I'm finally experimenting with writing macros in clojure. Learning macros is (for me at least) a 4 stage process:

1. Learn to use them (pretty straightforward)
2. Learn to read their implementations (including the quoting)
3. Learning to write them (in progress)
4. Learning when to write them (in progress)

Those last two are iterative; #4 is especially tricky -- the web is full of general considerations ("when a function won't do", "when you want new syntax", "when you need to make decisions at compile time", etc) - but actually making that judgment in practice, takes... well practice.

Hence this exercise. Anyway to the code:

Clojure offers the if-let and when-let macros that allow you to combine a let block with testing the binding for nil:

(when-let [a (some-fn)]  
   (do-something-with a))

(if-let [a (some-fn)]  
   (do-something-with a)  
   (otherwise-fn)) 

I found myself (on some real code) wanting to be able to do something similar with try:

(try-let [a (some-potentially-exceptional-fn)]
  (do-something-with a))

(try-let [a (some-potentially-exceptional-fn)]
  (do-something-with a)
  ArithmeticException ((println (.getMessage e)) 42)
  :else (do-something-by-default-fn)
  :finally (println "always"))

etc.

So I wrote this (non-hygenic) macro that seems to do the job:

(defmacro try-let [let-expr good-expr & {fin-expr :finally else-expr :else :as handlers}]
  (letfn [(wrap [arg] (if (seq? arg) arg (list arg)))]
  `(try (let ~let-expr ~good-expr)
    ~@(map #(apply list 'catch (key %) 'e (wrap (val %))) (dissoc handlers :finally :else))
    ~(if else-expr `(catch Exception ~'e ~else-expr))
    (finally ~(if fin-expr `~fin-expr ())))))

Thing is... I don't if it's a good idea or not. For one thing its not hygienic (it implicitly declares e that can be used in the handler clauses) though this seems the kind of case that sort of thing is for.

For another... I don't know if its correct. It seems to be (I've tested all the scenarios I can think of), but this is kinda like security -- I suspect anyone can write a macro that they themselves can't break, but that doesn't mean its correct.

Some things to note: - e is available to handler expressions
- the local function wrap allows for a complex expression or single value to be spliced in
- any number of handlers can be included
- ':else' (default) handler and ':finally' handlers are optional (as are any others!)

In short: I'm interested in any opinions/feedback that aim at learning steps 3 & 4 (writing and when to write). Fire away!

When testing functions, one tests: evaluation result side-effects (if any)

When testing macros, one may also wish to test: expander code expansion

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El emblemático convento de la Inmaculada Concepción de Ciudad Real, conocido como el convento de las Terreras, se prepara para una transformación significativa que lo convertirá en un centro de interpretación dedicado a la gastronomía manchega. Esta ambiciosa iniciativa, que busca revitalizar el patrimonio cultural de la ciudad, cuenta con una inversión de 1,5 millones […]

Artículo publicado en : Transformación Gastronómica: Antiguo Convento de Las Terreras en Ciudad Real Recibe 1,5 Millones en Inversión

Autor: Redakcija 031 Naslov: Servisne informacije: [13. studenog 2024.]
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DHMZ prognoza: Pretežno oblačno. Jutarnja temperatura 2°C, a dnevna do 6°C.

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Osijek
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od 8:30 do 12:00 sati - Martina Divalta 56-58 par, 60/a, 64-68 par, 70/a, 72, 76-78 par, 78/a, Vatrogasna ulica 15
od 9:00 do 10:00 sati - Park kraljice Katarine Kosača 8, 9, Ulica Josipa Huttlera 6/b, 27/a
od 9:00 do 14:00 sati - Šokačka ulica 16-18 par, Žumberačka ulica 6-10 par, 14-22 par, 7-15 nep, 21-31 nep, 37-49 nep

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od 11:30 do 14:00 sati - Biokovska 2-42 par, Dinarska 1-35 nep, 39, Ulica Petrove gore 2-24 par, 1-23 nep, Ulica Požeške gore 11-19 nep

