5 ms·

> ((if (< 1 0) + ) 42 100) This gives me an error in SBCL. Error below: ; in: (IF (< 1 0) ; + ; *) 42 ; ((IF (< 1 0) ; + ;

> ((if (< 1 0) + ) 42 100) This gives me an error in SBCL. Error below: ; in: (IF (< 1 0) ; + ; *) 42 ; ((IF (< 1 0) ; + ; *) ; 42 100) ; ; caught ERROR: ; illegal function call ; ; compilation unit finished ; caught 1 ERROR condition Why does it not work and how to make it work?

moonchild 4y ago
Common lisp has separate function and value namespaces, unlike scheme. Try (funcall (if (< 1 0) #'+ #'*) 42 100).
jlg23 4y ago
See moonchild's response for the why, here is the how: (funcall (if (< 1 0) #'+ #'*) 42 100) Edit: this is actually a pretty famous interview question for lisp jobs: write a program that is valid in CL and in scheme but produces different output. The solution is always to use the fact that scheme is a lisp-1, i.e. it has ONE namespace for variables and functions) and CL is a lisp-2, with TWO distinct namespaces.
- sph 4y ago
  Finding out about lisp-1 and lisp-2 when hacking in elisp turned me off so much I've decided to write my own editor in Scheme. This is my personal crusade for the next X months.
  - nerdponx 4y ago
    How about using Neovim and writing scripts and configs in Fennel? https://fennel-lang.org/ https://fennel-lang.org/
    - sph 4y ago
      vim is great but Emacs is on a whole other level for me. But I would love to spend some time to write a löve2d game in Fennel, for sure.
  - jlg23 4y ago
    I have yet to find a use-case where I really want a lisp-1 over a lisp-2. Though I'd really love to see some production code that exploits the single namespace.
    - Kamq 4y ago
      It's kinda nice when you want to, say, name a list "list" despite there being a list function. But I agree, it isn't super important.
      - Zambyte 4y ago
        I think you have it backwards. That's why you'd want a Lisp-2 over a Lisp-1.
        Kamq 4y ago
        That's what I was describing, yes.
      - kazinator 4y ago
        The thing is that Lisp-1 doesn't prevent you from naming a variable lisp; you may then have a problem if you try to use the list function in the same scope. It is a very important issue, because it interacts with hygiene in the face of macros. Lisp-1 dialects, and more broadly, single namespace languages, require hygienic macros if they have a macro system. Hygienic macros systems are gross; not everyone likes their complexity. They have referential transparency comes at the cost of sacrificing implementation transparency. A user of a Lisp-2 language can pretend that hygienic macros don['t exist, and be happy for the rest of their days. The reason is that if you do this in a Lisp-2, you're safe: (let ((list '(1 2 3)) (opaque-macro list)) Suppose opaque-macro has an expansion which contains (list ...) calls. Those are unaffected by the variable. opaque-macro would cause a problem if it tried to bind a variable called list. That is taken care of in unhygienic macro programming by using explicit gensyms. There could be a problem if there was a list variable that opaque-macro's expansion depends on. For that we have (1) nameing conventions like *list* for varaibles and (2) package system: macros coming from a foobar library, such that the variable they reference is foobar::list, unrelated to the user's (let ((list ...))). The whole Lisp-2 + unnhygienic macros (+ packages) is easy to work with and understand, simple and pragmatic.

moonchild 4y ago

Common lisp has separate function and value namespaces, unlike scheme. Try (funcall (if (< 1 0) #'+ #'*) 42 100).

jlg23 4y ago

See moonchild's response for the why, here is the how: (funcall (if (< 1 0) #'+ #'*) 42 100) Edit: this is actually a pretty famous interview question for lisp jobs: write a program that is valid in CL and in scheme but produces different output. The solution is always to use the fact that scheme is a lisp-1, i.e. it has ONE namespace for variables and functions) and CL is a lisp-2, with TWO distinct namespaces.

sph 4y ago
Finding out about lisp-1 and lisp-2 when hacking in elisp turned me off so much I've decided to write my own editor in Scheme. This is my personal crusade for the next X months.
- nerdponx 4y ago
  How about using Neovim and writing scripts and configs in Fennel? https://fennel-lang.org/ https://fennel-lang.org/
  - sph 4y ago
    vim is great but Emacs is on a whole other level for me. But I would love to spend some time to write a löve2d game in Fennel, for sure.
- jlg23 4y ago
  I have yet to find a use-case where I really want a lisp-1 over a lisp-2. Though I'd really love to see some production code that exploits the single namespace.
  - Kamq 4y ago
    It's kinda nice when you want to, say, name a list "list" despite there being a list function. But I agree, it isn't super important.
    - Zambyte 4y ago
      I think you have it backwards. That's why you'd want a Lisp-2 over a Lisp-1.
      - Kamq 4y ago
        That's what I was describing, yes.
    - kazinator 4y ago
      The thing is that Lisp-1 doesn't prevent you from naming a variable lisp; you may then have a problem if you try to use the list function in the same scope. It is a very important issue, because it interacts with hygiene in the face of macros. Lisp-1 dialects, and more broadly, single namespace languages, require hygienic macros if they have a macro system. Hygienic macros systems are gross; not everyone likes their complexity. They have referential transparency comes at the cost of sacrificing implementation transparency. A user of a Lisp-2 language can pretend that hygienic macros don['t exist, and be happy for the rest of their days. The reason is that if you do this in a Lisp-2, you're safe: (let ((list '(1 2 3)) (opaque-macro list)) Suppose opaque-macro has an expansion which contains (list ...) calls. Those are unaffected by the variable. opaque-macro would cause a problem if it tried to bind a variable called list. That is taken care of in unhygienic macro programming by using explicit gensyms. There could be a problem if there was a list variable that opaque-macro's expansion depends on. For that we have (1) nameing conventions like *list* for varaibles and (2) package system: macros coming from a foobar library, such that the variable they reference is foobar::list, unrelated to the user's (let ((list ...))). The whole Lisp-2 + unnhygienic macros (+ packages) is easy to work with and understand, simple and pragmatic.