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... 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))))
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