Improve usage of context

redraw/src/context.ffi.mjs

@@ -0,0 +1,21 @@
+import * as React from "react"
+import { jsx } from "./redraw.ffi.mjs"
+import * as gleam from "./gleam.mjs"
+import * as error from "./redraw/error.mjs"
+
+const contexts = {}
+
+export function contextProvider(context, value, children) {
+  return jsx(context.Provider, { value }, children)
+}
+
+export function createContext(name, defaultValue) {
+  if (contexts[name]) return new gleam.Error(new error.ExistingContext(name))
+  contexts[name] = React.createContext(defaultValue)
+  return new gleam.Ok(contexts[name])
+}
+
+export function getContext(name) {
+  if (!contexts[name]) return new gleam.Error(new error.UnknownContext(name))
+  return new gleam.Ok(contexts[name])
+}

redraw/src/redraw.ffi.mjs

@@ -174,10 +174,6 @@ export function coerce(value) {
   return value
 }
 
-export function contextProvider(context, value, children) {
-  return jsx(context.Provider, { value }, children)
-}
-
 export function setCurrent(ref, value) {
   ref.current = value
 }

redraw/src/redraw.gleam

@@ -1,5 +1,8 @@
+import gleam/function
 import gleam/javascript/promise.{type Promise}
 import gleam/option.{type Option}
+import gleam/string
+import redraw/error.{type Error}
 import redraw/internals/coerce.{coerce}
 
 // Component creation
@@ -313,18 +316,162 @@ pub fn use_context(context: Context(a)) -> a
 
 /// Let you create a [context](https://redraw.dev/learn/passing-data-deeply-with-context) that components can provide or read.
 /// [Documentation](https://redraw.dev/reference/redraw/createContext)
+@deprecated("Use redraw/create_context_ instead. redraw/create_context will be removed in 2.0.0. Unusable right now, due to how React handles Context.")
 @external(javascript, "react", "createContext")
 pub fn create_context(default_value default_value: Option(a)) -> Context(a)
 
 /// Wrap your components into a context provider to specify the value of this context for all components inside.
 /// [Documentation](https://redraw.dev/reference/redraw/createContext#provider)
-@external(javascript, "./redraw.ffi.mjs", "contextProvider")
+@external(javascript, "./context.ffi.mjs", "contextProvider")
 pub fn provider(
   context context: Context(a),
   value value: a,
   children children: List(Component),
 ) -> Component
 
