Joseph Ferano
d6fc15474b
The author: "I think I prefer length over len, because then I'll use len as the variable name". One arm in check.ml, one row in the table beside it, and every (len x) in lib, test, examples, vendor, spike, docs, web, emacs, plan.org and NEXT.md rewritten. Shadowing and builtin/ had already taken most of the sting out: a (defn len ...) was legal and won in its own file, and builtin/len reached past it. What was left is that len was still a builtin — the defn earned a warning, and a wrapper had to say builtin/ at every inner call. Now there is nothing under the short name: len is an ordinary identifier in every position, which is what (let [len (length xs)] ...) wants. length takes over as shadowing's worked example rather than the feature losing one. shadow-builtin.flan, builtin-qualified.flan, pkgs/shadowed and the builtin/ rows in test_flan move to it and go on testing shadowing. A call to a len nothing defines is answered where an unknown function is, after every table and after the shadowing guard, so a program with its own len never reaches it. The sentence is said rather than guessed at — len and length are three edits apart and the did-you-mean's net is one — and the call is written back out through spell_arg, as-slice's spelling lifted out of it and now shared, so what is printed compiles. sand.flan:33 still calls the old name and is the author's to change; until it does, test_acceptance and test_session abort there. Both were run green against a copy with that one line changed. FIX.org says so.
139 lines
5.9 KiB
Plaintext
139 lines
5.9 KiB
Plaintext
;;;; Classes and generic functions, the dyn side's two dispatch styles.
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;;;;
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;;;; A defclass is a named dyn map with a shape tag. The constructor is the
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;;;; class's own name, positional over the slots; the slots are ordinary map
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;;;; keys, so get and put are how one is read and written and nothing new was
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;;;; needed for either. What the class adds is the tag, which lives in the
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;;;; object's header and not in the entries: (length p) is the slot count, no
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;;;; key
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;;;; a program can write collides with it, and it shows up in exactly three
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;;;; places -- class-of, equality, and the printed form #point{ :x 1 :y 2}.
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;;;;
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;;;; The two dispatch styles are one mechanism. A defgeneric dispatches on the
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;;;; class of its first argument, which is Common Lisp's; a defmulti's body IS
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;;;; the dispatch, which is Clojure's. A class dispatcher is the shape tag of
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;;;; the first argument as the dispatch function, so the second spells the
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;;;; first, and a method is a branch either way.
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(defclass point [x y])
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(defclass circle [r])
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;; CLOS's half: dispatch on the class of the first argument. The generic
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;; declares the parameters and the return type once; a method states neither.
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(defgeneric area [self] dyn)
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(defmethod area point [p] (* (get p :x) (get p :y)))
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;; A method may name its parameter whatever it likes -- the generic's name is
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;; bound to it on the way in.
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(defmethod area circle [c] (* 3 (* (get c :r) (get c :r))))
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;; Clojure's half: the dispatch is a body, over the same parameter list every
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;; method has, answering the value the methods are keyed by.
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(defmulti describe [thing] dyn (get thing :kind))
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(defmethod describe :square [s] (get s :side))
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(defmethod describe "round" [s] "a round thing, keyed by a string")
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(defmethod describe 7 [s] "the one keyed by a number")
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;; The method that answers when no other does. It is :else, which is the word
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;; match already uses, and it is the last arm whatever order it is written in.
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(defmethod describe :else [s] "something else")
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;; A generic with no :else: a miss signals, and the program answers it.
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(defgeneric name-of [self] dyn)
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(defmethod name-of point [p] "a point")
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;; A method's parameter names are its own, and the rebinding that gives it
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;; them is parallel. [reorder] names them in the generic's order reversed, which
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;; a sequential binding would get wrong in the worst possible way -- it would
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;; read the name it had just bound and hand the method its first argument
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;; twice. [shift] is the same bug one step shorter: [b] there is the
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;; generic's second parameter and must not become the first.
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(defmulti reorder [a b] dyn (class-of a))
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(defmethod reorder point [b a] [b a])
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(defmulti shift [a b] dyn (class-of a))
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(defmethod shift point [b c] [b c])
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(defn main [] i32
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(let [p (point 3 4)
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c (circle 2)]
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;; The instance is a map, and prints as one with its tag in front.
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(println p)
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(println (length p))
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(println (get p :x))
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(put p :x 10)
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(println (get p :x))
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(println (has-key? p :x))
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(println (has-key? p :nothing))
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(println (get p :nothing))
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;; The shape tag, as a value. Every value can be asked; only an instance
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;; answers with a name.
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(println (class-of p))
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(println (class-of c))
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(println (class-of {:x 3 :y 4}))
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(println (class-of 1))
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(println (class-of nil))
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;; Equality takes the tag into account: two instances of one class compare
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;; by their slots, an instance and a plain map with the same entries do
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;; not, and two classes with the same slots are two classes.
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(println (= (point 1 2) (point 1 2)))
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(println (= (point 1 2) (point 1 3)))
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(println (= (point 1 2) {:x 1 :y 2}))
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(println (= (circle 2) (circle 2)))
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;; Class dispatch. Same call site, two classes, two methods.
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(println (area p))
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(println (area c))
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;; Arbitrary dispatch, over three kinds of dispatch value and the
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;; fallback. The dispatch runs on every call, so the value is whatever
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;; the map holds at the time.
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(println (describe {:kind :square :side 5}))
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(println (describe {:kind "round"}))
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(println (describe {:kind 7}))
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(println (describe {:kind :hexagon}))
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;; An empty map on its own is the zero-field struct literal, so it is
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;; bound first -- the dispatch answers nil for a map with no :kind, and
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;; nil finds no method either.
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(let [empty {:no :kind}]
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(println (describe empty)))
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;; An instance is an ordinary dyn value: it goes in a vec, keys a map,
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;; and is collected like anything else.
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(let [v [p c]]
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(println (length v))
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(println (class-of (at v 1))))
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;; The renamed parameters, in the generic's own order: the first element
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;; of each answer is the first argument. A sequential rebinding would
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;; print the point twice in the first and lose the 99 in the second.
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(println (reorder p 99))
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(println (shift p 99))
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;; The miss. No method and no :else, so the generic signals NoMethod, and
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;; handler-case answers the whole form with a value -- the condition
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;; carries the generic's name and the dispatch value that found nothing.
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(println (name-of p))
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(println
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(handler-case (name-of c)
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[(NoMethod [e] (.generic e))]))
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;; The condition carries the dispatch value that found nothing, which for
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;; a defgeneric over a value that is no instance at all is nil.
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(println
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(handler-case (name-of 42)
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[(NoMethod [e] (.value e))])))
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;; Instances under the collector: enough of them to pass the 1 MiB floor
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;; many times over, with one live instance in a rooted global. A marker that
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;; lost an instance's slots would free something live and the sum would come
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;; out wrong -- the tag itself is an interned keyword and immortal, which is
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;; why it needs no tracing.
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(let [total (point 0 0)]
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(dotimes [i 50000]
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(let [q (point i "forty-seven bytes of text to fatten each row")]
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(put total :x (+ (get total :x) (length q)))))
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(println (get total :x))
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(println (class-of total)))
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0)
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