Add context()

src/context.luau

@@ -0,0 +1,75 @@
+if not game then script = require "test/relative-string" end
+
+local throw = require(script.Parent.throw)
+local graph = require(script.Parent.graph)
+type Node<T> = graph.Node<T>
+local create_node = graph.create_node
+local get_scope = graph.get_scope
+local push_scope = graph.push_scope
+local pop_scope = graph.pop_scope
+local set_context = graph.set_context
+
+export type Context<T> = (() -> T) & ((T, () -> ()) -> ())
+
+local nil_symbol = newproxy()
+local count = 0
+
+local function context<T>(...: T): Context<T>
+    count += 1
+    local id = count
+
+    local has_default = select("#", ...) > 0
+    local default_value = ...
+
+    return function(...)
+        local scope: Node<unknown>? | false = get_scope()
+
+        if select("#", ...) == 0 then -- get
+            while scope do
+                local ctx = scope.context
+    
+                if not ctx then
+                    scope = scope.owner
+                    continue
+                end
+
+                local value = (ctx :: { unknown })[id]
+
+                if value == nil then
+                    scope = scope.owner
+                    continue
+                end
+                
+                return (if value ~= nil_symbol then value else nil) :: T
+            end
+
+            if has_default ~= nil then
+                return default_value
+            else
+                throw("attempt to get context when no context is set and no default context is set")
+            end
+        else -- set
+            if not scope then return throw("attempt to set context outside of a vide scope") end
+
+            local value, component = ...
+            
+            local new_scope = create_node(scope, false, false)
+            set_context(new_scope, id, if value == nil then nil_symbol else value)
+
+            push_scope(new_scope)
+
+            local function efn(err: string) return debug.traceback(err, 3) end
+            local ok, result = xpcall(component, efn)
+
+            pop_scope()
+
+            if not ok then
+                throw(`error while running context:\n\n{result}`)
+            end
+        end
+
+        return nil :: any
+    end
+end
+
+return context