From 68ecc10a0948aea9b7fb8db56cda270b48bf8177 Mon Sep 17 00:00:00 2001
From: Masaya Tojo <masaya@tojo.tokyo>
Date: Mon, 10 Jun 2024 00:59:14 +0900
Subject: Add monad examples

---
 examples/optional.scm | 71 +++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 71 insertions(+)
 create mode 100644 examples/optional.scm

(limited to 'examples/optional.scm')

diff --git a/examples/optional.scm b/examples/optional.scm
new file mode 100644
index 0000000..e3f2eec
--- /dev/null
+++ b/examples/optional.scm
@@ -0,0 +1,71 @@
+(module (data optional) (<some> some some? some-value <none> none none?)
+  (import scheme
+          (chicken base)
+          (chicken format))
+
+  (define-record-type <some>
+    (some value)
+    some?
+    (value some-value))
+
+  (set! (record-printer <some>)
+    (lambda (x out)
+      (fprintf out "#<(some ~S)>" (some-value x))))
+
+  (define-record-type <none>
+    (none)
+    none?)
+
+  (set! (record-printer <none>)
+    (lambda (_ out)
+      (fprintf out "#<(none)>"))))
+
+(module (data optional monad base) (pure map map2 >>=)
+  (import (except scheme map)
+          (prefix (data optional) opt:)
+          matchable)
+
+  (define (map f opt)
+    (match opt
+      [($ opt:<some> x) (opt:some (f x))]
+      [($ opt:<none>) (opt:none)]))
+
+  (define (pure x)
+    (opt:some x))
+
+  (define (map2 f opt1 opt2)
+    (match opt1
+      [($ opt:<some> x)
+       (match opt2
+         [($ opt:<some> y) (opt:some (f x y))]
+         [($ opt:<none>) (opt:none)])]
+      [($ opt:<none>) (opt:none)]))
+
+  (define (>>= opt f)
+    (match opt
+      [($ opt:<some> x) (f x)]
+      [($ opt:<none>) (opt:none)])))
+
+(import (only (algebraic-structs functor make))
+        (only (algebraic-structs applicative make))
+        (only (algebraic-structs monad make)))
+
+(module (data optional functor) = ((algebraic-structs functor make) (data optional monad base)))
+(module (data optional applicative) = ((algebraic-structs applicative make) (data optional monad base)))
+(module (data optional monad) = ((algebraic-structs monad make) (data optional monad base)))
+
+(import (prefix (data optional) opt:)
+        (prefix (data optional functor) opt:)
+        (prefix (data optional applicative) opt:)
+        (prefix (data optional monad) opt:))
+
+;; (opt:map (lambda (x) (* x x)) (opt:pure 5)) => (some 25)
+;; (opt:map (lambda (x) (* x x)) (opt:none)) => (none)
+
+;; (opt:map* + (opt:pure 1) (opt:pure 2) (opt:pure 3)) => (some 6)
+;; (opt:map* + (opt:pure 1) (opt:none) (opt:pure 3)) => (none)
+
+;; (opt:do (x <- (opt:pure 3))
+;;         (y <- (opt:pure 4))
+;;         (opt:pure (+ x y)))
+;; => (some 7)
-- 
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