Sandestin Registry Author

Name: fx-registry
Rating: 78
Author: brianium

Create effect registries for Sandestin projects.

About Sandestin

Sandestin is a Clojure effect dispatch library with schema-driven discoverability. Registries define effects, actions, and placeholders that can be composed and dispatched.

GitHub: https://github.com/brianium/sandestin

Check if Installed

Look for the dependency in deps.edn:

clojure

io.github.brianium/sandestin {:git/tag "v0.2.0" :git/sha "23ec7f0"}

Install if Missing

Add to deps.edn under :deps:

clojure

{:deps
 {io.github.brianium/sandestin {:git/tag "v0.2.0" :git/sha "23ec7f0"}}}

Workflow

1. Check for Existing Patterns

bash

# Find existing registries
find src -name "*.clj" | xargs grep -l "::s/effects" 2>/dev/null

# Check naming conventions
grep -r "defn registry" src/

2. Create the Registry

Simple Registry (no config)

clojure

(ns mylib.fx.logging
  "Logging effects."
  (:require [ascolais.sandestin :as s]))

(def registry
  {::s/effects
   {:mylib.log/info
    {::s/description "Log an info message"
     ::s/schema [:tuple [:= :mylib.log/info] :string]
     ::s/handler (fn [_ctx _system msg]
                   (println "[INFO]" msg))}}})

Configurable Registry (with dependencies)

clojure

(ns mylib.fx.database
  "Database effects."
  (:require [ascolais.sandestin :as s]))

(defn registry
  "Database effects registry.

   Requires a datasource."
  [datasource]
  {::s/effects
   {:mylib.db/query
    {::s/description "Execute a SQL query"
     ::s/schema [:tuple [:= :mylib.db/query] :string [:* :any]]
     ::s/system-keys [:datasource]
     ::s/handler (fn [_ctx system sql & params]
                   (jdbc/execute! (:datasource system) (into [sql] params)))}}

   ::s/system-schema
   {:datasource [:fn some?]}})

3. Registration Patterns

Effect (side-effecting):

clojure

{:<ns>/<verb>
 {::s/description "What this effect does"
  ::s/schema [:tuple [:= :<ns>/<verb>] <arg-schemas>]
  ::s/system-keys [:key1 :key2]
  ::s/handler (fn [{:keys [dispatch dispatch-data]} system & args]
                ;; Do side effect, optionally dispatch continuation
                )}}

Action (pure, returns effect vectors):

clojure

{:<ns>/<action>
 {::s/description "What this action does"
  ::s/schema [:tuple [:= :<ns>/<action>] <arg-schema>]
  ::s/handler (fn [state & args]
                [[:<ns>/effect1 arg]
                 [:<ns>/effect2 (:val state)]])}}

;; If actions need state from system:
::s/system->state (fn [system] @(:app-state system))

Placeholder (resolves from dispatch-data):

clojure

{:<ns>/<placeholder>
 {::s/description "What value this provides"
  ::s/schema <resolved-value-schema>
  ::s/handler (fn [dispatch-data & args]
                (:some-key dispatch-data))}}

Self-Preserving Placeholder (for async continuations):

When an effect dispatches continuation effects with new data, placeholders in those continuations must "self-preserve" — return themselves when the data isn't available yet, then resolve when re-interpolated with the actual data.

clojure

{:<ns>/<result>
 {::s/description "Result from async operation, self-preserving"
  ::s/handler (fn [dispatch-data]
                ;; Return self if data not yet available
                (or (:<ns>/<result> dispatch-data)
                    [:<ns>/<result>]))}}

Usage pattern:

clojure

;; Effect that dispatches continuation with result
{:<ns>/fetch
 {::s/handler
  (fn [{:keys [dispatch]} system url continuation-fx]
    (let [result (http/get url)]
      ;; Dispatch continuation with result in dispatch-data
      (dispatch {:<ns>/result result} continuation-fx))
    :fetch-started)}}

;; Calling code - placeholder resolves in continuation dispatch
(dispatch {} {}
  [[:<ns>/fetch "http://api.example.com"
    [[:<ns>/process [:<ns>/result]]]]])

Flow:

•Initial dispatch interpolates [:<ns>/result] → returns itself (no data yet)
•Effect runs, calls dispatch with {:<ns>/result actual-data}
•Continuation dispatch interpolates [:<ns>/result] → returns actual-data

4. Test via REPL

clojure

(require '[ascolais.sandestin :as s])
(require '[mylib.fx.logging :as logging])

;; Create a test dispatch
(def dispatch (s/create-dispatch [logging/registry]))

;; Verify registration
(s/describe dispatch :mylib.log/info)
(s/sample dispatch :mylib.log/info)

;; Test it
(dispatch {} {} [[:mylib.log/info "hello"]])

Required Fields

Field	Purpose
`::s/description`	Human-readable description
`::s/schema`	Malli schema for the effect vector
`::s/handler`	Implementation function

Optional Fields

Field	Purpose
`::s/system-keys`	Declare system map dependencies
`::s/system-schema`	Malli schemas for system keys
`::s/system->state`	Extract immutable state for actions

Placeholder Patterns

Pattern	When to Use
Simple	Value available at dispatch time (e.g., DOM event data)
Self-preserving	Value available later via continuation dispatch (e.g., async results)
Transforming	Wraps another placeholder to transform its value

Transforming placeholder example:

clojure

;; Extract field from async result
{:<ns>/result-name
 {::s/handler
  (fn [dispatch-data]
    ;; Check if result is available before transforming
    (if-let [result (:<ns>/result dispatch-data)]
      (:name result)
      [:<ns>/result-name]))}}

;; Usage: [:<ns>/result-name] instead of nesting