http://richhickey.github.com/clojure-contrib/types-api.html#types/match

This is a pattern-matching macro I wrote for working with algebraic data types, but it also knows about lists, vectors, and maps. The main feature that it lacks compared to your implementation is :when clauses.

The interesting question here is how often do things end up looking like conds with simple actions and how often do they look like funs with multiple arities. I was thinking the former (partially cause of the examples and some half-formed haskell in my head), but after musing on it, I’m not as sure.

@fogus: Thanks very much — enjoy. :-)

-m

On 1, I think that my suggestion has the advantage of consistency with other Clojure forms. It seems that everywhere in Clojure that Rich had the choice he followed the “single-layer model” rather than the “double-layer model” (which is usually the choice in CL). condp is another example here.

It would be nice if the match form (and the defnp form) felt as clojure-y as possible. The loss of the implicit do would be a small price to pay for that.

Tom

1. Yah, I went back and forth on this too. I eventually went with (pattern action) because a) it matches variable-arity function definitions and b) it allows for an implicit do-body for the actions (which I just realized doesn’t work right now, but I’ll fix that tonight). That said, I’m agnostic on the issue, and could be convinced one way or the other — would reducing the parens in the default case make up for having to have an explicit (do …) for the more complicated actions?

2. Oh, that’s a great idea — I’ll take a look at that.

Too small ideas/suggestions:

1) When writing Clojure, Rich wanted to reduce the number of parens (relative to Common Lisp), so

(let ((var1 val1)
(var2 val2))
…)

became

(let [var1 val1
var2 val2]
…)

and

(cond
((pred1 action1)
(pred2 action2)))

became

(cond
(pred1 action1
pred2 action2))

in a similar vein, it would seem to make sense for

(match var
(pat1 action1)
(pat2 action2))

to be simplified to

(match var
pat1 action1
pat2 action2)

though you’ll need to figure out exactly how best to express :when syntax.

2) This just screams out for a defnp like clojure.contrib.defnk:

(defnp signum
0 0
n :when (< n 0) -1
_ 1)