Merge pull request #20 from Innmind/identity-monad

Add `Identity` monad

Author: Baptouuuu

CHANGELOG.md

@@ -1,5 +1,12 @@
 # Changelog
 
+## [Unreleased]
+
+### Added
+
+- `Innmind\Immutable\Identity`
+- `Innmind\Immutable\Sequence::toIdentity()`
+
 ## 5.5.0 - 2024-06-02
 
 ### Changed

docs/structures/identity.md

@@ -0,0 +1,55 @@
+# `Identity`
+
+This is the simplest monad there is. It's a simple wrapper around a value to allow chaining calls on this value.
+
+Let's say you have a string you want to camelize, here's how you'd do it:
+
+=== "Identity"
+    ```php
+    $value = Identity::of('some value to camelize')
+        ->map(fn($string) => \explode(' ', $string))
+        ->map(fn($parts) => \array_map(
+            \ucfirst(...),
+            $parts,
+        ))
+        ->map(fn($parts) => \implode('', $parts))
+        ->map(\lcfirst(...))
+        ->unwrap();
+
+    echo $value; // outputs "someValueToCamelize"
+    ```
+
+=== "Imperative"
+    ```php
+    $string = 'some value to camelize';
+    $parts = \explode(' ', $string);
+    $parts = \array_map(
+        \ucfirst(...),
+        $parts,
+    );
+    $string = \implode('', $parts);
+    $value = \lcfirst($string);
+
+    echo $value; // outputs "someValueToCamelize"
+    ```
+
+=== "Pyramid of doom"
+    ```php
+    $value = \lcfirst(
+        \implode(
+            '',
+            \array_map(
+                \ucfirst(...),
+                \explode(
+                    ' ',
+                    'some value to camelize',
+                ),
+            ),
+        ),
+    );
+
+    echo $value; // outputs "someValueToCamelize"
+    ```
+
+!!! abstract ""
+    In the end this monad does not provide any behaviour, it's a different way to write and read your code.

docs/structures/index.md

@@ -1,6 +1,6 @@
 # Structures
 
-This library provides the 10 following structures:
+This library provides the following structures:
 
 - [`Sequence`](sequence.md)
 - [`Set`](set.md)
@@ -10,6 +10,7 @@ This library provides the 10 following structures:
 - [`Maybe`](maybe.md)
 - [`Either`](either.md)
 - [`Validation`](validation.md)
+- [`Identity`](identity.md)
 - [`State`](state.md)
 - [`Fold`](fold.md)
 

