LISP 1.5 How lisp is like a machine language?

Question

I wish that John McCarthy was still alive, but...

From LISP 1.5 Programmer's Manual :

LISP can interpret and execute programs written in the form of S- expressions. Thus, like machine language, and unlike most other higher level languages, it can be used to generate programs for further execution.

I need more clarification about how machine language can used to generate programs and how Lisp can do it?

Answer 1

All that is saying is that machine code can directly write machine instructions to memory and jump to those instructions to execute them; this is the basis of many attack vectors to break into software, in fact.

The point is, when you're writing machine code, it's easy to generate machine code. But when you're writing in a compiled language like C, you can't just generate C code at run time and then execute it - unless your program includes a C compiler.

Lisp - and, these days, many other languages, especially "scripting languages" like Perl, Python, Ruby, Tcl, Javascript, and command shells - have the ability to execute code that is generated at runtime. In Lisp, since code and data have the same structure, this is usually less work than it is in the other languages, where the code to be evaluated is generally a string that has to be parsed. (Though Perl has the ability to eval a block instead of a string, which lets the compiler do the parsing ahead of time for literal code.)

Answer 2

A machine language can alter itself while running. The last assembly programming i did was for MS DOS and resident program that i used to run before testing other programs. When my program misbehaved, a keystroke switched to the resident program and could peek into the running program and alter it directly before resuming. It was quite handy since I didn't have a debugger.

LISP had this from the very beginning since it was originally interpreted. You could change the definition of a function while you were running and the whole langugage was always available at runtime, even eval and define. When it started getting compiled it wasn't compiled like Algol, but partially, allowing for interpreted and compiled code to intermix at the same time. The fact that its code structure was list structure and that symbols are a data type contributed to this.

Last interview I saw with McCarthy he was asked about what he thought of modern programming languages (Not LISP family but the Algol family language Ruby, that is said to be influenced by LISP), and before answering he asked if they could represent code as data (like list structure). Since it didn't, Ruby is still behind what LISP was in the 60s in his opinion.

Many new programming languages are emerging in the Algol family and some of the most promising ones, like Perl6 and Nemerle, are getting closer to the features LISP had in the 60s.

Answer 3

Machine language programs can fill memory regions with arbitrary bytes. Then they can just jump to the start of such region which will thus get executed right away.

Lisp language programs can easily create arbitrary S-expressions in memory, using cons. Then they can just call eval on these S-expressions to evaluate (interpret) them.

High level languages programs can easily fill memory regions with characters representing new code in the language's syntax. But they can not run such a code.