A Steaming Pile of Study

by baggers

TLDR: If you add powerful features to your language you must be sure to balance them with equally powerful introspection

I haven't got much done this week, I have some folks coming from the uk soon so cleaning up and getting ready for that. I have decided to take on more projects though :D

Both revolve around shipping code which was the theme last week as well. At first I was looking at some basic system calls and it seemed that, if there wasn't a good library for this already then I should make one. Turns out that was mostly me not knowing what I'm doing. I hadn't appreciated that the posix api specifies itself based on certain types and that those types can be different size/layouts/etc on different platforms, which means that we can just define lisp equivalents without knowing those details.

To get those our FFI has a groveler. A groveler's job is to make a C program which emits all the details you need to know about, you then compile and run that C program and use the output to generate the type definitions & function declarations for you language.

This all works great but requires you to have a C compiler set up and ready to go which for Windows, OSX and quite a few linux's is not the case. This, in my opinion, sucks as the user now has to think about more than the language they are trying to work in. Also because all libraries are delivered though the package manager you tend not to know about the C dependencies until you get an error during build which makes for pretty poor user experience.

To mitigate this what we can do is cache the output of the C program along with info on what platforms it is valid for. That second part is a little more fiddly as the specification that is given to the groveler is slightly different for different platforms and those difference are generally expressed with read-time conditionals.

Some background

In lisp you have 4 main times your code can run:

  • Read time: When the compiler is reading the text that represents your program you can say "Hey, if you see this pattern of characters then please give me control of the reader for a bit my calling my function". This means you can do arbitrary execution whilst your program is still being read in. This functionality is called reader macros
  • Macro Expansion Time: A regular macro is a function that is given a chunk of your code, and returns new code that will replace the original.
  • Compiler Macro Expansion Time: You can say "Hey if you see the function foo, let me have a look at the code as I might know ways to optimize it". Like with regular macros this is done with a function that is given and returns code.
  • Runtime: The time we are all used to, when the program runs.

The are some reader macros already defined, they specify how parens work for example!

And back to the story

The read-time conditionals are the read macros #+ and #- these allow you to include or exclude code from being read based on the features of the lisp implementation you are running.

For example lets say we want to have a function that does something depending on whether there are threads or not.

#+threads        ;; this says that if threads are present add (read) this code
(defun foo (x)
  (print x))

#-threads        ;; this says that if threads are present subtract (not read) this code
(defun foo (x)
  (print "No threads"))

As we all know 'the worst thing about trivial examples is that they are trivial' however it get's the idea across. If you can detect and handle a feature at runtime then it's usually more robust to do so, however if you have a situation where 'reading' the code itself makes no sense under a certain condition then read-time conditionals are very useful.

The problem

Remember I said things get fiddly? Here's why. If we are caching some code we need to know when that cache is valid, if people used reader conditionals in the specification then the cache is only valid if the result of the reader-conditionals matches. The problem is that the code that doesnt match the condition is never even read so there is nothing to introspect.

One solution would be tell people not to use reader-conditionals and use some other way of specifying the features required, we would make this a standard and we would have to educate people on how to use it.

Or maybe we can get clever, remember that reader-conditionals #+ #- are just reader macros, they are just functions. We could replace them with our own implementation which would work exactly the same except that it would also record the conditions and the results of the conditions.

This turned out to be REALLY easy!

The mapping between a character pattern like #+ and the function it calls is kept in an object called a readtable. We don't want to screw up the global readtable so we need our own copy.

    (let ((*readtable* (copy-readtable)))

*readtable* is the global variable where the readtable lives, so now an use of the lisp function read inside the scope of the let will be using our readtable (this is effect is thread local).

Next we replace the #+ & #- reader macros with our own function my-new-reader-cond-func:

    (set-dispatch-macro-character #\# #\+ my-new-reader-cond-func *readtable*)
    (set-dispatch-macro-character #\# #\- my-new-reader-cond-func *readtable*)

And that's it! my-new-reader-cond-func is actually a closure over an object that the conditions/results are cached into but that's just boring details.

The point is we can now introspect the reader conditions and know for certain what features were required in the spec file, and we do this without having to add any new practices for library writers.

This the reason for the TLDR at the top:

If you add powerful features to your language you must be sure to balance them with equally powerful introspection

Or at least trust programmers with some of the internals so they can get things done.

You can totally shoot your foot off with this feature, but this is lisp, that ship sailed with macros anyway.

I wrapped all this up in a library you can find here: https://github.com/cbaggers/with-cached-reader-conditionals

Other Stuff

Aside from this I:

  • Pushed a whole bunch of code from master to release for CEPL, Varjo, rtg-math & CEPL.SDL2

  • Requested that that new with-cached-reader-conditionals library and two others (for loading images) are added to quicklisp (the lisp package manager) So more code shipping! yay!

Enough for now

Like I said, next week I'll have people over so I won't get much done, however my next goals are:

  • Add groveler caching to the FFI

  • Add functionality to the build system to copy dynamic libraries to the build directory of a compiled lisp program. This will need to handle the odd stuff OSX does with frameworks

Seeya folks, have a great week