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Comment by HarHarVeryFunny | Hacker News Reader

Reminds me of Rational Software's Instant-C from the 1980's which combined an incremental function-at-a-time compiler and runtime link/loading. Basically you just edited your code and ran it - kind of like using an interpreter, except it really was compiled. It really was instant even back on the slower computers of that era. You could even run code that had missing functions, then write them and continue when the runtime system complained!

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This is how Julia works today. We sometimes like to call this "Just Ahead of Time" compilation as a riff on "Just In Time" compilation since most JIT languages are operating very differently.

that's how common lisp and clojure still work too!

How does it work in Clojure? Attempting to compile/evaluate this:

  (defn foobar [] (undef-fn))

gives:

  1. Caused by java.lang.RuntimeException
  Unable to resolve symbol: undef-fn in this context

Attempting to call foobar:

  (foobar)

gives:

  1. Unhandled java.lang.IllegalStateException
  Attempting to call unbound fn: #'user/foobar

Environment info:

  ;; CIDER 1.3.0 (Ukraine), nREPL 0.9.0
  ;; Clojure 1.11.1, Java 18.0.2

Only change is that you have to tell the compiler that 'undef-fn' exists but you don't have to provide an implementation. So, before all of that just do

    (def undef-fn)

It should work after that.

Pre-declaring the undefined function does prevent the first of those exceptions. I guess it just feels weird to include Clojure here, because the CL experience is like it brings up an interactive debugger and asks if you wanna define the function now or abort.

Re-reading the original post I see what you mean now. But Clojure definitely fits the first part of the definition by compiling your methods instantly as you're writing them in the repl. For the second part I'm not aware of a prompt which would let you define a method in response to a missing function call. Might be an interesting feature.

For the second part: with restarts and conditions, CL pulls up an interactive debugger that asks if you would like to abort, continue/retry calling undefined or supply a definition now, amongst others.

Evalutating this in CL (SBCL)

  (undef-fn)

gives:

  Restarts:
   0: [CONTINUE] Retry calling UNDEF-FN.
   1: [USE-VALUE] Call specified function.
   2: [RETURN-VALUE] Return specified values.
   3: [RETURN-NOTHING] Return zero values.
   4: [RETRY] Retry SLY interactive evaluation request.
   5: [\*ABORT] Return to SLY's top level.
   6: [ABORT] abort thread (#<THREAD "slynk-worker" RUNNING {100288C7E3}>)
  [... as well as a clipped stack trace ...]

Pressing 1 will allow you to name an alternative function in a minibuffer prompt, 2 lets you set the return value of the undefined fn, etc.

Defining the function after the error and retrying does I think fit the description too.

I guess the seamless continue without retry possibility could be useful if you are doing important side effects, but Clojure is quite functional unlike CL so it doesn't come up as much.

HolyC from TempleOS also has some similar facilities, I believe. I just know it from Terry's videos, but for example the TempleOS shell is just a HolyC REPL.

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