Spreadable Elisp

shadchen 1.0

pattern matching for elisp

M-x package-install shadchen
Vincent Toups

Shadchen: A pattern matching library ====================================

shadchen: Noun matchmaker from Yiddish

(note: there is an emacs lisp port of this library [here][shadchen-el]) (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 `_` to the `car` of a list, `x` to the `car` of that list's `cdr`, and `rest` to the `cdr` of that list.

If we don't pass in a list, the match fails. (Because of the behavior of CL's `car` and `cdr`, which return `NIL` on `NIL`, the form `cons` doesn't enforce a length requirement on the input list, and will return `NIL` for an empty list. This corresponds with the fact that in Common Lisp `(car nil)` is `nil` and `(cdr nil)` is `nil`.)

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.

Built-in Patterns -----------------

Shadchen supports the following built-in patterns.


Matches anything, binding <SYMBOL> 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.


Matches any `CONS` cell, or `NIL`, then matches `<PATTERN1>` and `<PATTERN2>`, executing the body in a context where their matches are bound. If the match value is NIL, then each `PATTERN` matches against NIL.

(LIST <P1> ... <PN>)

Matches a list of length N, then matches each pattern `<PN>` to the elements of that list.


Matches <P1> - <PN> to elements in at list, as in the `LIST` pattern. The final `<REST-PATTERN>` is matched against the rest of the list.


Only succeeds when `DATUM` is `EQUALP` to the match-value. Binds no values.

(AND <P1> .. <PN>)

Tests all `<PN>` against the same value, succeeding only when all patterns match, and binding all variables in all patterns.

(OR <P1> .. <PN>)

Tries each `<PN>` in turn, and succeeds if any `<PN>` succeeds. The body of the matched expression is then executed with that `<PN>'s` bindings. It is up to the user to ensure that the bindings are relevant to the body.


Succeeds when `(FUNCALL PREDICATE MATCH-VALUE)` is true and when `<PATTERN>` matches the value. Body has the bindings of `<PATTERN>`.


Applies `FUN` to the match value, then matches `<PATTERN>` against _the result_.


Matches as if by `BACKQUOTE`. If `EXPR` is an atom, then this is equivalent to `QUOTE`. If `EXPR` is a list, each element is matches as in `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 `10` and `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 not-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)))

Will produce `(1 (4))`. Note that a similar functionality can be provided with `funcall`.

(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. 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.

Match-let ---------

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 `(14 91)`.

If you like this feature, please let me know if you would like it to check that `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 non-tail position.

User feedback will vary how I approach this.

defun-match -----------

This special form allows the definition of functions using pattern matching where bodies can be specified over multiple `defun-match` invokations:

(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 documentation.

Extending shadchen ------------------

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 behavior as:

(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 `AND`, `FUNCALL`, and `?` allow the definition of arbitrary matchers without exposing the guts of the matching system.

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Copyright 2012, Vincent Toups This program is distributed under the terms of the GNU Lesser General Public License (see license.txt).