From 35fc7886539c423e0eebdfe4e16df457bd835d70 Mon Sep 17 00:00:00 2001
From: d-plaindoux <d.plaindoux@free.fr>
Date: Sun, 17 Sep 2023 20:09:25 +0200
Subject: [PATCH] Writer functor

---
 README.md              |  7 +++++--
 src/specs/mod.rs       |  2 ++
 src/specs/monoid.rs    | 11 +++++++++++
 src/specs/semigroup.rs | 10 ++++++++++
 src/standard/writer.rs | 16 ++++++++++++++++
 5 files changed, 44 insertions(+), 2 deletions(-)
 create mode 100644 src/specs/monoid.rs
 create mode 100644 src/specs/semigroup.rs

diff --git a/README.md b/README.md
index 2309e20..631d447 100644
--- a/README.md
+++ b/README.md
@@ -2,6 +2,8 @@
 
 Maitar is a rust library providing some functional programming constructions 
 like:
+- Semigroup,
+- Monoid,
 - Functor,
 - Applicative;
 - Bind and
@@ -12,8 +14,9 @@ Some incarnations are available like:
 - Option,
 - Result,
 - Either
-- Vector and
-- Reader
+- Vector,
+- Reader and
+- Writer.
 
 # Philosophy
 
diff --git a/src/specs/mod.rs b/src/specs/mod.rs
index ab75467..53cd9ae 100644
--- a/src/specs/mod.rs
+++ b/src/specs/mod.rs
@@ -8,3 +8,5 @@ pub mod applicative;
 pub mod bind;
 pub mod functor;
 pub mod monad;
+pub mod monoid;
+pub mod semigroup;
diff --git a/src/specs/monoid.rs b/src/specs/monoid.rs
new file mode 100644
index 0000000..20cedd1
--- /dev/null
+++ b/src/specs/monoid.rs
@@ -0,0 +1,11 @@
+/*
+ * MIT License
+ *
+ * Copyright (c) 2023 Didier Plaindoux
+ */
+
+use crate::specs::semigroup::Semigroup;
+
+pub trait Monoid: Semigroup {
+    fn neutral() -> Self::T;
+}
diff --git a/src/specs/semigroup.rs b/src/specs/semigroup.rs
new file mode 100644
index 0000000..e9f8e56
--- /dev/null
+++ b/src/specs/semigroup.rs
@@ -0,0 +1,10 @@
+/*
+ * MIT License
+ *
+ * Copyright (c) 2023 Didier Plaindoux
+ */
+
+pub trait Semigroup {
+    type T;
+    fn compose(a: Self::T, b: Self::T) -> Self::T;
+}
diff --git a/src/standard/writer.rs b/src/standard/writer.rs
index f824427..e2af7ef 100644
--- a/src/standard/writer.rs
+++ b/src/standard/writer.rs
@@ -5,7 +5,9 @@
  */
 
 use crate::core::hkp::HKP;
+use crate::specs::applicative::Applicative;
 use crate::specs::functor::Functor;
+use crate::specs::monoid::Monoid;
 use std::marker::PhantomData;
 
 pub struct Writer<'a, E: 'a, F: HKP<'a>, A: 'a>(F::T<(A, E)>);
@@ -25,3 +27,17 @@ impl<'e, E, F: Functor<'e>> Functor<'e> for WriterK<'e, E, F> {
         Writer(F::map(move |(a, e)| (f(a), e), va))
     }
 }
+
+impl<'e, E: Monoid<T = E>, F: Applicative<'e>> Applicative<'e> for WriterK<'e, E, F> {
+    fn pure<A>(a: A) -> Self::T<A> {
+        Writer(F::pure((a, E::neutral())))
+    }
+
+    fn apply<A, B, MAP>(mf: Self::T<MAP>, ma: Self::T<A>) -> Self::T<B>
+    where
+        A: Clone,
+        MAP: Fn(A) -> B,
+    {
+        todo!()
+    }
+}