added userlib reduce function

Signed-off-by: Ava Hahn <ava@sunnypup.io>

Readme.org

@@ -507,7 +507,6 @@ Note: this section only tracks the state of incomplete TODO items. Having everyt
 - Custom ast pretty print
 - Implement Compose for lambdas
   Document this in relevant readme sections
-- Userlib Reduce function
 - File operations
   - read-to-string
   - write-to-file

snippets/userlib-tests.rls

@@ -35,6 +35,7 @@
        (let ((test-val 0))
          (set (q test-val) 3)
          (eq? test-val 3))))
+
    ('prepend prepends to list'
      (quote
        (let ((list (2 3 4))
@@ -48,6 +49,12 @@
              (adder (lambda (x) (add 1 x))))
          (eq? (map adder list) (2 3 4)))))
 
+   ('reduce function adds numbers'
+     (quote
+       (let ((list (1 2 3))
+             (adder (lambda (x y) (add x y))))
+         (eq? (reduce adder list) (add 1 2 3)))))
+
      ;; add more test cases here
    ))
 

snippets/userlib.rls

@@ -64,3 +64,24 @@ result is added to a new list. Returns the new list.'
       (set (q result) (cons result current-res))
       (set (q list-iter) (pop remaining))))
   result))
+
+(def reduce
+'Takes two arguments: a function and a list.
+The function is expected to take two arguments:
+* the current list item
+* the previous result
+Initially the function will take element1 and element2, outputting result1.
+Then the function will take result1 and element3, outputting result2.
+this will continue iuntil the list is exhausted.'
+(func list)
+(let ((list-iter (pop list))
+      (result (car list-iter)))
+  (set (q list-iter) (pop (cdr list-iter)))
+  (if (lt? (len list) 2)
+      (echo "list too short!")
+      (while (gt? (len list-iter) 1)
+        (let ((current (car list-iter))
+              (remaining (cdr list-iter)))
+          (set (q result) (func result current))
+          (set (q list-iter) (pop remaining)))))
+  result))