Utilities beyond Alexandria.
Table of Contents
- A note about strings
- Dividing sequences
- Binding values in the function namespace
- Internal definitions
- Compile-time exhaustiveness checking
- Function reference
Serapeum is a conservative library of Common Lisp utilities. It is a supplement, not a competitor, to Alexandria. That means it is safe to do:
(defpackage ... (:use #:cl #:alexandria #:serapeum),
without package conflicts.
There may already be too many utility libraries for Common Lisp. Releasing another has become something to apologize for, not celebrate. But I would rather make my apologies than have to maintain copy-pasted versions of the same utilities across a dozen systems. And, though Serapeum is justified even if only I ever use it, the best way to ensure its quality is to write it as if for general use.
Serapeum is conservative: its goal is to fill in gaps in Common Lisp, not to redesign it. But it is less conservative than Alexandria. Alexandria limits itself to utilities with a Common Lisp pedigree. Serapeum casts a wider net: other dialects of Lisp, and other languages in the functional and array families, have been drafted.
Alexandria is self-contained. It exists in splendid isolation, without depending on, or even acknowledging, other libraries. Serapeum tries to be a good citizen of the Quicklisp era: whenever possible, it avoids duplicating functionality that can be had elsewhere.
Many of the utilities in Serapeum are original ideas; many ideas are borrowed from other languages, or from other Lispers. I try to give credit in the docstrings, but sometimes I have forgotten where I got an idea or a name. I regard missing credits as bugs: please report them.
Serapeum is intended to be portable, but it is principally tested where it is developed, on SBCL and Clozure CL. Patches and bug reports for other Lisps are always welcome, however, including implementation-specific optimizations.
One goal of Serapeum is to have excellent documentation. A utility library is a fork of its language; it deserves documentation of the same quality as a language reference. If a utility is not worth documenting, it is not worth having.
The full function reference will be found here. (It is in a separate file in deference to documentation browsers, which often print the README as a preamble to their own function reference).
Most utilities in Serapeum stand alone, but there are some families that deserve separate introduction.
- Dividing sequences
- Binding values in the function namespace
- Internal definitions and block compilation
- Compile-time exhaustiveness checking
A note about strings
Beginning Lispers often ask about Lisp: “Where are the string utilities?” Since strings in Lisp are a kind of vector, which is in turn a kind of sequence, the right place to look is under the documentation for sequences (and vectors).
All recent functional programming languages share a family of useful sequence-related functions with terrible names. All of them are called something like “split”, “divide”, or “group”, more or less at random.
For each function, we ensure:
- It is efficient.
- It returns like sequences for like (lists for lists, strings for strings, &c.).
- It accommodates generic sequences (
vectorare not necessarily an exhaustive partition of
- It has a distinctive name which does not use any of the weasel words “split,” “divide,” or “group.”
The function that returns runs of like elements in a sequence is
(runs '(head tail head head tail)) => '((head) (tail) (head head) (tail))
The function that returns a sequence in batches of a certain maximum size is
(batches (iota 11) 2) => ((0 1) (2 3) (4 5) (6 7) (8 9) (10))
The function which groups the like elements of a sequence is called
assort (because it returns a sequence assorted by some property).
(assort (iota 10) :key (lambda (n) (mod n 3))) => '((0 3 6 9) (1 4 7) (2 5 8))
The function that takes a predicate and a sequence, and returns two
sequences – one sequence of the elements for which the function
returns true, and one sequence of the elements for which it returns
false – is (still) called
(partition #'oddp (iota 10)) => (1 3 5 7 9), (0 2 4 6 8)
The generalized version of
partition, which takes a number of
functions and returns the items that satisfy each condition, is called
(partitions (list #'primep #'evenp) (iota 10)) => ((2 3 5 7) (0 4 6 8)), (1 9)
Items that do not belong in any partition are returned as a second value.
Serapeum simply re-exports
split-sequence, which seems to be firmly
rooted under its present name.
