CODEWITHSUNDEEP
macrosclojureoperators
prosseek
prosseek

Reputation: 191189

Clojure's -> and ->> macro

Clojure's ->> macro thread the form from the last argument, when -> form from the first.

user=> (->> a (+ 5) (let [a 5]))
10

However, I get an exception when I used the operations exchanged. 

user=> (-> a (let [a 5]) (+ 5))

CompilerException java.lang.IllegalArgumentException: let requires a vector for its binding in user:1, compiling:(NO_SOURCE_PATH:1:7)

Furthermore, I expect these two operations will get me the same results, which is not. 

user=> (-> 0 (Math/cos) (Math/sin))
0.8414709848078965
user=> (->> 0 (Math/sin) (Math/cos))
1.0

What's wrong? How's the -> and ->> macros work?

Upvotes: 0

Views: 126

Answers (1)

prosseek
prosseek

Reputation: 191189

The -> macro inserts the argument as the first argument for the given function, not giving the argument to the last function. Likewise ->> inserts as the last argument. 

user=> (macroexpand '(-> x (- 1)))
(- x 1)
user=> (macroexpand '(->> x (- 1)))
(- 1 x)

Two simple examples: 

user=> (-> 1 (- 1) (- 2)) 
-2
user=> (->> 1 (- 1) (- 2))
2

As for the first example, -2 == (- (- 1 1) 2), and for the second 2 == (- 2 (-1 1))

As a result, we get the same results for the unary functions. 

user=> (macroexpand '(-> 0 Math/sin Math/cos))
(. Math cos (clojure.core/-> 0 Math/sin))
user=> (macroexpand '(->> 0 Math/sin Math/cos))
(. Math cos (clojure.core/->> 0 Math/sin))

So, only ->> makes sense in the question. 

user=> (macroexpand '(->> a (+ 5) (let [a 5])))
(let* [a 5] (clojure.core/->> a (+ 5)))
user=> (macroexpand '(-> a (+ 5) (let [a 5])))

IllegalArgumentException let requires a vector for its binding in user:1  clojure.core/let (core.clj:4043)

Upvotes: 5

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