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od 8:30 do 9:30 sati - Dolica 4-6 par, 26, 30, 1-7 nep, 11-15 nep, 19, 23-25 nep, Dunavska obala 12/a, 16-20 par, 24, 28-30 par, 40, 11-15 nep, 19-21 nep, 27, 37-39 nep, Hrvatskih mučenika 12, 16, 18/a, 20, 30, 5-7 nep, 7/a, 9-11 nep, 19, 25, Ivana Kesmičkog 6-14par,3, Kalvarija 4, Lipin dol 2, 6-16 par, 20, 20/c, 22, 22/a, 24-26 par, 28/a, 30-32 par, 42-44 par, 76-78 par, 88, 146, 1-5 nep, 5/a, 7-9 nep, 13, 17, 37, 41-43 nep, 49/b, Matije Gupca 2-18 par, 18/b, 22-24 par, 28, 52, 1-25 nep, 25/a, 27, Mladena Palinkaša 2-6 par, 6/a, 8-16 par, 20-30 par, 34-38 par, 48, 92, 94/e, 100-102 par, 1, 1/a, 3-27 nep, 31, 35-39 nep, 47, 87, Podunavlje 8, 8/a, 10, 12/a, 14, 20, 20/c, 20/g, 22-26 par, 32, 46, 52, 1/a, 3-5 nep, 5/a, 7, 7/a, 9-13 nep, 17, 31/a, 33-35 nep, Puarov dol 20/a, Rudina Balinac 20, Rudina crkvina 2, 6-14 par, 22-30 par, 36-50 par, 58-64 par, 68-74 par, 120-124 par, 128, 132, 208, 400, 3-9 nep, 13-15 nep, 23, 27, 31, 39-51 nep, 67-69 nep, 75, 81, 101, 117-121 nep, 125-129 nep, 133, Rudina Dolečak1, Rudina kod Vilme 40, 83, Rudina provalija 2, 14, Rudina selišće 16-22 par, 3-9 nep, Rudina stepanićevac 26, 99, 107-109 nep, Slavka Frangeša 9, 15, 21, Trg braće Radića 6-30 par, 34, 44, 1-7 nep, 15, 19, 23-27 nep, 27/a, 33-35 nep, 39-43 nep,47-49nep, 51/a, ušće 2-22 par, 22/a, 22/c, 24-28 par, 32-38 par, 42, 42/b, 44-56 par, 60-66 par, 72-76 par, 76/a, 78, 78/b, 80-82 par, 82/a, 84, 88-96 par, 100-108 par, 112-118 par, 126, 1, 1/a, 3-21 nep, 21/a, 21/b, 21/e, 21/f, 21/g, 23-29 nep, 37, 41,41/a,43-49 nep, 53, 53/a, 61, 63/a, 67-69 nep, 73, 77, 79/c, 83, 87, 87/a, 89, 89/h, 91-93 nep, 101-109 nep, 113-117 nep, 117/a, 121-125 nep, Vrška 28-30 par, 52/a, 58, 86, 1-3 nep, 7-9 nep, 13, 19-21 nep, 25-27 nep, 37, 41-45 nep, 49, 55, zlatna 2, 2/a,4-16par, 20-24 par, 32, 44-52 par, 58-64 par, 66/a, 68-70 par, 74, 78, 104, 104/a, 106-110 par, 118/a, 122, 142, 142/a, 144, 1-3 nep, 9/a, 11-15 nep, 19-31 nep, 31/a, 33-45 nep, 61-63 nep, 63/a, 65-71 nep, 75, 75/a, 79/a, 79/b, 81, 81/a, 81/c, 95,

Napomena: U slučaju lošeg vremena radovi se odgađaju.

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Bez plina
Višnjevac
od 09:00 do 12:00 sati - Ulica Petra Svačića (kbr. 37A-43), Ulica Vatroslava Lisinskog

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Tekst i foto: Osijek031.com

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How to do the incline chest press on Iso HammerStrength machine with video.

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Apple has made a much larger comeback in Yale student and faculty computing than its 10 to 12 percent national market share would indicate.