Added an abstraction for interest rates.

1 files changed, 63 insertions, 0 deletions

diff --git a/stdlib/source/library/lux/world/finance/interest/rate.lux b/stdlib/source/library/lux/world/finance/interest/rate.lux
new file mode 100644
index 000000000..61fd9f9a5
--- /dev/null
+++ b/stdlib/source/library/lux/world/finance/interest/rate.lux
@@ -0,0 +1,63 @@
+... This Source Code Form is subject to the terms of the Mozilla Public License, v. 2.0.
+... If a copy of the MPL was not distributed with this file, You can obtain one at https://mozilla.org/MPL/2.0/.
+
+(.require
+ [library
+  [lux (.except)
+   [abstract
+    [monoid (.only Monoid)]]
+   [control
+    [function
+     [predicate (.only Predicate)]]]
+   [data
+    [text
+     ["%" \\format]]]
+   [math
+    [number
+     ["n" nat]
+     ["f" frac]]]]]
+ [///
+  ["[0]" money (.only Money)]])
+
+... https://en.wikipedia.org/wiki/Interest_rate
+(type .public Rate
+  Frac)
+
+... https://en.wikipedia.org/wiki/Break-even
+(def .public break_even
+  Rate
+  +1.0)
+
+... https://en.wikipedia.org/wiki/Compound_interest
+(def .public compound
+  (-> Rate Rate
+      Rate)
+  f.*)
+
+(with_template [<order> <name>]
+  [(def .public <name>
+     (Predicate Rate)
+     (<order> ..break_even))]
+
+  [f.< loss?]
+  [f.> gain?]
+  [f.= break_even?]
+  )
+
+(def .public monoid
+  (Monoid Rate)
+  (implementation
+   (def identity ..break_even)
+   (def composite ..compound)))
+
+(def .public format
+  (%.Format Rate)
+  (|>> (f.- ..break_even)
+       %.percentage))
+
+(def .public (rate before after)
+  (All (_ $)
+    (-> (Money $) (Money $)
+        Rate))
+  (f./ (n.frac (money.amount before))
+       (n.frac (money.amount after))))