condp

macro

since v0.0-927

clojure.core/condp

Edit

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Details:

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See Also:

• cond
• if

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Source docstring:

Takes a binary predicate, an expression, and a set of clauses.
Each clause can take the form of either:

test-expr result-expr

test-expr :>> result-fn

Note :>> is an ordinary keyword.

For each clause, (pred test-expr expr) is evaluated. If it returns
logical true, the clause is a match. If a binary clause matches, the
result-expr is returned, if a ternary clause matches, its result-fn,
which must be a unary function, is called with the result of the
predicate as its argument, the result of that call being the return
value of condp. A single default expression can follow the clauses,
and its value will be returned if no clause matches. If no default
expression is provided and no clause matches, an Error is thrown.

(core/defmacro condp
  {:added "1.0"}

  [pred expr & clauses]
  (core/let [gpred (gensym "pred__")
             gexpr (gensym "expr__")
             emit (core/fn emit [pred expr args]
                    (core/let [[[a b c :as clause] more]
                               (split-at (if (= :>> (second args)) 3 2) args)
                               n (count clause)]
                      (core/cond
                        (= 0 n) `(throw (js/Error. (cljs.core/str "No matching clause: " ~expr)))
                        (= 1 n) a
                        (= 2 n) `(if (~pred ~a ~expr)
                                   ~b
                                   ~(emit pred expr more))
                        :else `(if-let [p# (~pred ~a ~expr)]
                                 (~c p#)
                                 ~(emit pred expr more)))))
             gres (gensym "res__")]
    `(let [~gpred ~pred
           ~gexpr ~expr]
       ~(emit gpred gexpr clauses))))

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