From 959ead049e6f22438902399eb0925e8f60e0f3e0 Mon Sep 17 00:00:00 2001
From: Masaya Tojo <masaya@tojo.tokyo>
Date: Sat, 15 Jun 2024 01:47:35 +0900
Subject: Add `member?` function

---
 algebraic-structures.foldable.scm | 17 +++++++++++++++--
 tests/run.scm                     |  6 ++++--
 2 files changed, 19 insertions(+), 4 deletions(-)

diff --git a/algebraic-structures.foldable.scm b/algebraic-structures.foldable.scm
index be3d615..25c66bb 100644
--- a/algebraic-structures.foldable.scm
+++ b/algebraic-structures.foldable.scm
@@ -3,10 +3,11 @@
      length
      count
      any
-     every)
+     every
+     member?)
   (import (except scheme length)
           F
-          (only (chicken base) add1 call/cc))
+          (only (chicken base) add1 call/cc assert))
 
   (define (length xs)
     (fold (lambda (_ acc) (add1 acc))
@@ -36,4 +37,16 @@
        (fold (lambda (e acc)
                (or (pred e) (return #f)))
              #t
+             xs))))
+
+  (define (member? x xs #!optional (= equal?))
+    (call/cc
+     (lambda (return)
+       (fold (lambda (e _)
+               (if (= e x)
+                   (return #t)
+                   #f))
+             #f
              xs)))))
+
+
diff --git a/tests/run.scm b/tests/run.scm
index d316f75..be0845b 100644
--- a/tests/run.scm
+++ b/tests/run.scm
@@ -107,8 +107,10 @@
 (test #f (list:every (cut member 'x <>) '((a b c) (d x f))))
 (test '(x f) (list:every (cut member 'x <>) '((a x c) (d x f))))
 
-(test 7 (list:maximum '(1 3 7 5) <))
-(test -3 (list:minimum '(4 -3 0 1 7 8) <))
+(test #t (list:member? 3 '(1 3 7 5) =))
+(test #f (list:member? 3 '(1 7 5) =))
+(test #t (list:member? 'c '(a b c) eq?))
+(test #f (list:member? 'c '(a b) eq?))
 
 (test-end "foldable")
 
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