Binding values in the function namespace
fbindrec* bind values in the
fbindrec are like
(fbind ((fn (lambda ....))) ...) ≡ (flet ((fn ...)) ...) (fbindrec ((fn (lambda ...))) ...) ≡ (labels ((fn ...)) ...)
fbindrec* have no exact parallels: they bind functions
in sequence, so that each can be used in the construction (not just
the definition, as with
fbindrec) of the next.
(fbind* ((flip2 (lambda (fn) (lambda (x y) (funcall fn y x)))) (xcons (flip2 #'cons))) (xcons 2 1)) => (1 . 2)
These are non-trivial implementations. In many cases,
produce code that is more efficient than using
funcall, and even
eliminate the overhead of higher-order functions like
fbindrec, which builds on
fbind, further implements
the optimizing transformation from Waddell et. al., Fixing Letrec. (Note that the macroexpansion of
fbind may sometimes appear simplistic, using
macroexpand; this is the happy case when we can prove that the function is never used as a value.)
For binding values in the function namespace at the top level,
(defalias xcons (flip #'cons))
This is equivalent to
(setf (fdefinition ...)), but also gives the
function a compile-time definition so compilers don’t complain about
its being undefined.
local form lets you use top-level definition forms to create local
bindings. You can use
defun instead of
defmacro instead of
def (which is Serapeum’s macro for top-level lexical
bindings) instead of
let, and so forth.
This has three advantages:
Given a set of variable, function, and macro bindings, you can leave it to the compiler to figure out how to nest them. (This could be because you are porting a function from a language that uses flat bindings, or just because you are writing a very complicated function.)
You can use macro-defining macros (macros that expand into
defmacro), as well as macros that expand into
defunforms, to create local bindings.
You can (using
block-compile) easily switch to block compilation of top-level functions.
Serapeum’s implementation of internal definitions is as complete as it can be while remaining portable. That means full support for variables, functions, and symbol macros, but restricted support for macros.
Example: macros that work locally and globally
For example, memoizing local functions is usually clumsy; given
you can define a single
defmemo form that supports both
(defmacro defmemo (name params &body body) (with-gensyms (memo-table args result result?) `(let ((,memo-table (make-hash-table :test 'equal))) (defun ,name (&rest ,args) (multiple-value-bind (,result ,result?) (gethash ,args ,memo-table) (if ,result? ,result (setf (gethash ,args ,memo-table) (apply (lambda ,params ,@body) ,args))))))))
At the top level, this expands into an example of “let over defun” (gensyms elided for readability):
;; This source form (defmemo fibonacci (n) (if (<= n 1) 1 (+ (fibonacci (- n 1)) (fibonacci (- n 2))))) ;; Expands into... (let ((memo-table (make-hash-table :test 'equal))) (defun fibonacci (&rest args) (multiple-value-bind (result result?) (gethash args memo-table) (if result? result (setf (gethash args memo-table) (apply (lambda (n) (if (<= n 1) 1 (+ (fibonacci (- n 1)) (fibonacci (- n 2))))) args))))))
But within a
local form, it expands differently. This nearly
identical source form:
(local (defmemo fibonacci (n) (if (<= n 1) 1 (+ (fibonacci (- n 1)) (fibonacci (- n 2))))) (fibonacci 100))
Expands into this very different code (simplified for readability):
(let (fn) (labels ((fibonacci (&rest args) (apply fn args))) (let ((memo-table (make-hash-table :test 'equal))) (setf fn (named-lambda fibonacci (&rest args) (multiple-value-bind (result result?) (gethash args memo-table) (if result? result (setf (gethash args memo-table) (apply (lambda (n) (if (<= n 1) 1 (+ (fibonacci (- n 1)) (fibonacci (- n 2))))) args)))))) (fibonacci 100))))
Example: block compiling
local* is almost the same as
local, except that it
leaves the last form in the body intact. This is useful for obtaining
block compilation in Lisps that don’t have a syntax for it.
During development, you define functions at the top level inside a
(progn (defun aux-fn-1 ...) (defun aux-fn-2 ...) (defun entry-point ...))
Then, when you decide you want block compilation, simply switch the
progn to a
(local* (defun aux-fn-1 ...) (defun aux-fn-2 ...) (defun entry-point ...))
Which expands into something like:
(labels ((aux-fn-2 ...) (aux-fn-1 ...)) (defun entry-point ...))
