I had a pattern that kept popping up in code of:
(let [x (foo)
y (bar)]
(when y
(let [ ....]
....)))
that check jarred me, so I put this together:
https://gist.github.com/1347312. On reflection, discomfort with indentation
levels probably isn't near the top of the "to macro or not to macro?"
checklist.
(defmacro let?
[bindings & body]
(let [[bind [kwd pred & more]] (split-with (complement #{:ensure})
bindings)]
`(let [~@bind]
~@(cond (and kwd more) [`(when ~pred (check-let [~@more] ~@body))]
kwd [`(when ~pred ~@body)]
:else body))))
(let? [x 100
y 300
:ensure (pos? y)
z (- y 250)]
z)
;; expands to
(let [x 100
y 300]
(when (pos? y)
(let [z (- y 250)]
z))) ;; => 50
;; and returns 50. The following returns nil:
(let? [x 100
y 300
:ensure (neg? y)
z (- y 250)]
z) ;; => nil
On Tue, Dec 6, 2011 at 11:51 AM, Evan Gamble <[email protected]> wrote:
> I noticed in my code that I often nest a let inside an if-let, or vice-
> versa, so I wrote a macro let-else that expands into nested lets,
> except where there's an :else <expr> after a binding, in which case
> that binding expands into an if-let.
>
> E.g.
>
> (let-else
> [foo (f1) :else (e)
> bar (f2)]
> (b1)
> (b2))
>
> expands into
>
> (if-let [foo (f1)]
> (let [bar (f2)]
> (b1)
> (b2))
> (e))
>
> The jar is at https://clojars.org/org.clojars.egamble/let-else
> The code is at https://github.com/egamble/let-else
>
> - Evan
>
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