratching the surface re: the fun that can be had.

Here's one in Clojure using its core.logic library. I'd say it's pretty neat. You can do something similar in something like Prolog, but a Java implementation would look very different.

https://github.com/sideshowcoder/core-logic-sudoku-solver/bl...

One doesn't have to invoke Turing or Church to show all languages can do the same things.

Any code that runs on a computer (using the von Neumann architecture) boils down to just a few basic operations: Read/write data, arithmetic (add/subtract/etc.), logic (and/or/not/etc.), bit-shifting, branches and jumps. The rest is basically syntactic sugar or macros.

If your preferred programming language is a pre-compiled type-safe object oriented monster with polymorphic message passing via multi-process co-routines, or high-level interpreted purely functional archetype of computing perfection with just two reserved keywords, or even just COBOL, it's all going to break down eventually to the ops above.

Isn’t this just a cheeky joke? I.e. “if Einstein is right about this whole theory of relatively thing”

Common Lisp at its base is usually written in Common Lisp.

There are several Lisp implementations (including fully-fledged operating systems) which are implemented in Lisp top to bottom.

This is conflating slightly different things, though? One is that you can build a program that does the same thing. The other is that you can do the same things with the language.

There are special forms in LISP, but that is a far cry from the amount of magic that can only be done in the compiler or at runtime for many languages out there.

Yes, but sometimes doing the things Lisp can do in another language as easily and flexibly as they are done in Lisp has, as a first step, implementing Lisp in the target language.

For a famous example, see Clasp: https://github.com/clasp-developers/clasp

I believe ‘twas a joke.

It doesn't have to be wrong to be irrelevant.

Many things can be viewed as coordination problems. All of life can be viewed as being about coordination between tasks.

But I want to engage in good faith and assume you have some way of making this productive. What angle are you going for?

Recent, compared to Lisp

The word “properly” is not only working hard here, but perhaps pointing to deeper concepts.

In particular, it implies a coherent design around scope and extent. And, much more indirectly, it points to time. EVAL-WHEN has finally made a bit of a stir outside Lisp.

> "properly working lambda expressions were only available in Lisp until recently."

> until -> since

I think "only since recently" is not standard English, but, even if it were, I think it would change the intended meaning to say that they were not available in Lisp until recently, the opposite of what was intended. I find it clearer to move the "only": "were available only in Lisp until recently."

"properly working lambda expressions were available only in lisp until recently."

I have about 6 years of ruby experience and if you're saying that ruby has "easily predictable syntax"...

You really should try lisp. I liked clojure a lot coming from ruby because it has a lot of nice ergonomics other lisps lack. I think youd get a lot out of it.

Common Lisp has compilers that produce fast code.

If you want something similar to Ruby but more functional, try Elixir. The similarities are superficial but might be enough to ease you in.

Haskell is weird. You can express well defined problems with relative ease and clarity, but performance can be kind of wonky and there's a lot more ceremony than your typical Lisp or Scheme or close relative of those. F# can give you a more lispish experience with a threshold about as low as Haskell, but comes with the close ties to an atrocious corporation and similar to Clojure it's not exactly a first class citizen in the environment.

Building stuff in Lisp-likes typically don't entail the double programming languages of the primary one and a second one to program the type system, in that way they're convenient in a way similar to Ruby. I like the parens and how they explicitly delimit portions that quite closely relate to the AST-step in compilation or whatever the interpreter sees, it helps with moving things around and molding the code quickly.

Llm content is trained in popularity. If we use that as a metric there will never be any improvements or changes again.

how is that anywhere close to a given?