rontolisp.wit/provide

(provide interface provider) with [rontolisp.wit :as wit]

Binds the implementation of an imported interface where the program provides it itself: the interpreter and the JVM (WIT contracts). provider is a function of the member's name (a string such as "open" or "bucket-get") followed by that member's arguments, a resource method's handle first; what it answers is the call's value, and what it throws, the call throws. A later provide of the interface replaces it. The answer is interface.

interface is a string: the full id of an interface a rontolisp.wit/import above binds, or the spelling that import wrote ("wasi:keyvalue/store"). Any other interface, or one computed when the program runs, is refused when the program compiles: the import's WIT is what the provider's values are converted by. For the same reason provide has no value.

$ cat hits.clj
(ns hits (:require [rontolisp.wit :as wit]))

(wit/import "kv.wit" {:interface "wasi:keyvalue/store@0.2.0-draft" :as kv})

(def store (atom {}))

(wit/provide "wasi:keyvalue/store@0.2.0-draft"
             (fn [member & [_ k v]]
               (cond
                 (= member "open") 0
                 (= member "bucket-get") (get @store k)
                 (= member "bucket-set") (do (swap! store assoc k v) nil))))

(let [bucket (kv/open "")]
  (kv/bucket-set bucket "hits" (.getBytes "42"))
  (println (String. (kv/bucket-get bucket "hits"))))
$ rontolisp hits.clj
42

A WASM build's host provides every import, so there provide binds nothing: the same source compiled with --component runs against wasmtime's own wasi:keyvalue (wasmtime run -S keyvalue=y). With no provider bound, a call throws naming the interface.

A provider sees and answers Clojure values, the ones a caller passes and gets (What crosses): a bool arrives as true or false, a record as a map, a variant's case as a keyword or [:case payload]. A member answering a result answers its ok value; its error arm is an ExceptionInfo holding the error value under :rontolisp.wit/error, the exception a caller catches: (throw (ex-info "no such store" {::wit/error :no-such-store})). A Common Lisp program calling the interface receives that arm as rontolisp:wit-error, its payload in the Common Lisp spelling (:NO-SUCH-STORE).