🚡 Lifted

A small Clojure library for lifting functions into protocols. It allows an easy way to protocolize a (default) implementation, which can be useful for testing (mocking) purposes for example.

Usage

The library offers two macros and a function.

(require '[lifted.core :refer [lift-as lift-on lifted]])

The lift-as macro declares a protocol. Its sole argument is the name of the protocol to declare. The macro "lifts" the functions in the current namespace which names are prefixed with the - character into the protocol. The prefix is stripped from the protocol function names. Only those functions which take at least one argument are lifted.

For example, let's create a simple prefixed function:

(defn -incr
  "Incrementer function."
  ([this] (-incr this 1))
  ([this n] (+ this n)))

(lift-as Incrementer)

The lift-as macro above expands to:

(defprotocol Incrementer
  (incr [this] [this n]
    "Incrementer function."))

The lift-on macro creates a protocol implementation. Its two arguments are the protocol to implement and the object to use as this. The protocol simply forwards the calls to the prefixed functions, passing the this object as the first argument. For example:

(def i (lift-on Incrementer 5))

The lift-on macro above expands to:

(def i
  (let [G__3427 5]
    (reify Incrementer
      (incr [this] (-incr G__3427))
      (incr [this n] (-incr G__3427 n))

      lifted.core/Lifted
      (lifted [_] G__3427)))

Now you can use the protocol implementation as you would with any protocol.

(incr i)
;=> 6

(incr i 10)
;=> 15

To retrieve the object on which the protocol implementation was lifted on, you can use the lifted function. For example:

(lifted i)
;=> 5

Closing remarks

Varargs

Clojure protocols do not support varargs. Therefore the lifted function argument lists with a vararg are normally stripped from the protocol. If however one needs varargs, you can pass an option map to lift-as as follows:

(defn -vararg-test [y & ys]
  ...)

(lift-as VarargTest {:expand-varargs-for #{vararg-test}})

This will expand the vararg into concrete arguments, up to a maximum of 20 total arguments. For example, above lift-as macro would expand to:

(defprotocol VarargTest
  (vararg-test [y] [y ys_0] [y ys_0 ys_1] . . . [y ys_0 ys_1 .. ys_18]))

The rule still applies that for a function to be lifted it should at least take one argument.

Destructuring

The macros do not support destructuring syntax in the signatures of the to-be lifted functions. Please do the destructuring inside the function, using a let for example.

That's it. Enjoy! 🚀

License

Copyright © 2020 Functional Bytes

This program and the accompanying materials are made available under the terms of the Eclipse Public License 2.0 which is available at http://www.eclipse.org/legal/epl-2.0.

This Source Code may also be made available under the following Secondary Licenses when the conditions for such availability set forth in the Eclipse Public License, v. 2.0 are satisfied: GNU General Public License as published by the Free Software Foundation, either version 2 of the License, or (at your option) any later version, with the GNU Classpath Exception which is available at https://www.gnu.org/software/classpath/license.html.