Another feature of CL, specifically CLOS, is that it actually has a formal protocol for changing classes.
A formal protocol you can tap into. While hardly omniscient, I don’t know of another language that does offer this. I don’t even think Smalltalk has this. (Smalltalk has a fundamental primitive related to this called “become:”, which can be used for this purpose, but that’s less formal than what CLOS provides.)
What does this mean? It means that you can change the structure of classes, AND their instances, in a live, running system. Not just their structure, but how the system transmutes from the old structure to the new structure. How the conversion is done.
What does this have to do with the REPL? It’s part and parcel of the kind of environment and functionality of the system that is exposed by the REPL.
The REPL is not just a console that you can type into, and use backspace, and what not. It’s the door to the very rich world underlying the system.
And this is the key point. In CL, the REPL is not an afterthought. It’s not an add-on. It’s a core competency. Much of this no doubt came from the Lisp Machine experience, where all you had was a running image that, like a surgeon, you had your arms elbow deep into. A system where you could not trivially just stop and restart for every little change. A system where you had to have the ability to change the tires on a running vehicle.
The vast majority of modern REPL environments don’t have that burden, so when things get tough, they can punt. “Eh, just restart and do over.”
It’s absolutely fair to consider whether that quality is actually still germane in the modern era. Hard to imagine a scenario where you might do something like a change like this on a running server, especially in our age of “cattle, not pets”.
But the legacy is still there from that past time. A time when not only were they pets, they were coddled and spoiled. So the folks back then had to think this stuff through.
That is why I always give Lisp enviroments and Smalltalk as example, when someone comes up with the common excuse that Python cannot have good JITs, as it is too dynamic.
If AWS had an R&D Labs division with zero expectation of profit I think you could sell them an idea of Elastic Lisp Machine where a deployment is not a restart but whatever you need to do in the lisp world. (Patch an image…? No idea but sounds cool!)
The REPL is not an add-on in the sense that it comes with the implementation, but it is in the sense that if it were not provided, it could be implemented using only things available standardly in a Common Lisp implementation.
A formal protocol you can tap into. While hardly omniscient, I don’t know of another language that does offer this. I don’t even think Smalltalk has this. (Smalltalk has a fundamental primitive related to this called “become:”, which can be used for this purpose, but that’s less formal than what CLOS provides.)
What does this mean? It means that you can change the structure of classes, AND their instances, in a live, running system. Not just their structure, but how the system transmutes from the old structure to the new structure. How the conversion is done.
What does this have to do with the REPL? It’s part and parcel of the kind of environment and functionality of the system that is exposed by the REPL.
The REPL is not just a console that you can type into, and use backspace, and what not. It’s the door to the very rich world underlying the system.
And this is the key point. In CL, the REPL is not an afterthought. It’s not an add-on. It’s a core competency. Much of this no doubt came from the Lisp Machine experience, where all you had was a running image that, like a surgeon, you had your arms elbow deep into. A system where you could not trivially just stop and restart for every little change. A system where you had to have the ability to change the tires on a running vehicle.
The vast majority of modern REPL environments don’t have that burden, so when things get tough, they can punt. “Eh, just restart and do over.”
It’s absolutely fair to consider whether that quality is actually still germane in the modern era. Hard to imagine a scenario where you might do something like a change like this on a running server, especially in our age of “cattle, not pets”.
But the legacy is still there from that past time. A time when not only were they pets, they were coddled and spoiled. So the folks back then had to think this stuff through.