Shadchen: A pattern matching library
shadchen: Noun matchmaker from Yiddish
(note: there is an emacs lisp port of this library here) (note: if you are reading this README for the emacs version of the library, keep in mind that emacs symbols are case sensitive. Symbols are all lowercase in this library.)
I love pattern-matching, which I find to be a great way to combine destructuring data with type-checking when used in dynamic languages. If you aren't familiar with how pattern matching works, here is an example:
(defun second (lst) (match lst ((cons _ (cons x rest)) x)))
MATCH introduces a pattern matching expression, which takes a value,
in this case
LST and a series of lists, whose first elements are
descriptions of a data structure and whose subsequent elements are
code to execute if the match succeeds. Pattern matching takes the
description of the data and binds the variables that appear therein to
the parts of the data structure they indicate. Above, we match
car of a list,
x to the
car of that list's
cdr of that list.
If we don't pass in a list, the match fails. (Because of the behavior
cdr, which return
NIL, the form
doesn't enforce a length requirement on the input list, and will
NIL for an empty list. This corresponds with the fact that
in Common Lisp
(car nil) is
(cdr nil) is
We might instead write:
(defun second-of-two (lst) (match lst ((list _ x) x)))
Which returns the second element of a list only when a two element
list is passed in.
MATCH can take multiple pattern/body sets, in
which case patterns are tried in order until one pattern matches, and
the result of evaluating the associated forms is returned. If no
patterns match, an error is raised.
Shadchen supports the following built-in patterns.
Matches anything, binding to that value in the body expressions.
Matches only when the value is the same keyword.
Matches only when the value is the same number.
Matches only when the value is
string= is the same string.
(CONS <PATTERN1> <PATTERN2>)
CONS cell, or
NIL, then matches
<PATTERN2>, executing the body in a context where their matches are
bound. If the match value is NIL, then each
PATTERN matches against
(LIST <P1> ... <PN>)
One may also write
(LIST <P1> ... <PN> (TAIL <PT>))
Where the pattern is matched against the tail of the list. This is an alternative to LIST-REST.
Matches a list of length N, then matches each pattern
<PN> to the
elements of that list.
(LIST-REST <P1> ... <PN> <REST-PATTERN)
Matches - to elements in at list, as in the
<REST-PATTERN> is matched against the rest of the list.
Only succeeds when
EQUALP to the match-value. Binds no
(AND <P1> .. <PN>)
<PN> against the same value, succeeding only when all
patterns match, and binding all variables in all patterns.
(OR <P1> .. <PN>)
<PN> in turn, and succeeds if any
<PN> succeeds. The
body of the matched expression is then executed with that
bindings. Each sub-pattern in an OR pattern must bind an identical
set of identifiers or an error is raised.
(? PREDICATE <PATTERN>)
(FUNCALL PREDICATE MATCH-VALUE) is true and when
<PATTERN> matches the value. Body has the bindings of
(FUNCALL FUN <PATTERN>)
FUN to the match value, then matches
<PATTERN> against the
(MAYBE-FUNCALL FUN PATTERN)
FUNCALL but if the application of
FUN to the object being
matched against is
*match-fail* than the match fails, other wise the
match succeeds only when
PATTERN matches the result.
Matches as if by
EXPR is an atom, then this is
EXPR is a list, each element is matches
QUOTE, unless it is an
(UQ <PATTERN>) form, in which case it
is matched as a pattern. Eg:
(match (list 1 2 3) ((BQ (1 (UQ x) 2)) x))
Will succeed, binding
X to 2.
(match (list 10 2 20) ((BQ (1 (UQ x) 2)) x))
Will fail, since
1 don't match.
(values <P1> ... <PN>)
Will match multiple values produced by a
(values ...) form.
(let (n1 v1) (n2 v2) ... (nn vn))
Not a pattern matching pattern, per se.
let always succeeds and
produces a context where the bindings are active. This can be used to
provide default alternatives, as in:
(defun non-nil (x) x) (match (list 1) ((cons hd (or (? #'non-nil tl) (let (tl '(2 3))))) (list hd tl)))
Will result in
(1 (2 3)) but
(match (list 1 4) ((cons hd (or (? #'non-nil tl) (let (tl '(2 3))))) (list hd tl)))
(1 (4)). Note that a similar functionality can be
(concat P1 ... PN)
Concat is a powerful string matching pattern. If each pattern is a string, its behavior is simple: it simply matches the string that is the concatenation of the pattern strings.
