Tuesday, March 26, 2024

With- vs. call-with-

In Common Lisp, there are a lot of macros that begin with the word “with-”. These typically wrap a body of code, and establish a context around the execution of the code.

In Scheme, they instead have a lot of functions that begin with the words “call-with-”. They typically take a thunk or receiver as an argument, and establish a context around a call to the thunk or receiver.

Both of these forms accomplish the same sort of thing: running some user supplied code within a context. The Scheme way accomplishes this without a macro, but “call-with-” functions are rarely used as arguments to higher order functions. Writing one as a function is slightly easier than writing one as a macro because the compiler takes care of avoiding variable capture. Writing one as a macro leaves it up to the implementor to use appropriate gensyms. Writing one as a macro avoids a closure and a function call, but so does inlining the function. The macro form is slightly more concise because it doesn’t have a lambda at every call site. The function form will likely be easier to debug because it will probably involve a frame on the stack.

There’s no need to commit to either. Just write a “with-” macro that expands into a call to an inline “call-with-” function. This should equally please and irritate everyone.


A. V. said...

But what reason would anyone have for being irritated?

Ember said...

Some further benefits from this old blog post here: http://random-state.net/log/3390120648.html