I tried stepping in various free implementations, but I couldn't really follow the source forms and execute them one by one. Also, I couldn't find much information online. Maybe your experience is different?
In most of those implementations (certainly in SBCL) it's either you break or step; you can't start stepping from a breakpoint. SBCL got some support for that this year, see https://news.ycombinator.com/item?id=43791709. It, however, doesn't allow stepping into any functions called after the break.
Also, the compilers are allowed to make the code unsteppable in some cases, depending on optimization declaration: generally, debug needs to be >=2 and > speed/compilation-speed/space. In some circumstances, you land in decompiled/macroexpanded code, which is also quite unhelpful.
Anyway, it's not that source-level stepping isn't there at all, it's just quirky and somewhat inconvenient. A fresh implementation that does comparatively little optimization and is byte-code based can probably support debuggers better. I hope such support won't go away later when the native code compiler is implemented.
If I recall correctly, there are macros to control the level of code optimization? And some implementations can turn it off entirely for interactive use?
If I recall correctly, there are macros to control the level of code optimization?
Yup, you can either `(proclaim (optimize (debug 3) (speed 1)))` somewhere, which will take effect globally, or you can `(declare (optimize ...))` inside a particular function. It sounds great in theory - and it is great, in some respects - but this granularity makes it harder to ensure all interesting code is steppable when you need it.
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I think most free CL implementations have a stepper. Which ones do not?
I tried stepping in various free implementations, but I couldn't really follow the source forms and execute them one by one. Also, I couldn't find much information online. Maybe your experience is different?
I haven’t used CL recently so I can’t speak from experience. But it looks like:
CMU CL, SBCL, and LispWorks have steppers.
Clozure does not. (Edit: an answer on https://stackoverflow.com/questions/37754935/what-are-effici... suggests it does...)
As I understand it, those are the big 4.
Clisp, ABCL, and Allegro also appear to have steppers.
Always cool to see a new implementation, though!
In most of those implementations (certainly in SBCL) it's either you break or step; you can't start stepping from a breakpoint. SBCL got some support for that this year, see https://news.ycombinator.com/item?id=43791709. It, however, doesn't allow stepping into any functions called after the break.
Also, the compilers are allowed to make the code unsteppable in some cases, depending on optimization declaration: generally, debug needs to be >=2 and > speed/compilation-speed/space. In some circumstances, you land in decompiled/macroexpanded code, which is also quite unhelpful.
Anyway, it's not that source-level stepping isn't there at all, it's just quirky and somewhat inconvenient. A fresh implementation that does comparatively little optimization and is byte-code based can probably support debuggers better. I hope such support won't go away later when the native code compiler is implemented.
Thanks!
If I recall correctly, there are macros to control the level of code optimization? And some implementations can turn it off entirely for interactive use?
Or am I off-base?
Yup, you can either `(proclaim (optimize (debug 3) (speed 1)))` somewhere, which will take effect globally, or you can `(declare (optimize ...))` inside a particular function. It sounds great in theory - and it is great, in some respects - but this granularity makes it harder to ensure all interesting code is steppable when you need it.