If any of the patterns are a more complex pattern, then, starting from the left-most pattern, the shortest substring matching the first pattern is matched, ad then matching proceeds on the subsequent patterns and the unmatched part of the string. If this fails, a longer initial match is searched for. Eg:
(match "bobcatdog" ((concat (and (or "bobcat" "cat") which) "dog") which))
will produce "bobcat", but the pattern will also match "catdog", returning "cat".
This is a handy pattern for simple parsers.
(append P1 ... PN)
concat except for lists rather than strings:
(match (number-sequence 1 10) ((append (list 1) _ (list y)) y)) => 10
the interveening numbers are matched away.
This pattern is a bit unusual in that if the value fails to match
pattern, then the match will raise an error indicating this fact.
This is useful if you have compound patterns which are strict in some
parts and for which you wish to raise an error, for instance,
(match `(if (< x 10) true-case false-case nonsense) ((list 'if (tail (must-match (list a b c)))) <compile if>))
In the above, we assert that if the head of a list is the symbol
if, then the tail must consist of three elements and we want to know
about it immediately if a list with head
if shows up with a
(must-match pattern fail-binding-pattern expression)
This is like the pattern above, except in the case that
fails to match,
fail-binding-pattern is matched, and then
is evaluated and passed to the
error raising mechanism, coerced to a
string if need be. We migth write the above as
(match `(if (< x 10) true-case false-case nonsense) ((list 'if (tail (must-match (list a b c) actual-tail (format "if should have three arguments, but got %s" actual-tail)))) <compile if>))
A full shadchen pattern can be used in the fail-binding-pattern, but usually you are going to provide just a symbol - after all, if the pattern failed, you don't know much about the value.
Match let is a form which behaves identically to a let expression
with two extra features: first, the each variable can be an arbitrary
shadchen pattern and secondly, one can invoke
recur in any tail
position of the body to induce a trampolined re-entry into the let
expression, so that self-recursive loops can be implemented without
blowing the stack.
(match-let (((list x y) (list 0 0))) (if (< (+ x y) 100) (recur (list (+ x 1) (+ y x))) (list x y)))
Will result in
If you like this feature, please let me know if you would like it to
recur is in tail position. This is an expensive step
which requires walking the body after macro-expansion, which may also
introduce subtle bugs. The upside of doing this is that you avoid the
possibly strange bugs encountered when
recur is invoked in a
User feedback will vary how I approach this.
This special form allows the definition of functions using pattern
matching where bodies can be specified over multiple
(defun-match- product (nil) "The empty product." 1) (defun-match product (nil acc) "Recursion termination." acc) (defun-match product ((cons (p #'numberp n) (p #'listp rest)) (p #'numberp acc)) "Main body of the product function." (recur rest (* n acc))) (defun-match product (lst) "Calculate the product of the numbers in LST." (recur lst 1))
Note that different bodies can
recur to eachother without growing
the stack. Documentation for each body is accumulated, along with the
pattern associated with the body, into the function's complete
The argument list of a
defun-match form is syntactically equivalent
to the body of a
list pattern, so you can use
(tail pattern) to
match against the tail of the arguments passed in.
Users can define their own patterns using the
defpattern form. For
instance, the behavior of
CONS, which matches the empty list, may
not be desired. We can define a match which doesn't have this
(defun non-nil (x) x) (defpattern cons* (car cdr) `(? #'non-nil (cons ,car ,cdr)))
A pattern is a function which takes the arguments passed into the custom pattern, and expands them into a new pattern in the language of the built-in pattern-matching.
We can now say:
(match (cons 10 11) ((cons* a b) a))
Which will produce 10, but:
(match nil ((cons* a b) a))
Will raise a no-match error.
Judicious application of the matchers
the definition of arbitrary matchers without exposing the guts of the
Copyright 2012, Vincent Toups This program is distributed under the terms of the GNU Lesser General Public License (see license.txt).