From cbb00fb43701ef7e78655b99b67b3a5a5dfec7ea Mon Sep 17 00:00:00 2001
From: Chris Rink <chrisrink10@gmail.com>
Date: Mon, 9 Dec 2024 09:56:56 -0500
Subject: [PATCH 1/2] Add support for pre- and post-conditions on fn forms

---
 CHANGELOG.md          |  1 +
 src/basilisp/core.lpy | 20 ++++++++++++++++++++
 2 files changed, 21 insertions(+)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 424ed166..bbfec162 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased]
 ### Added
  * Added support for the optional `attr-map?` on the `ns` macro (#1159)
+ * Added support for the optional pre- and post-conditions in `fn` forms (#1167)
 
 ### Fixed
  * Fix a bug where `#` characters were not legal in keywords and symbols (#1149)
diff --git a/src/basilisp/core.lpy b/src/basilisp/core.lpy
index d8e34312..7684b22b 100644
--- a/src/basilisp/core.lpy
+++ b/src/basilisp/core.lpy
@@ -5940,6 +5940,25 @@
   (let [args         (first body)
         arg-vec-meta (meta args)
         body         (rest body)
+        conditions   (when (and (next body) (map? (first body)))
+                       (first body))
+        body         (if conditions
+                       (rest body)
+                       body)
+
+        conditions (or conditions arg-vec-meta)
+        pre-conds  (:pre conditions)
+        post-conds (:post conditions)
+        body       (if post-conds
+                     [`(let [~'% (do ~@body)]
+                         ~@(map (fn* [c] `(assert ~c)) post-conds)
+                         ~'%)]
+                     body)
+        body       (if pre-conds
+                     (concat
+                      (map (fn* [c] `(assert ~c)) pre-conds)
+                      body)
+                     body)
 
         arg-groups (split-with (partial not= '&) args)
         args       (first arg-groups)
@@ -5964,6 +5983,7 @@
                       (filter #(not= :symbol (:type %)))
                       (mapcat destructure-binding)
                       (concat rest-binding))
+
         new-body (if (seq bindings)
                    [`(let* [~@bindings]
                        ~@body)]

From 3bae58dd5ce3dd7f277a18a19153df70f0156293 Mon Sep 17 00:00:00 2001
From: Chris Rink <chrisrink10@gmail.com>
Date: Mon, 9 Dec 2024 10:23:11 -0500
Subject: [PATCH 2/2] Tests

---
 tests/basilisp/test_core_macros.lpy | 98 +++++++++++++++++++++++++++++
 1 file changed, 98 insertions(+)

diff --git a/tests/basilisp/test_core_macros.lpy b/tests/basilisp/test_core_macros.lpy
index 68685902..2926f1e6 100644
--- a/tests/basilisp/test_core_macros.lpy
+++ b/tests/basilisp/test_core_macros.lpy
@@ -1795,6 +1795,104 @@
                         (conj accum [a b c d e f v]))
                  [coll accum])))))))
 
+(deftest fn-pre-and-post-condition-test
+  (testing "fn with map as body member does not count as condition map"
+    (let [f (fn [v]
+              {:pre  [v]
+               :post [:nothin]})]
+      (is (= {:pre [1] :post [:nothin]} (f 1)))))
+
+  (testing "pre-condition only"
+    (testing "empty pre-condition vec"
+      (let [f (fn [x]
+                {:pre []}
+                (* x x))]
+        (is (= 1 (f 1)))
+        (is (= 100 (f 10)))
+        (is (= 0 (f 0)))
+        (is (= 25 (f -5)))))
+
+    (testing "single pre-condition"
+      (let [f (fn [x]
+                {:pre [(pos? x)]}
+                (* x x))]
+        (is (= 1 (f 1)))
+        (is (= 100 (f 10)))
+        (is (thrown? python/AssertionError
+                     (f 0)))
+        (is (thrown? python/AssertionError
+                     (f -5)))))
+
+    (testing "multiple pre-condition"
+      (let [f (fn [x]
+                {:pre [(pos? x) (even? x)]}
+                (* x x))]
+        (is (= 4 (f 2)))
+        (is (= 100 (f 10)))
+        (is (thrown? python/AssertionError
+                     (f 1)))
+        (is (thrown? python/AssertionError
+                     (f 0)))
+        (is (thrown? python/AssertionError
+                     (f -5)))
+        (is (thrown? python/AssertionError
+                     (f -6))))))
+
+  (testing "post-condition only"
+    (testing "empty post-condition vec"
+      (let [f (fn [x]
+                {:post []}
+                (* x x))]
+        (is (= 1 (f 1)))
+        (is (= 100 (f 10)))
+        (is (= 0 (f 0)))
+        (is (= 25 (f -5)))))
+
+    (testing "single post-condition"
+      (let [f (fn [x]
+                {:post [(> % 16)]}
+                (* x x))]
+        (is (= 25 (f 5)))
+        (is (= 100 (f 10)))
+        (is (thrown? python/AssertionError
+                     (f 1)))
+        (is (thrown? python/AssertionError
+                     (f -3)))))
+
+    (testing "multiple post-condition"
+      (let [f (fn [x]
+                {:post [(> % 16) (< % 225)]}
+                (* x x))]
+        (is (= 25 (f 5)))
+        (is (= 25 (f -5)))
+        (is (= 196 (f 14)))
+        (is (thrown? python/AssertionError
+                     (f 1)))
+        (is (thrown? python/AssertionError
+                     (f 0)))
+        (is (thrown? python/AssertionError
+                     (f 15)))
+        (is (thrown? python/AssertionError
+                     (f 100))))))
+
+  (testing "pre- and post-conditions"
+    (let [f (fn [x]
+              {:pre  [(pos? x)]
+               :post [(> % 16) (< % 225)]}
+              (* x x))]
+      (is (= 25 (f 5)))
+      (is (= 196 (f 14)))
+      (is (thrown? python/AssertionError
+                   (f 1)))
+      (is (thrown? python/AssertionError
+                   (f 0)))
+      (is (thrown? python/AssertionError
+                   (f 15)))
+      (is (thrown? python/AssertionError
+                   (f 100)))
+      (is (thrown? python/AssertionError
+                   (f -5))))))
+
 (defmacro ^:private variadic-fn []
   `(fn [& r#] r#))