Common Lisp is case sensitive

One Common Lisp feature that needs more publicity is case sensitivity. A common misconception is that Common Lisp is case insensitive, when in fact symbols in Common Lisp are case sensitive.

By default, the Common Lisp reader is case-converting: all unescaped characters in a symbol name get upper-cased. This gives the practical effect of making it seem as though symbol case doesn't matter. This is desirable behavior for interfacing with other case-insensitive languages (such as Fortran; from what I understand the main motivation for the default Common Lisp behavior), but a pain to interface with case-sensitive ones (such as C).

The behavior of the reader can be customized via readtable-case.

The one that might seem to be most useful for having case-sensitive symbols at first glance is :preserve, however remember that all code read in with the default setting (:upcase) is in upper-case, as are all the standard Common Lisp symbols (this is defined by the standard), so this means you will need to spell out all CL and external symbols IN ALL UPPERCASE. To make this less annoying, the :invert readtable-case is the most practical - all-lowercase symbol names become uppercase, all-uppercase become lowercase, and mixed-case stays mixed-case (the important part for case sensitivity). The Lisp printer outputs symbol names correctly this way by default. The only problem now becomes inconsistent spelling of a symbol in all lowercase or all uppercase in old code that expects case conversion. But otherwise you can get case sensitivity for your software by setting readtable-case to :invert today.

An easy way to manage the readtable-case is by using the named-readtables library. I've recommended named-readtables before; besides case sensitivity, it helps manage reader macros.

[This blog post is adapted from the case sensitivity CLiki FAQ entry I wrote. Feel free to make corrections and other suggestions on the CLiki page.]