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Lisp creates lisp

I've heard a lot that writing a function in lisp will yield a function equivalent to any of the built in operators, therefore it is entirely possible to re-create any of the built in operators using lisp code. However, this can't be done in C because of its syntax rules. But if C did use prefix notation then I don't see a reason why you couldn't re-create any of the built in operators using C code. So basically what I am wondering is if it is prefix notation alone that makes this claim true in lisp or is there something deeper going on here that I am not seeing clearly?

By qweasdzxc at 2008-03-16 03:37 | LtU Forum | previous forum topic | next forum topic | other blogs | 6810 reads

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The immediate point is that

The immediate point is that all Lisp function calls look the same syntactically: there is no visible difference between (+ a b) and (foo a b) corresponding to the C distinction between a + b and foo(a, b).

The deeper point is that because Lisp code is uniformly constructed, and has a one-to-one correspondence with built-in Lisp objects (namely symbols and pairs/conses), it is straightforward to (a) process code at run time, and (b) to run user-specified transformer programs (called macros, but not to be confused with C macros) at compile time. That is what makes Lisp the programmable programming language.

By johnwcowan at Sun, 2008-03-16 18:39 | login or register to post comments

Lisp objects not directly representable in source code

Common Lisp at least also has self-evaluating objects that can be passed to EVAL but have no direct representation in source code (i.e. don't have a canonical way to be printed readably). Examples are readtables, structures, CLOS objects, etc. Emacs Lisp is much the same: buffers, windows, etc evaluate to themselves but can't be represented in source code.

What's the Scheme take on this? I probably need a Scheme refresher course because I honestly don't remember whether it has EVAL or not.

By Luke Gorrie at Fri, 2008-03-21 02:28 | login or register to post comments

R5RS

eval was introduced to scheme in R5RS.

(I actually found out that when reading a critique of R5RS on the MIT scheme page)

By Timothy Beyer at Fri, 2008-03-21 11:50 | login or register to post comments

Not the Language, the Implementation

Being able to rewrite built in operators is a feature of the language implementation, not the language (or the syntax rules as you stated in your question). Most LISP systems are designed to be highly configurable and would allow for such major modifications at runtime by the program.

C systems are much less dynamic and do not allow those types of modifications at run time. Technically, you can rewrite the built ins in C, but that means hacking the compiler which is a separate program (generally written in C) and not the actual program you are writing. This is a different type of operation, although similarly equivalent and much less convenient than if the system is designed to support it.

LISP systems were designed for dynamic programming and flexibility, and although most of the time redefining + is a "bad idea", especially when working on a large project with many programmers, like the niche C eventually too over in history. But sometimes, on programs written by single developers with an extensive understanding of the complete system, such language modification abilities were a practical requirement at times.

So, syntax has nothing to do with it. :)

By Burton Samograd at Sun, 2008-03-16 18:43 | login or register to post comments

_

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By oldaccount at Wed, 2008-03-19 04:39 | login or register to post comments

A Language is More Than It's Standard

And sometimes the standard is the implementation.

By Burton Samograd at Thu, 2008-03-20 02:18 | login or register to post comments

There have been other

There have been other systems (than lisp) with reflective abilities.

IIRC, Self was such a system. You actually could redefine + and the system then run several hours to recompile everything inside it which used it.

By Basile STARYNKEVITCH at Mon, 2008-03-17 06:39 | login or register to post comments