Hi MK-list, I'm trying all kinds of nasty-hackery stuff in my mk
implementation.
I want to share this one.
Maybe some one already came up with this.
Use case: Pattern matching/destructuring a cons.
In order to destructure a list I tend to create fresh vars for matching the
head and the tail of the list.
(defn some-goal [alist]
(fresh [h t]
(conso h t alist) ;; if alist is actually a pair/list then h and t are
introduce only for capture,
(conde [(some-goal-onhead h)]
[(some-goal-ontail t)]))
This may introduce h and t into the substitution. But it can be avoided
with using continuations-ish form.
(defn match-lcons [maybe-alist fun]
(fn [a]
(let [maybe-alist (walk maybe-alist a)]
(cond
(pair? maybe-alist) (fun (car maybe-alist) (cdr maybe-alist))
(lvar? maybe-alist) (fresh [ h t ]
(conso h t maybe-alist)
(fun h t))
:else u#)))
Now i can write the original example as:
(defn some-goal [alist]
(match-lcons alist
(fn [h t]
(conde [(some-goal-onhead h)]
[(some-goal-ontail t)])))
And it will introduce lvars only when they are actually needed.
Cheers.
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