how to avoid nesting in clojure - clojure

Question

when my write a function to check a user can delete a post by clojure,I get this
(defn delete!
{:arglists}
[^String id]
(if (valid-number? id)
(let [result {:code 200 :status "error" :messag "delete success"}]
(if-let [user (session/get :userid)]
(if-let [post (pdb/id id)]
(if (= user (post :user_id))
(do
(pdb/delete! (Long/valueOf id))
(assoc result :status "ok"))
(assoc result :message (emsg :not-own)))
(assoc result :message (emsg :post-id-error))))
(assoc result :message (emsg :not-login)))))
so i want to fix it,i get this
https://github.com/4clojure/4clojure/blob/develop/src/foreclojure/register.clj#L27
https://github.com/4clojure/4clojure/blob/develop/src/foreclojure/utils.clj#L32
but it is line,but not a nest.
the delete! function is nest ugly and it is very hard to understand it,how to write a macro to avoid the nesting a lot.or other way to avoid it.

Answer 1

This doesn't need a macro. I guess cond is a macro, but it is the only one we need to make this code readable.
(defn delete!
;; {:arglists} ; this line will not compile
[^String id]
(let [result {:code 200 :status "error" :message "delete success"}
user (session/get :userid)
post (and user (valid-number? id) (pbd/id id))]
(cond
(not user)
(assoc result :message (emsg :not-login))
(not post)
(assoc result :message (emsg :post-id-error))
(not= user (:user_id post))
(assoc result :message (emsg :not-own))
:else
(do
(pdb/delete! (Long/valueOf id))
(assoc result :status "ok")))))

Answer 2

This is something a lot of people run into, so don't feel bad.
Check out this blog by Christophe Grand, which I think is a pretty nice (and concise!) solution.
Edit: you only need something fancy like this (or alternatively the version using delay in this other post) if you need to short-circuit execution like the original - otherwise noisesmith's answer is the way to go.

Answer 3

Here's how you could do this sort of thing with the Either monad -- I'm sure there are libraries for it already but I'll implement it here for completeness. (Note: this code hasn't been validated.)
(defn success? [v]
(contains? v :success))
(defn inject [v]
{:success v})
(defn bind [v f]
(if (success? v)
(apply f (:success v))
v))
(defmacro >>= [v & body]
(let [binds (map #(list 'bind %) body)]
`(-> ~v ~#binds)))
(defn delete!
{:arglists}
[^String id]
(if (valid-number? id)
(let [result {:code 200 :status "error" :message "delete success"}
check
(>>= (inject {:id id})
#(if-let [user (session/get :userid)]
{:success (assoc % :user user)}
(:failure (assoc result :message (emsg :not-login))))
#(if-let [post (pdb/id (:id %))]
{:success (assoc % :post post)}
{:failure (assoc result :message (emsg :post-id-error))})
#(if (= (:user %) ((:post %) :user_id))
{:success %}
{:failure (assoc result :message (emsg :not-own))}))]
(if (success? check)
(do
(pdb/delete! (Long/valueOf id))
(assoc result :status "ok"))
(:failure check)))))
The >>= macro works like the -> macro (obviously, since it uses it), but if any of the functions return a {:failure ...} then the chain short-circuits (thanks to bind) and the failure value of the function that failed becomes the value returned by >>=.
Edit
I should note that the function I have named inject is actually called return, but I decided to name it inject here since that's more along the lines of what it does in this monad.