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There was some discussion[0] about this a few months ago... (where I was involved & sparked the discussion), and the conclusion was that SBCL & C interop is not as good for emacs.

SBCL doesn’t allow lisp functions to be called from C without crossing the conversion line two times, which would be inefficient (esp. since the C functions are written in C for fast speed).

I personally think that even with the overhead, adopting SBCL would be an overall improvement, but I’m not maintaining emacs so...

[0] https://lists.gnu.org/archive/html/help-gnu-emacs/2019-05/ms... and the thread following




I have a hard time following that line of argument. I'd think much of the C code has been written a quarter century ago "for speed". Computers are orders of magnitudes faster these days, so we can waste a few cycles here and there. I wouldn't think it sensible to require Emacs to be fluid on computers slower than e.g. an original Raspberry Pi, which is much faster than the early Unix workstations the C code was written for.

Said that, while I personally might prefer an efficient implementation of a well documented and standardized variant of Lisp, Emacs Lisp is arguably easier to learn and more appealing to those who just want to customize the editor, which I'd presume it the vast majority of users.


Not only that, but the runtime semantics are different. ‘foo isn’t a function in CL. But in emacs lisp, it might be.

It’s not just a pedantic destination either. ‘(do-foo do-bar) is far easier to write than (list #’do-foo #’do-bar). And you can pass it around more easily. These are all things users want and need.

Emacs lisp probably wasn’t designed as a lisp. It was probably designed to be a configuration language, and lisp happened to be the most flexible way of designing it. That’s why stuff like lexical scoping or compiler semantics took a back seat (as it should in this case).

Look, I love lisp. You’d be hard pressed to find someone more fanatical about lisp. But CL is... well. CL is CL. You can make it into a config language, but even basics like (require ‘foo) leaves much to be desired.

Emacs Lisp is fundamentally simple; most people just don’t take the time to learn the design, and they dismiss it due to the long identifiers and global scope. But the global scope is the power. The global namespace matters. A lot.


> ‘foo isn’t a function in CL. But in emacs lisp, it might be.

In Common Lisp it might be.

  CL-USER 72 > (funcall (first '(sin cos)) pi)
  1.2246063538223773D-16
In Common Lisp all symbols denoting functions can be called via FUNCALL and APPLY.

Thus one can also pass symbols for functions to higher order functions like MAPCAR:

  CL-USER 73 > (mapcar 'sin '(12 34 56))
  (-0.53657294 0.5290827 -0.521551)

  CL-USER 75 > (mapcar (lambda (f arg) (funcall f arg))
                       '(sin cos tan)
                       '(1.3d0 1.4d0 1.5d0))
  (0.963558185417193D0 0.16996714290024104D0 14.101419947171719D0)

  CL-USER 76 > (mapcar 'funcall
                       '(sin cos tan)
                       '(1.3d0 1.4d0 1.5d0))
  (0.963558185417193D0 0.16996714290024104D0 14.101419947171719D0)
A restriction of Common Lisp is that local lexical functions can't be called via symbols. But anything defined by DEFUN, DEFMETHOD, DEFGENERIC uses symbols as names and those can be used by FUNCALL and APPLY.

> basics like (require ‘foo) leaves much to be desired.

REQUIRE isn't much used in Common Lisp in the last decades. Individual implementations may still support it as a calling interface to some more complex machinery to locate code. Much of the current code uses ASDF to describe and load code files.


Heh. You’re right of course. Thank you for correcting me.

I was going to retract most of my criticism due to being blatantly mistaken. But, the namespace thing is actually the part that seems like it would cause the most friction for CL as a configuration language. Specifically: in emacs lisp, if you write ‘foo-frobber, you know it’s probably from the package ‘foo, and that it’s a frobber. Whether it’s a function or a variable can often be inferred correctly from context.

Whereas in CL, isn’t the norm to use the package system everywhere? (Perhaps I am mistaken about this as well.) So you can’t just pass ‘foo-frobber to arbitrary packages and expect it to work. For example, (intern “foo-frobber”) returns a symbol. (Or does it? Now I’m second guessing everything I thought I remembered about CL.) But can you just pass that plain old symbol between packages without having to explicitly indicate its origin?

Hmmm... now that I’ve talked through the problem, perhaps the namespace issue isn’t such a big deal. Maybe the friction was moreso ASDF’s peculiarities rather than the fundamental behaviors of CL.


> in emacs lisp, if you write ‘foo-frobber, you know it’s probably from the package ‘foo, and that it’s a frobber.

It's a convention, not an actual mechanism. But it's open to interpretations. What does ANIMATED-WINDOW-DRAW mean? Is it supposed to be WINDOW-DRAW in the ANIMATED 'package' or is it DRAW from the ANIMATED-WINDOW package? Probably there is another convention for that.

Note then that in Emacs Lisp the concept package means a software library, where in Common Lisp the concept package means a symbol namespace.

> Whereas in CL, isn’t the norm to use the package system everywhere?

It's widely used, but in slightly different ways. Some use it as a poor module system replacement. Often it is used a way to structure code into larger symbol namespaces. Sometimes you'll find code bases which do everything in one package.

