From ff60d0d3daed364588187b523a903a9abbdc2a4a Mon Sep 17 00:00:00 2001
From: Mike Cifelli <michael.cifelli@gmail.com>
Date: Wed, 24 Jan 2018 20:28:29 -0500
Subject: [PATCH] Add MAP and FLAT-MAP functions

---
 lisp/lang/functions.lisp | 101 +++++++++++++++++++++++++--------------
 1 file changed, 64 insertions(+), 37 deletions(-)

diff --git a/lisp/lang/functions.lisp b/lisp/lang/functions.lisp
index d00dffc..d222b99 100644
--- a/lisp/lang/functions.lisp
+++ b/lisp/lang/functions.lisp
@@ -1,39 +1,66 @@
-(defun mapcar (function-name the-list)
-  (if the-list
-    (cons
-      (funcall function-name (first the-list))
+(defun mapcar (function-name the-list)
+  (if the-list
+    (cons
+      (funcall function-name (first the-list))
       (mapcar function-name (rest the-list)))))
 
-(defun maplist (function-name the-list)
-  (cond
-   ((null the-list) nil)
-   (t (cons (funcall function-name the-list)
-       (maplist function-name (rest the-list))))))
-
-(defun append (list-one list-two)
-  (append-tail (reverse list-one) list-two))
-
-(defun append-tail (reversed-list list-two)
-  (if (null reversed-list) list-two
-    (recur (rest reversed-list) (cons (car reversed-list) list-two))))
-
-(defun reverse (the-list)
-  (reverse-tail () the-list))
-
-(defun reverse-tail (accumulator the-list)
-  (if (null the-list) accumulator
-    (recur (cons (first the-list) accumulator) (rest the-list))))
-
-(defun deep-reverse (the-list)
-  (if the-list
-    (append
-      (deep-reverse (rest the-list))
-      (list
-        (if (listp (first the-list))
-          (deep-reverse (first the-list))
-          (first the-list))))))
-
-(defun nth (n listA)
-  (cond
-   ((equal 0 n) (first listA))
-   (t (nth (- n 1) (rest listA)))))
+(defun maplist (function-name the-list)
+  (cond
+   ((null the-list) nil)
+   (t (cons (funcall function-name the-list)
+       (maplist function-name (rest the-list))))))
+
+(defun map (function the-list)
+  (reverse (map-tail function the-list nil)))
+
+(defun map-tail (function the-list accumulator)
+  (if the-list
+    (recur
+      function
+      (rest the-list)
+      (cons (call function (first the-list)) accumulator))
+    accumulator))
+
+(defun flat-map (function the-list)
+  (reverse (flat-map-tail function the-list nil)))
+
+(defun flat-map-tail (function the-list accumulator)
+  (if the-list
+    (recur
+      function
+      (rest the-list)
+      (append
+        (let ((result (call function (first the-list))))
+          (if (list? result)
+            (reverse result)
+            (list result)))
+        accumulator))
+    accumulator))
+
+(defun append (list-one list-two)
+  (append-tail (reverse list-one) list-two))
+
+(defun append-tail (reversed-list list-two)
+  (if (null reversed-list) list-two
+    (recur (rest reversed-list) (cons (car reversed-list) list-two))))
+
+(defun reverse (the-list)
+  (reverse-tail () the-list))
+
+(defun reverse-tail (accumulator the-list)
+  (if (null the-list) accumulator
+    (recur (cons (first the-list) accumulator) (rest the-list))))
+
+(defun deep-reverse (the-list)
+  (if the-list
+    (append
+      (deep-reverse (rest the-list))
+      (list
+        (if (listp (first the-list))
+          (deep-reverse (first the-list))
+          (first the-list))))))
+
+(defun nth (n listA)
+  (cond
+   ((equal 0 n) (first listA))
+   (t (nth (- n 1) (rest listA)))))