This is a post I wrote a long time ago and never posted, but I’ve started getting back into doing some work in Common Lisp and thought it’d be good to send this one off.

On my recent “(re)learn Common Lisp” journey, I’ve happened across a few things that I’ve found frustrating or confusing: this post is a collection of them, in hopes that other people don’t struggle with them:

  • Implementing an existing generic function for a class of your own, and have other callers specialize use your method implementation you must import the generic function, otherwise other callers will (might?) fall back to another method. This makes sense in retrospect, but definitely wasn’t clear on the first go.

  • As a related follow on, you don’t have to define a generic function in order to write or use a method, and I’ve found that using methods is actually quite nice for doing some type checking, at the same time, it can get you into a pickle if you later add the generic function and it’s not exported/imported as you want.

  • Property lists seem cool for a small light weight mapping, but they’re annoying to handle as part of public APIs, mostly because they’re indistinguishable from regular lists, association lists are preferable, and maybe with make-hash even hash-tables.

  • Declaring data structures inline is particularly gawky. I sometimes want to build a list or a hash map in-line an alist, and it’s difficult to do that in a terse way that doesn’t involve building the structure programatically. I’ve been writing (list (cons "a" t) (cons "b" nil)) sort of things, which I don’t love.

    You could render this as:

    `(("a" . t) ("b" . nil))

    Having said that, I’ve always found the back-tick hard to read, so I tend to disprefer it.

  • If you have a variadic macro (i.e. that takes &rest args), or even I suppose any kind of macro, and you have it’s arguments in a list, there’s no a way, outside of eval to call the macro, which is super annoying, and makes macros significantly less appealing as part of public APIs. My current conclusion is that macros are great when you want to add syntax to make the code you’re writing clearer or to introduce a new paradigm, but for things that could also be a function, or are thin wrappers on for function, just use a function.