Jumping to definitions with M-. is wonderful. The downside — everything has a downside — is that it is all too easy to accidentally edit a source file in the Quicklisp distribution. Here’s what I do. Emacs supports directory-local variables. If you create a file named .dir-locals.el in a directory, Emacs will apply the settings …

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Sometimes you want to write a macro that communicates information to macros inside of its compile-time “scope,” not unlike the way special variables can communicate information to functions inside of another function’s run-time scope. Pre-ANSI Common Lisp actually had a special form for this purpose, compiler-let, but it was removed in ANSI CL, and for …

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Sure, it’s nice to have exhaustiveness checking when dispatching on the type of a single object. But this is Lisp. What about multiple dispatch? Lisp already has a syntax for multiple dispatch: generic functions. Could you layer exhaustiveness checking on top of generic functions? Of course. But while many Lisp types have corresponding built-in classes, …

Continue reading “Exhaustiveness checking in Common Lisp, part 4: dispatch-case”

Programmers are not to be measured by their ingenuity and their logic but by the completeness of their case analysis. Alan Perlis Let’s start with a simplified definition for etypecase-of (assuming type= from Part I). This looks very similar to ecase-of: In fact ecase-of is, conceptually, a special case of etypecase-of. You could define ecase-of …

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The term exhaustiveness checking comes from functional languages, and programmers experienced with functional languages may feel confused — where are the algebraic data types? In, say, Ocaml, to get exhaustiveness checking, you first have to define an ADT: This superficially resembles the deftype form above, but what it does is very different. The Ocaml defines …

Continue reading “Exhaustiveness checking in Common Lisp, part 2: The Case of the Missing ADTs”

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Continue reading “Exhaustiveness checking in Common Lisp, part 1: ecase-of”

In Common Lisp it is idiomatic for functions to allow their behavior to be overridden with keyword arguments. So Lispers often write functions whose sole purpose is to call a function that takes keyword arguments, but with different defaults. Since keyword arguments are processed from left to right, and the leftmost occurrence takes precedence, this …

Continue reading “The multiple-value-call trick”

Suppose you have a parser that reads from a stream, but the stream is missing something. E.g. you have a function that reads a string from a stream, but while the function expects the next character to be a opening quote, the opening quote has already been read from the stream. The obvious solution is …

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Continue reading “The Iterate clause trick”