Add draft type class container idea

Author: thgs

src/Typeclass/Container.php

@@ -0,0 +1,90 @@
+<?php
+
+namespace thgs\Functional\Typeclass;
+
+use thgs\Functional\Data\Just;
+use thgs\Functional\Data\Maybe;
+use thgs\Functional\Data\Nothing;
+
+use function thgs\Functional\c;
+
+/**
+ * Contains type class implementations here
+ */
+class Container
+{
+    /**
+     * @var array<string, array{predicate: \Closure, impl: \Closure}>
+     */
+    private static array $definitions;
+
+    /**
+     * @param \Closure(mixed):bool $predicate
+     * @param \Closure $implementation
+     *
+     * @todo Make the container a state monad?
+     */
+    public static function register(string $identifier, \Closure $predicate, \Closure $implementation): void
+    {
+        self :: $definitions [$identifier] [] = ['predicate' => $predicate, 'impl' => $implementation];
+    }
+
+    public function get(string $identifier, mixed $target): \Closure
+    {
+        if (!isset( self :: $definitions [$identifier] )) {
+            throw new \Exception("Missing definition for $identifier");
+        }
+
+        foreach (self :: $definitions [$identifier] as ['predicate' => $predicate, 'impl' => $impl]) {
+            if ($predicate($target)) {
+                return $impl;
+            }
+        }
+
+        throw new \Exception("No definition for $identifier on given target");
+    }
+}
+
+/**
+ * Maybe we can still use interfaces like FunctorInstance but can also
+ * provide the ability to register more fancy things that otherwise
+ * would end up in `Instance` namespace by using the callable
+ * predicate?
+ *
+ * This will work best when we can compile/build, so it changes to
+ * static dispatch.
+ *
+ * The main benefit is that the type declaration does not have
+ * to define interfaces OR be wrapped around to be used.
+ *
+ * example: Register `fmap` for some new X object that some imported
+ * lib defines and then call fmap. Otherwise we would have to use
+ * the `FunctorAdapter` or the `Wrapper` or implement a new class to
+ * basically wrap around the X object and provide `fmap`.
+ *
+ * @see https://terbium.io/2021/02/traits-typeclasses/
+ */
+Container::register(
+    'fmap',
+    fn ($x) => $x instanceof Maybe,
+    /**
+     * @template A1
+     * @template B1
+     * @param \Closure(A1):B1 $f
+     * @param Maybe<A1> $fa
+     * @return Maybe<B1>
+     */
+    function (\Closure $f, Maybe $fa): Maybe {
+        $value = $fa->getValue();
+        if ($value instanceof Nothing) {
+            return $fa;
+        }
+
+        /** @var Composition<A1,B1> */
+        $c = c ($f);
+
+        /** @phpstan-assert B1 $value->getValue() */
+
+        return new Maybe(new Just( $c ($value->getValue()) ));
+    }
+);