+/// Create a [context](https://redraw.dev/learn/passing-data-deeply-with-context)
+/// that components can provide or read.
+/// Each context is referenced by its name, a little bit like actors in OTP
+/// (if you're familiar with Erlang). Because Gleam cannot execute code outside of
+/// `main` function, creating a context should do some side-effect at startup.
+///
+/// In traditional React code, Context usage is usually written like this.
+///
+/// ```javascript
+/// import * as react from 'react'
+///
+/// // Create your Context in a side-effectful way.
+/// const MyContext = react.createContext(defaultValue)
+///
+/// // Create your own provider, wrapping your context.
+/// export function MyProvider(props) {
+///   return <MyContext.Provider>{props.children}</MyContext.Provider>
+/// }
+///
+/// // Create your own hook, to simplify usage of your context.
+/// export function useMyContext() {
+///   return react.useContext(MyContext)
+/// }
+/// ```
+///
+/// To simplify and mimic that usage, Redraw wraps Context creation with some
+/// caching, to emulate a similar behaviour.
+///
+/// ```gleam
+/// import redraw
+///
+/// const context_name = "MyContextName"
+///
+/// pub fn my_provider(children) {
+///   let assert Ok(context) = redraw.create_context_(context_name, default_value)
+///   redraw.provider(context, value, children)
+/// }
+///
+/// pub fn use_my_context() {
+///   let assert Ok(context) = redraw.get_context(context_name)
+///   redraw.use_context(context)
+/// }
+/// ```
+///
+/// Be careful, `create_context_` fails if the Context is already defined.
+/// Choose a full qualified name, hard to overlap with inattention. If
+/// you want to get a Context in an idempotent way, take a look at [`context()`](#context).
+/// [Documentation](https://redraw.dev/reference/redraw/createContext)
+@external(javascript, "./context.ffi.mjs", "createContext")
+pub fn create_context_(
+  name: String,
+  default_value: a,
+) -> Result(Context(a), Error)
+
+/// Get a context. Because of FFI, `get_context` breaks the type-checker. It
+/// should be considered as unsafe code. As a library author, never exposes
+/// your context and expect users will call `get_context` themselves, but rather
+/// exposes a `use_my_context()` function, handling the type-checking for the
+/// user.
+///
+/// ```gleam
+/// import redraw
+///
+/// pub type MyContext {
+///   MyContext(value: Int)
+/// }
+///
+/// /// `use_context` returns `Context(a)`, should it can be safely returned as
+/// /// `Context(MyContext)`.
+/// pub fn use_my_context() -> redraw.Context(MyContext) {
+///   let context = case redraw.get_context("MyContextName") {
+///     // Context has been found in the context cache, use it as desired.
+///     Ok(context) -> context
+///     // Context has not been found. It means the user did not initialised it.
+///     Error(_) -> panic as "Unitialised context."
+///   }
+///   redraw.use_context(context)
+/// }
+/// ```
+@external(javascript, "./context.ffi.mjs", "getContext")
+pub fn get_context(name: String) -> Result(Context(a), Error)
+
+/// `context` emulates classic Context usage in React. Instead of calling
+/// `create_context_` and `get_context`, it's possible to simply call `context`,
+/// which will get or create the context directly, and allows to write code as
+/// if Context is globally available. `context` also tries to preserve
+/// type-checking at most. `context.default_value` is lazily evaluated, meaning
+/// no additional computations will ever be run.
+///
+/// ```gleam
+/// import redraw
+///
+/// const context_name = "MyContextName"
+///
+/// pub type MyContext {
+///   MyContext(count: Int, set_count: fn (Int) -> Nil)
+/// }
+///
+/// fn default_value() {
+///   let count = 0
+///   les set_count = fn (_) { Nil }
+///   MyContext(count:)
+/// }
+///
+/// pub fn provider() {
+///   use _, children <- redraw.component()
+///   let context = redraw.context(context_name, default_value)
+///   let #(count, set_count) = redraw.use_state(0)
+///   redraw.provider(context, MyContext(count:, set_count:), children)
+/// }
+///
+/// pub fn use_my_context() {
+///   let context = redraw.context(context_name, default_value)
+///   redraw.use_context(context)
+/// }
+/// ```
+///
+/// `context` should never fail, but it can be wrong if you use an already used
+/// name.
+pub fn context(name: String, default_value: fn() -> a) -> Context(a) {
+  case get_context(name) {
+    Ok(context) -> context
+    Error(get) ->
+      case create_context_(name, default_value()) {
+        Ok(context) -> context
+        Error(create) -> {
+          let get = "  get_context: " <> string.inspect(get)
+          let create = "  create_context_: " <> string.inspect(create)
+          let head = "[Redraw Internal Error] Unable to find or create context."
+          let body =
+            function.flip(string.join)(" ", [
+              "context should never panic.",
+              "Please, open an issue on https://github.com/ghivert/redraw,",
+              "and join the error details.\n",
+            ])
+          let details = "Error details:"
+          let msg = string.join([head, body, details, get, create], "\n")
+          panic as msg
+        }
+      }
+  }
+}
+
 // API
 //
 /// Test helper to apply pending React updates before making assertions.

redraw/src/redraw/error.gleam

@@ -0,0 +1,4 @@
+pub type Error {
+  ExistingContext(name: String)
+  UnknownContext(name: String)
+}