> So you can’t just pass ‘foo-frobber to arbitrary packages and expect it to work.

'Passing to packages' is not a concept in Common Lisp. But you can use symbols from all packages everywhere. There is no enforced hiding mechanism or module system which hides things. The only thing packages actually do is being a namespace for symbols. Thus it is possible to have many symbols with the same name, but they need to be in different packages or need to belong to no package.

> For example, (intern “foo-frobber”) returns a symbol.

It does. It uses at runtime the package bound to the current package to intern the symbol.

The current package is the value of the symbol

  *package*
> But can you just pass that plain old symbol between packages without having to explicitly indicate its origin?

You can pass the symbol around. Its package can be queried. If you want to refer to the symbol in written code while being in another package (another symbol namespace) one has four options:

1) use the symbol fully qualified FOO::BAR. Note the two colons.

2) use exported symbols as FOO:BAR. Note the one colon.

3) use the symbol after importing it into the current package as BAR.

4) programmatically lookup the symbol: (find-symbol "BAR" "FOO")


There's only one problem[0] that could bite someone used to the way elisp code is written: when the reader sees e.g. FOO:BAR, or FOO::BAZQUUX, the package FOO must already exist (and AFAIK have BAR exported), otherwise you'll get a reader error. You can't work around it with regular macros, because the error is at read-time, way before macroexpansion-time or compile-time.

This shows up whenever you need to set up some functions using a package before the code defining that package is loaded. This might be a rare use case in general, but it's something that shows up frequently enough when configuring Emacs. The only workaround for CL that I know is programmatic lookup of the symbol via FIND-SYMBOL, or just keeping the problematic code as string and running it through (EVAL (READ-FROM-STRING ...)) when it's safe for reader to parse it. Since Elisp doesn't have a package system, the problem doesn't exist there; at worst you'll get a warning from the byte compiler/interpreter that it doesn't (yet) know what variable or function a symbol represents.

--

[0] - Or at least what I believe to be a problem; I'm half-hoping there's a trivial solution that I'm missing.


Right, these are two limitations in CL at read time (when symbols are read by Lisp):

the package needs to exist and for an exported symbol notation, the symbol actually needs to be exported. The latter makes sense in some way, because the mechanism should warn, when a symbol is used, which is not exported.

One way one might be able to deal with that would be to catch the reader error and create a package on the fly, which then would have these symbols and/or exports some of these. Later, when a DEFPACKAGE is seen, the package could be adjusted to the new package definition. Note that modifying an existing package via DEFPACKAGE has undefined consequences in the standard. Implementations may deal with that, though.

But there are problems with the approach: imagine someone uses FOO:BAR in his source code. But that was a symbol from an old version of that software and now FOO:BAR is no longer exported or even doesn't exist. What now? Should this modify the package? Should it warn? Be an error?

Currently when using Common Lisp and reading a symbol with a non-existing package, the Lisp system shows an error. I can't remember what the Lisp Machine did, but it easily could have presented a restart option to create the package and go on.

LispWorks does that: it shows an error and provides a restart.

  CL-USER 89 > (read-from-string "LOL::ROTFL")

  Error: Reader cannot find package LOL.
    1 (continue) Create the LOL package.
    2 Use another package instead of LOL.
    3 Try finding package LOL again.
    4 (abort) Return to top loop level 0.

  Type :b for backtrace or :c <option number> to proceed.
  Type :bug-form "<subject>" for a bug report template or :? for other options.

  CL-USER 90 : 1 > :C 1
  LOL::ROTFL
  10
In SBCL one can create the package via DEFPACKAGE and use the RETRY restart:

  * (read-from-string "LOL::ROTFL")

  debugger invoked on a SB-INT:SIMPLE-READER-PACKAGE-ERROR in thread
  #<THREAD "main thread" RUNNING {10005184C3}>:
    Package LOL does not exist.

      Line: 1, Column: 9, File-Position: 9

      Stream: #<SB-IMPL::STRING-INPUT-STREAM {1002F8ADF3}>

  Type HELP for debugger help, or (SB-EXT:EXIT) to exit from SBCL.

  restarts (invokable by number or by possibly-abbreviated name):
    0: [CONTINUE ] Use the current package, COMMON-LISP-USER.
    1: [RETRY    ] Retry finding the package.
    2: [USE-VALUE] Specify a different package
    3: [UNINTERN ] Read the symbol as uninterned.
    4: [SYMBOL   ] Specify a symbol to return
    5: [ABORT    ] Exit debugger, returning to top level.

  (SB-IMPL::READER-FIND-PACKAGE "LOL" #<SB-IMPL::STRING-INPUT-STREAM {1002F8ADF3}> T)
  0] (defpackage "LOL")   

  #<PACKAGE "LOL">
  0] 1
  LOL::ROTFL
  10


I am not arguing that converting Emacs to Common Lisp is a good idea, but it would seem that much less low level C code would be required.

Also, the startup time for compiled and packaged SBCL applications is very fast because loading a prepared Lisp image file is very fast.




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