utils.modular

Utilities for creating modular systems: functions related to Component, or protocols.

Installation

[com.nedap.staffing-solutions/utils.modular "2.2.0-alpha3"]

Synopsis

nedap.utils.modular.api/implement

implement is a safer layer over raw metadata-based protocol extension.

Metadata-based protocol extension has recently proven to be a reliable solution against this problem.

In plain Clojure you might be tempted to do:

(defn start [this] ...)

(defn stop [this] ...)

(def my-component
  "A com.stuartsierra/component implementing some functionality"
  ^{`component/start start
    `component/stop stop}
  {})

However, several things might go wrong:

• What if the protocol lacks a :extend-via-metadata directive?
• What if you're running Clojure < 1.10?
• What if the component/stop quoted symbol does not get expanded to its fully-qualified name?
  • Would happen if you forget the :require in your (ns ...) declaration
• What if component/start does not resolve to a protocol function?
  • e.g. to a function of not emitted by defprotocol
• What if start does not evaluate to a function?
  • i.e. any other kind of value

implement guards you against all of those, preventing the lack of errors (or opaque errors, at best) that you'd get otherwise.

It also provides some sugar (symbols don't have to be quoted) and enforcement (you cannot pass anything other than a symbol; this aims to avoid deeply nested, non-reusable code).

This is how it looks like:

(implement {}
  component/start start
  component/stop  stop)

nedap.utils.modular.api/add-method

clojure.core/defmethod does the following:

Creates and installs a new method of multimethod associated with dispatch-value.

add-method does the same exact thing, but skipping the Creates part. i.e., it merely associates an existing function to a multimethod.

This has multiple advantages:

• One can code with plain defns, making things more homogeneous
  • And decoupled, reusable
• Said defns can be speced.def ones
• Importantly, one should understand that defmethod is a side-effect, and as such should be controlled.
  • Better to add-method in a Component start definition.

nedap.utils.modular.api/dependent

Helper fn for com.stuartsierra.component/using which takes a dependency collection and optionally a map with renames. Note that the dependencies can be passed as a vector or a map.

(dependent (my-component/new)
           :on my-component/dependencies
           :renames {:internal ::my-component/external})

This allows the user to keep using the my-component/dependencies-def while maintaining the flexibility to rename some keys.

nedap.utils.modular.api/omit-this [f]

Creates a replacement for f which drops the first argument, presumed to be of "this" type. Apt for protocol extensions, when f is an arbitrary function which may not participate in our protocols at all.

Refer to its test for an example.

ClojureScript compatibility

All the offered API is compatible with vanilla ClojureScript (i.e. non-self-compiled).

However, implement offers weaker guarantees in its cljs version, since cljs has fewer introspection capabilities, particularly at macroexpansion time.

At the same time, as long as your cljs code is defined as a .cljc file and it is compiled for the two possible targets (JVM, js), then the JVM target will provide the guarantees that cljs cannot provide. i.e. cross-compilation can act as a "linter", even if only using in production just a single target.

ns organisation

There is exactly 1 namespace meant for public consumption:

• nedap.utils.modular.api

By convention, api namespaces are deliberately thin so you can browse them comfortably.

Documentation

Please browse the public namespaces, which are documented, speced and tested.

License

Copyright © Nedap

This program and the accompanying materials are made available under the terms of the Eclipse Public License 2.0.