The Magnet Is a Predicate You Drag Through a River

A magnet on a rope has no idea where anything is on the riverbed. It knows one thing: given an object, does it stick or not? You drag it through a heap of mud, aluminum cans, quartz pebbles, one lost bike chain, and it silently drops everything that fails the test and keeps everything that passes. That is a predicate — a function from a thing to yes-or-no — and pulling it through a collection is filter.

In Clojure the magnet is a value you can hold and pass around:

(defn ferrous? [item]
  (contains? #{:iron :steel :nickel} (:metal item)))

(def riverbed
  [{:name "horseshoe"  :metal :iron}
   {:name "beer can"   :metal :aluminum}
   {:name "old key"    :metal :steel}
   {:name "quartz"     :metal :none}
   {:name "bike chain" :metal :steel}])

(doseq [x (filter ferrous? riverbed)]
  (println (:name x) "->" (:metal x)))

horseshoe -> :iron
old key -> :steel
bike chain -> :steel

ferrous? gets handed to filter like a magnet clipped onto a rope. The rope is generic; the magnet decides.

Ada 83, which the U.S. Department of Defense was busy mandating across every avionics contract in the same decade, won’t let you do that. Subprograms aren’t first-class values there — you can’t pass Ferrous into a general dragging routine — so the test lives welded inside the loop:

with Ada.Text_IO; use Ada.Text_IO;

procedure Magnet is
   type Metal is (Iron, Steel, Nickel, Aluminum, Glass);
   type Find  is record
      Name : String (1 .. 9);
      Made : Metal;
   end record;
   Riverbed : constant array (1 .. 4) of Find :=
     (("horseshoe", Iron),
      ("beer  can", Aluminum),
      ("old   key", Steel),
      ("quartz   ", Glass));
begin
   for I in Riverbed'Range loop
      if Riverbed (I).Made = Iron
        or Riverbed (I).Made = Steel
        or Riverbed (I).Made = Nickel
      then
         Put_Line (Riverbed (I).Name & " sticks");
      end if;
   end loop;
end Magnet;

horseshoe sticks
old   key sticks

Same catch, same discard pile. But notice where the magnet went: in Clojure it’s an object you carry between routines; in Ada 83 it’s fused to the loop and can’t be lifted out. To change what sticks, the Clojure fisher swaps the magnet; the Ada fisher rewrites the boat.