+reduce

orsinium · orsinium · commit f33de7eff86c · 2019-09-03T15:39:22.000+02:00
diff --git a/lambdas.py b/lambdas.py
@@ -144,6 +144,22 @@
     (lambda _: PREPEND(f(INC(a))(b))(a))
     (TRUE)
 )
+# REDUCE(r)(l)(v):
+# 1. Apply pass head of `l` and `v` into `r` and save result into `v`.
+# 2. Do it for every element into lest from left to right.
+# 3. Return `v` (accumulated value)
+REDUCE = LFOLD = Y(
+    lambda f: lambda r: lambda l: lambda v:
+    EMPTY(l)
+    (lambda _: v)  # if list is empty, return accumulated value (v)
+    (
+        lambda _: (
+            f(r)(TAIL(l))  # do reucing on tail of list (l) with a new accumulated value (v)
+            (r(HEAD(l))(v))  # pass accumulated value (v) and head into reducer (r)
+        )
+    )
+    (TRUE)
+)
 
 
 # helpers
diff --git a/test_lambdas.py b/test_lambdas.py
@@ -9,7 +9,7 @@
 from lambdas import CONS, CAR, CDR
 from lambdas import SIGN, UNSIGN, NEG, ISPOS, ISNEG, SADD, SSUB, SMUL
 from lambdas import FAC, FIB
-from lambdas import LIST, APPEND, PREPEND, REVERSE, MAP, RANGE
+from lambdas import LIST, APPEND, PREPEND, REVERSE, MAP, RANGE, REDUCE
 from lambdas import decode_number, decode_list
 
 
@@ -328,6 +328,19 @@ def test_reverse(given):
     assert decode_list(REVERSE(lst)) == given
 
 
+@pytest.mark.parametrize('given, expected', [
+    ([], 0),
+    ([ONE], 1),
+    ([ONE, TWO], 3),
+    ([ONE, TWO, THREE], 6),
+])
+def test_reduce(given, expected):
+    lst = LIST
+    for el in given:
+        lst = PREPEND(lst)(el)
+    assert decode_number(REDUCE(ADD)(lst)(ZERO)) == expected
+
+
 @pytest.mark.parametrize('given, expected', [
     ([], []),
     ([ONE], [3]),
