defnw

A Clojure library that defines defnw, a macro that replaces defn so as to handle special cases in the pre-post map.

Rationale

A common pattern is to test arguments for certain conditions that preclude the execution of the function's main purpose.

    (defn do-that 
      [ arg-1 arg-2 ]
        (cond (that-is-inapropriate? arg-1)
              special-case-return-value
              (that-is-impossible-with? arg-2)
              special-case-return-value-2
              :else 
              (actually-do-that arg-1 arg-2))

defnw increases readability by limiting the body of the function to its main purpose (the :else clause), moving argument testing to the pre-post map.

This is partiallly similar to pre. But, unlike conditions in the :pre vector of the pre-post map, conditions in the :cond vector do not trigger assert failures. Each condition must be followed by an expression to be evaluated and returned when the condition is met.

Local bindings can be set for the whole function including the :cond vector in :let vector.

Usage

Dependencies

Leiningen

[gabrielash.defnw "0.1.0"]

Clojure CLI/deps.edn

gabrielash/defnw {:mvn/version "0.1.0"}

Example

  (ns my-namespace 
    (:require [gabrielash.defnw.core :refer [defnw]]))

  (defnw add-display-name
    [{:keys [ first-name last-name name-withwheld? ] :as person}]

      {:pre [(string? last-name)
              (seq last-name)]
       :cond [name-withheld? person]}

      (assoc person
             :display-name
             (str first-name " " last-name)))

Caveats

• body is wrapped in do
• if you're redefining cond, defn, or do, well ...
• :cond/:let with a nil value are ignored
• This is a learning exercise; use at your own risk

License

Copyright © 2020 Gabriel Ash

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.

This code is provided as is, without any guarantee whatsoever.