This has the slight disadvantage that calls to the entry points,
including self calls, will still be compiled as global calls. If you
want calls to the entry points to be compiled as local calls, you can
block-compile macro instead.
block-compile, you can write:
(block-compile (:entry-points (entry-point)) (defun aux-fn-1 ...) (defun aux-fn-2 ...) (defun entry-point ...))
And have it expand into something like:
(labels ((aux-fn-2 ...) (aux-fn-1 ...) (entry-point ...)) (defalias entry-point #'entry-point))
Compile-time exhaustiveness checking
etypecase-of is just like
etypecase, except that it takes an
additional argument – the type to be matched against – and warns, at
compile time, if the clauses in its body are not an exhaustive
partition of that type.
ecase-of is a succint variant of
etypecase with the same syntax as
respectively, except that they expect, and enforce, the presence of an
There are also continuable versions of these macros –
We may call a type defined using
member an enumeration. Take an
enumeration like this:
(deftype switch-state () '(member :on :off :stuck :broken))
Now we can use
ecase-of to take all the states of the switch into
(defun flick (switch) (ecase-of switch-state (state switch) (:on (switch-off switch)) (:off (switch-on switch)))) => Warning (defun flick (switch) (ecase-of switch-state (state switch) (:on (switch-off switch)) (:off (switch-on switch)) ((:stuck :broken) (error "Sorry, can't flick ~a" switch)))) => No warning
Even more usefully, we don’t have to worry about bugs caused by misspellings:
(defun flick (switch) (ecase-of switch-state (state switch) (:on (switch-off switch)) (:offf (switch-on switch)) ;Gotcha! ((:stuck :broken) (error "Sorry, can't flick ~a" switch)))) => Warning
Example: union types
(defun negative-integer? (n) (etypecase-of t n ((not integer) nil) ((integer * -1) t) ((integer 1 *) nil))) => Warning (defun negative-integer? (n) (etypecase-of t n ((not integer) nil) ((integer * -1) t) ((integer 1 *) nil) ((integer 0) nil))) => No warning
Serapeum includes some utilities for CLOS. These utilities do nothing earthshaking, but since the function reference does not include them, they should be documented somewhere.
Method combination: standard with context
Serapeum exports a method combination,
You may recognize it as the
wrapping-standard method combination
due to Tim Bradshaw.
Generic functions defined with
standard/context behave the same as
ordinary generic functions, except that they allow an extra
:context. This extra qualifier works almost like
:around, except instead of being run in most-specific-first order,
like methods defined with
:around, methods defined with
are run in most-specific-last order. Furthermore,
take priority over any other methods, including
The big idea is that a class can use
:context methods to make sure
that any methods defined by subclasses – even
:around methods – run
in a certain dynamic context.
In most cases, when I write a metaclass, I want all of the classes defined using that metaclass to inherit from a specific class. Injecting a topmost class is not difficult to do, but it involves a certain amount of boilerplate.
To eliminate that boilerplate, Serapeum exports a metaclass,
topmost-object-class, to use as a base class for your metaclasses.
When you define a metaclass, all you have to do to ensure that classes
defined using your metaclass inherit from a specific class is to
supply the name of the class to inherit from in the definition of the
metaclass. This is much better demonstrated than explained:
;;; The class to inherit from. (defclass my-topmost-object () ()) ;;; The metaclass. (defclass my-metaclass (serapeum:topmost-object-class) () (:default-initargs :topmost-class 'my-topmost-object)) (defclass my-class () () (:metaclass my-metaclass)) (typep (make-instance 'my-class) 'my-topmost-object) => t
Note that, since the topmost object is usually a standard class, there
validate-superclass method which allows an instance of
topmost-object-class to inherit from a standard class.
The contrib/ directory holds large contributions to Serapeum (ones that add a lot of new exports).
:serapeum/contrib/hooks holds an enhanced implementation of hooks (extension points) contributed by the maintainers of the Nyxt web browser. This is fully compatible with the existing Serapeum functions for hooks (
The complete reference is in a separate file.
(Note that the reference is generated from docstrings, and should not be edited by hand.)