docs/structures/sequence.md

@@ -572,6 +572,34 @@ $sequence = Sequence::ints(1, 2, 3, 4);
 $sequence->reverse()->equals(Sequence::ints(4, 3, 2, 1));
 ```
 
+### `->toIdentity()`
+
+This method wraps the sequence in an [`Identity` monad](identity.md).
+
+Let's say you have a sequence of strings representing the parts of a file and you want to build a file object:
+
+=== "Do"
+    ```php
+    $file = Sequence::of('a', 'b', 'c', 'etc...')
+        ->toIdentity()
+        ->map(Content::ofChunks(...))
+        ->map(static fn($content) => File::named('foo', $content))
+        ->unwrap();
+    ```
+
+=== "Instead of..."
+    ```php
+    $file = File::named(
+        'foo',
+        Content::ofChunks(
+            Sequence::of('a', 'b', 'c', 'etc...'),
+        ),
+    );
+    ```
+
+??? note
+    Here `Content` and `File` are imaginary classes, but you can find equivalent classes in [`innmind/filesystem`](https://packagist.org/packages/innmind/filesystem).
+
 ### `->safeguard()`
 
 This method allows you to make sure all values conforms to an assertion before continuing using the sequence.

mkdocs.yml

@@ -14,6 +14,7 @@ nav:
     - Maybe: structures/maybe.md
     - Either: structures/either.md
     - Validation: structures/validation.md
+    - Identity: structures/identity.md
     - State: structures/state.md
     - Fold: structures/fold.md
   - Monoids: MONOIDS.md

proofs/identity.php

@@ -0,0 +1,74 @@
+<?php
+declare(strict_types = 1);
+
+use Innmind\Immutable\Identity;
+use Innmind\BlackBox\Set;
+
+return static function() {
+    yield proof(
+        'Identity::unwrap()',
+        given(Set\Type::any()),
+        static fn($assert, $value) => $assert->same(
+            $value,
+            Identity::of($value)->unwrap(),
+        ),
+    );
+
+    yield proof(
+        'Identity::map()',
+        given(
+            Set\Type::any(),
+            Set\Type::any(),
+        ),
+        static fn($assert, $initial, $expected) => $assert->same(
+            $expected,
+            Identity::of($initial)
+                ->map(static function($value) use ($assert, $initial, $expected) {
+                    $assert->same($initial, $value);
+
+                    return $expected;
+                })
+                ->unwrap(),
+        ),
+    );
+
+    yield proof(
+        'Identity::flatMap()',
+        given(
+            Set\Type::any(),
+            Set\Type::any(),
+        ),
+        static fn($assert, $initial, $expected) => $assert->same(
+            $expected,
+            Identity::of($initial)
+                ->flatMap(static function($value) use ($assert, $initial, $expected) {
+                    $assert->same($initial, $value);
+
+                    return Identity::of($expected);
+                })
+                ->unwrap(),
+        ),
+    );
+
+    yield proof(
+        'Identity::map() and ::flatMap() interchangeability',
+        given(
+            Set\Type::any(),
+            Set\Type::any(),
+            Set\Type::any(),
+        ),
+        static fn($assert, $initial, $intermediate, $expected) => $assert->same(
+            Identity::of($initial)
+                ->flatMap(static fn() => Identity::of($intermediate))
+                ->map(static fn() => $expected)
+                ->unwrap(),
+            Identity::of($initial)
+                ->flatMap(
+                    static fn() => Identity::of($intermediate)->map(
+                        static fn() => $expected,
+                    ),
+                )
+                ->unwrap(),
+        ),
+    );
+};

proofs/sequence.php

@@ -0,0 +1,18 @@
+<?php
+declare(strict_types = 1);
+
+use Innmind\Immutable\Sequence;
+
+return static function() {
+    yield test(
+        'Sequence::toIdentity()',
+        static function($assert) {
+            $sequence = Sequence::of();
+
+            $assert->same(
+                $sequence,
+                $sequence->toIdentity()->unwrap(),
+            );
+        },
+    );
+};

src/Identity.php

@@ -0,0 +1,69 @@
+<?php
+declare(strict_types = 1);
+
+namespace Innmind\Immutable;
+
+/**
+ * @psalm-immutable
+ * @template T
+ */
+final class Identity
+{
+    /** @var T */
+    private mixed $value;
+
+    /**
+     * @param T $value
+     */
+    private function __construct(mixed $value)
+    {
+        $this->value = $value;
+    }
+
+    /**
+     * @psalm-pure
+     * @template A
+     *
+     * @param A $value
+     *
+     * @return self<A>
+     */
+    public static function of(mixed $value): self
+    {
+        return new self($value);
+    }
+
+    /**
+     * @template U
+     *
+     * @param callable(T): U $map
+     *
+     * @return self<U>
+     */
+    public function map(callable $map): self
+    {
+        /** @psalm-suppress ImpureFunctionCall */
+        return new self($map($this->value));
+    }
+
+    /**
+     * @template U
+     *
+     * @param callable(T): self<U> $map
+     *
+     * @return self<U>
+     */
+    public function flatMap(callable $map): self
+    {
+        /** @psalm-suppress ImpureFunctionCall */
+        return $map($this->value);
+    }
+
+    /**
+     * @return T
+     */
+    public function unwrap(): mixed
+    {
+        return $this->value;
+    }
+}

src/Sequence.php

@@ -591,6 +591,14 @@ public function toSet(): Set
         return $this->implementation->toSet();
     }
 
+    /**
+     * @return Identity<self<T>>
+     */
+    public function toIdentity(): Identity
+    {
+        return Identity::of($this);
+    }
+
     /**
      * @return list<T>
      */