Experimental lib for Clojure conditional feature reader using tagged literals
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Experimental lib for Clojure conditional features.

Nota bene: This library has been superseded by the official Clojure/core team plan to implement Feature Expressions in the upcoming Clojure 1.7 release.




leiningen dependency [com.velisco/wilkins "a.b.c"]

where "a.b.c" stands for the version number shown below (from Clojars.org)

Wilkins on clojars.org

The main namespace is miner.wilkins.

There are two ways to use wilkins. First, there is a data-reader that implements conditional compilation at read-time. Second, there is a macro that implements conditional compilation at compile time. In both cases, feature expressions determine which form is actually evaluated.


In your data_readers.clj you can assign a tag to enable the conditional reader like this:

{feature/condf miner.wilkins/condf-reader}

Then in your source:

(require '[miner.wilkins :as w])

#feature/condf [
    (and jdk-1.6+ clj-1.5.*) (call-my-fast-reducer-code)
	else (some-old-fashioned-code)	]

Note that all the clause are contained in a vector literal so the data reader applies to the whole vector as its single argument. The pairs of forms are like a cond but the test is a feature requirement expression. The data-reader will return the single form following the first matching feature requirement. That is the only form that will be seen by the compiler.

The macro compile-condf is similar but doesn't have require the vector notation around the clauses. It reads more like a cond but the feature expressions are evaluated at compile-time.

	(and jdk-1.6+ clj-1.5.*) (call-my-fast-reducer-code)
	else (some-old-fashioned-code))

If you want to evaluate feature expressions at runtime, use feature?.

(w/feature? 'clj-1.5+)

Conditional Feature Reader

condf is short for "conditional feature". The condf tag marks a sequence of clauses similar to cond, but each test is a feature requirement. The first feature requirement to be satisfied causes the reader to return the following expression. As a special case, the symbol else always qualifies. If no feature requirement is satisfied, the reader effectively returns nil.

The feature requirement specifies a feature name and an optional version. The feature requirements are evaluated by the condf-reader data-reader at read-time so the compiler never sees unsuccessful clauses. The compile-condf macro is similar but all its forms are read by the reader so it's slightly less flexible with expressions that might not be legal in certain Clojure variants. Note: We intend to support Clojurescript, but that is not currently implemented.

Feature Notation

A canonical "feature requirement" is defined by an identifier (symbol) and version information in a map following the same scheme as *clojure-version*. We'll give the details below, but typically we use a compact notation for versions and feature requirements.

The compact form of a feature requirement uses a single symbol. For example, clj-1.4 means Clojure 1.4. The alphabetic part is the feature identifier and the dotted number part is the version number, separated by a hyphen (-). A trailing + means "or greater". A trailing .* means "any increment", but the previous parts must match exactly. Only one + or * is allowed in a feature requirement. A qualifier string may follow the version number (separated by a hyphen), but in that case an exact match is required. For example, clj-1.5-RC1 matches exactly Clojure "1.5.0-RC1", and not any other version. A hyphen is required to separate the feature name from version, but the version information is optional. Also, hyphens may appear in the feature name. In the rare case in which your feature name itself actually contains a hyphen followed by digit, you can either quote the symbol or use the vector form of the feature requirement to avoid parsing confusion.

The vector form of a feature requirement contains an identifier symbol (not subject to version parsing) and a version string. This allows you to use a feature name that contains a digit, such as [lucky-7 "1.2+"]. The version string can be omitted if you don't care about versioning.

Feature Declaration

A feature declaration is simply a Clojure var with metadata declared for the :feature key.

(require '[miner.wilkins :as w])
(def ^{:feature (w/version "1.2.3")} foo 42)

The version macro expands a version string into a map form, similar to the style of *clojure-version*, with the keys :major, :minor, :incremental and :qualifier. A map-entry may be omitted if the value is nil. No wildcards are allowed when declaring a version.

A feature requirement without a namespace implicitly refers to the current namespace (*ns*) or the miner.wilkins.features namespace which has a few predefined features. For example, miner.wilkins/clj is a var with metadata defining the current Clojure version (taken from *clojure-version*). clojure is a synonym for clj. The vars java and jdk have the version of the JDK.

A feature requirement without a version simply requires that var exist. You can also require that a Java class exists by using its name as the feature symbol (without any version). As in Clojure, a Java class is named by a simple symbol (no namespace) containing internal periods -- for example, java.util.concurrent.ForkJoinPool.

Boolean combinations are encoded as list expressions beginning with and, or or not. For example: (and jdk-1.6+ clj-1.5+).


A feature version requirement is one of:

  • else and true immediately succeed
  • nil and false immediately fail
  • a namespaced symbol, for example my.ns/my-feature-2.3+ (checks against :feature metadata of #'my.ns/my-feature)
  • unqualified symbol, for example: clj-1.4 (implicitly checks *ns* and miner.wilkins.features)
  • a vector of [name "ver"], for example: [clj "1.4+"] (the feature name is a literal symbol)
  • a list of (quote name) to suppress the parsing of version information in the compact symbol form.

A boolean feature requirement is

  • a list starting with and, or, or not combining other feature requirements.

Here are a few examples to show the how the parsing works for the compact symbol form:

  • lucky-7 (no quote) expands into {:feature lucky :major 7}
  • 'lucky-7 (quoted) expands into {:feature lucky-7 :major :*}
  • [lucky-7] (vector without version) also expands into {:feature lucky-7 :major :*}

Canonical Feature Requirement

The canonical feature requirement is a map similar to *clojure-version*.

The "version" part has possible keys (with value types): :major (int), :minor (int), :incremental (int), and :qualifier (string). For informational purposes it might also contain :version (string).

The feature requirement has the key :feature with a symbol as the value, and the version keys described above. It can also have the key :plus (boolean). The least significant of the :major, :minor, or :incremental values may be :* (a keyword) in addition to the usual int value. The :* matches any value. For example, 1.4.* matches any version in the 1.4 series (1.4.0, 1.4.1, etc.) but not 1.5. If the version doesn't matter, then :major should be given as :*. (See as-feature-request.) When the :plus value is true, then the version matches exactly the given :major, :minor, and :incremental keys or anything greater. For example, 1.4+ matches 1.4.1, 1.5.0, etc. If a :qualifier (string) is specified as a requirement, then the match must be exact, and wildcards are not allowed.


If you're using AOT ("ahead of time") compilation, you have to be careful that your actual runtime corresponds to the compilation environment. If the conditional compilation depended on JDK 1.7, but you later use that jar with JDK 1.6 you might have a problem. If you want to do your feature tests at runtime (or in a REPL), use the feature? predicate and pass it feature expression. (Note the you typically have to quote a symbolic requirement since this is a normal function call, not a macro.)

   (w/feature? '(and clj (not foo-2.4+)))  (do-something 1 2)
   (w/feature? '(or clj-1.5 [clj "1.4"]))  :clj 
   :else  :something-else)

Clojurescript not implemented yet

Wilkins is intended to be useful for conditional code across variants of Clojure (such as ClojureScript) where the platform or host language may be so different that runtime checks would not be feasible. We have not yet implemented the ClojureScript version of condf-reader so that needs a bit more work to validate the approach.

Bob Wilkins

Named in honor of Bob Wilkins, host of KTVU's "Creature Features" (1971-79).


Copyright © 2014 Stephen E. Miner

Distributed under the Eclipse Public License, the same as Clojure.