core.match with interaction between fields

[Emlyn Corrin]

Hi,

I have a series of map-like objects that I need to translate into keywords, and I've found core.match works really well for most cases, but there are a few cases where I need to match based on properties of combinations of fields, for example something like:

(match obj

  {:a 0, :b 0, :c _} :foo

  {:a 0, :b _, :c _} :bar

  {:a a, :b b, :c (+ a b)} :baz)

Except that's not valid, and results in the error: AssertionError: Invalid list syntax a in (+ a b).

I tried using a guard instead, like:

(match obj

  {:a 0, :b 0, :c _} :foo

  {:a 0, :b _, :c _} :bar

  {:a a, :b b, :c (_ :guard #(= % (+ a b)))} :baz)

But the bound symbols a and b are not available in the guard function: RuntimeException: Unable to resolve symbol: a in this context

Is there a way to do this with core.match? Or is there a better way?

Thanks in advance for any help.

Emlyn

[Adam Clements]

Hey Emlyn!

For your specific example, couldn't you simply:

(match obj

  {:a 0, :b 0, :c _} :foo

  {:a 0, :b _, :c _} :bar

  {:a a, :b b, :c (+ (:a obj) (:b obj))} :baz)

I have hit this before myself and IIRC when I asked then, this is expressly not supported by core.match because of the particular way the macro expands for good performance. There are other pattern matching libraries which I think *do* support this though might not have the same performance profile as core.match. matchure is the one that springs to mind, though I think there were others.

Adam

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[Emlyn Corrin]

Hey Adam!

Thanks, that almost works - I can't put the + form in the pattern, as parentheses have special meaning in there, but I can just let it outside, like:

(let [aplusb (+ (:a obj) (:b obj))]

  (match obj

    {:a 0, :b 0, :c _} :foo

    {:a 0, :b _, :c _} :bar

    {:a a, :b b, :c aplusb} :baz))

I also have some cases with inequalities, for which I think I need to use guards, something like:

(match obj

  {:a 0, :b (_ :guard #(< % min_b))} :foo

  {:a 0, :b (_ :guard #(<= min_b % max_b)} :bar

  {:a 0, :b _} :baz

  ...)

Not the prettiest, but overall still much nicer (and hopefully efficient) than a huge cond. I might have a look into other pattern matching libraries, matchure looks like it might be a bit clearer for the :guard cases.

Cheers,

Emlyn