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predicate dispatching for clojure / clojurescript

[selfsame/pdfn "1.0.3"]

a multimethod style macro for compiling core.match style conditionals from ordered methods with predicate:argument patterns

Many languages have dispatching based on both arity and argument type. Predicate dispatching is a similar system where methods variants define unary predicate patterns for the arguments. When the results of applying the predicates to the args are truthy the method is called.

(defpdfn open)

(pdfn open [subject ^:contents object] 
  (str "opening reveals" (:contents object)))

(pdfn open [subject object] 
  {object (and* :contents :locked)}
  (str "it's locked"))

(pdfn open [^:key subject object] 
  {object (and* :contents :locked)}
  (str "unlocking and opening reveals" (:contents object)))

(open {} {:locked true :contents [:boots]})
>"it's locked"

(open {:key true} {:locked true :contents [:boots]})
>"unlocking and opening reveals[:boots]"

Method declarations are ordered. The user must reason about the specificity of the methods, but gains the ability to declare methods that override parts of a system without having to know the specifics. The absence of a predicate check is a 'wildcard' (nil).

(defpdfn ^:inline foo)

(pdfn foo 
  ([^pos?  a        b ^map?   c] :fish)
  ([^pos?  a ^neg?  b ^empty? c] :snail)
  ([^neg?  a ^zero? b         c] :mouse)
  ([       a ^neg?  b ^map?   c] :bird)
  ([^neg?  a        b ^set?   c] :dog)
  ([^odd?  a ^pos?  b         c] :lion)
  ([^even? a ^neg?  b ^map?   c] :horse))

(foo 4 -1 [])

pdfn uses an implementation of Compiling Pattern Matching to good Decision Trees, which is used/explained in depth by core.match.

The compiled conditional has a unique path of (p v) evaluations for every method.

(inspect foo)
>(fn ([a b c]
  (if (and (even? a) (neg? b) (map? c))
    (if (and (odd? a) (pos? b))
      (if (and (set? c) (neg? a))
        (if (neg? b)
          (if (map? c)
            (if (and (neg? a) (zero? b))
              (if (and (pos? a) (empty? c)) 
          (if (and (neg? a) (zero? b))
            (if (and (pos? a) (map? c)) 


(ns foo.core (:require [pdfn.core :refer :all]))

(ns foo.core 
  (:require [pdfn.core :refer [and* or* not* is*] :refer-macros [defpdfn pdfn compile! inspect benchmark]]))



(defpdfn ^{:inline false :stub-arity true} foo)

Declares a symbol that will be bound to a compiled dispatch fn. Meta data can configure options.


(pdfn baz [a b] :body)

(pdfn baz 
  ([a] :x)
  ([a b c & ^empty? more] :z))
  • The pdfn macro defines a method variant for an existing fn binding.
  • The order of definition matters - last defined is the first to be considered.
  • Multiple methods can be defined in one pdfn.
  • Different arities are compiled separately and grouped into the main fn.
(pdfn ^:inline baz [^int? a b ^:kw c]
  {b #{nil 0 false}}

The vector binding uses meta data to define predicates. You can use &, destructuring is not supported.

  • Meta tags only support ^symbol, ^:keyword, and ^{:map :literals}.
  • Note: ^:keyword is notation for {:keyword true} but pdfn interperates ( {foo true} ) meta as foo in the vector binding.

An optional map of {arg predicate} can follow the vector binding, as long as it is not the last form.

  • allows most forms for predicates (excepting #() (fn []))
  • merges onto meta predicates.

by default, every pdfn emits compiled code.


(compile! f)

Explicit build command for use with the :defer-build meta option (used when small code emission is desired)


(inspect fizz)
(inspect fizz :methods)

pprints code or method map, assumes user has a pprint alias.


(benchmark 100000 (foobar 5 8))
;"Elapsed time: 13 msecs"

Convenience macro for (time (dotimes [i n] code))

and* or* not*

(and* map? (not* empty?) (or* :red :blue))

predicate composition fns.


((is* 5) 5)

makes equiv predicate


meta data on the defpdfn or pdfn symbol can configure the following:

  • :inline when false (default) methods will be externally defined. External methods are usefull for debugging exceptions, but inline is easier to read when inspecting and is a bit faster.
  • :stub-arity (default false) when true the compiled fn will fill unused arities with blank variants. (not including [& more])
  • :defer-compile (default false) when true requires user to explicitly (compile! f), avoiding code generation for every pdfn method step.

REPL workflow

pdfn has a few caveats due to the ordered nature of it's parts.

  • defpdfn eval removes pdfn methods and built code

pdfn re-evaluation depends on the predicate identity

  • unchanged - original method entry updated in the dispatch map
  • changed - new method added to the end of the dispatch map


clojure predicate dispatch




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