From 6b8be199ae88979e508f98fb8a8b6fc17c2a7f2d Mon Sep 17 00:00:00 2001
From: Rose Hogenson <rhogenson@posteo.net>
Date: Sun, 9 Jan 2022 13:17:20 -0800
Subject: Define helper intercalate.

---
 list-test.csc | 32 ++++++++++++++++++++++++++++++++
 list.csc      | 14 ++++++++++++--
 2 files changed, 44 insertions(+), 2 deletions(-)

diff --git a/list-test.csc b/list-test.csc
index ec4b4ed..e4981e5 100644
--- a/list-test.csc
+++ b/list-test.csc
@@ -125,3 +125,35 @@
           (unless (equal? got (want tc))
             (errorf t "(revappend {} {}) = {}, want {}." (a tc) (b tc) got (want tc)))))
       tests)))
+
+
+(define-test (test-intercalate t)
+  (define-record-type <test-case>
+    (test-case desc x l want)
+    test-case?
+    (desc desc)
+    (x x)
+    (l l)
+    (want want))
+  (let ((tests (list
+                 (test-case
+                   "simple"
+                   ","
+                   '("a" "b" "c")
+                   '("a" "," "b" "," "c"))
+                 (test-case
+                   "empty"
+                   ","
+                   '()
+                   '())
+                 (test-case
+                   "singleton"
+                   ","
+                   '(1)
+                   '(1)))))
+    (for-each
+      (lambda (tc)
+        (let ((got (intercalate (x tc) (l tc))))
+          (unless (equal? got (want tc))
+            (errorf t "(intercalate {} {}) = {}, want {}." (x tc) (l tc) got (want tc)))))
+      tests)))
diff --git a/list.csc b/list.csc
index c4bc013..799b070 100644
--- a/list.csc
+++ b/list.csc
@@ -1,9 +1,12 @@
 (define-library (csc list)
   (export
+    intercalate
     revappend
     split-at
     take)
-  (import (scheme base))
+  (import (scheme base)
+          (only (csc match)
+            match))
   (begin
 
 
@@ -30,4 +33,11 @@
                  (acc b))
         (if (null? xs)
             acc
-          (loop (cdr xs) (cons (car xs) acc)))))))
+          (loop (cdr xs) (cons (car xs) acc)))))
+
+
+    (define (intercalate x l)
+      (match l
+        ('() '())
+        ((_) l)
+        ((head . tail) (cons head (cons x (intercalate x tail))))))))
-- 
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