1 files changed, 37 insertions, 0 deletions
diff --git a/stdlib/source/lux/control/contract.lux b/stdlib/source/lux/control/contract.lux
new file mode 100644
index 000000000..2f347dfa5
--- /dev/null
+++ b/stdlib/source/lux/control/contract.lux
@@ -0,0 +1,37 @@
+(;module:
+  lux
+  (lux (control monad)
+       (data text/format)
+       [compiler #+ Monad<Lux>]
+       (macro [ast]
+              ["s" syntax #+ syntax:])))
+
+(def: #export (assert! message test)
+  (-> Text Bool [])
+  (if test
+    []
+    (error! message)))
+
+(syntax: #export (@pre test expr)
+  {#;doc (doc "Pre-conditions."
+              "Given a test and an expression to run, only runs the expression if the test passes."
+              "Otherwise, an error is raised."
+              (@pre (i.= 4 (i.+ 2 2))
+                    (foo 123 456 789)))}
+  (wrap (list (` (exec (assert! (~ (ast;text (format "Pre-condition failed: " (%ast test))))
+                                (~ test))
+                   (~ expr))))))
+
+(syntax: #export (@post test expr)
+  {#;doc (doc "Post-conditions."
+              "Given a predicate and an expression to run, evaluates the expression and then tests the output with the predicate."
+              "If the predicate returns true, returns the value of the expression."
+              "Otherwise, an error is raised."
+              (@post i.even?
+                     (i.+ 2 2)))}
+  (do @
+    [g!output (compiler;gensym "")]
+    (wrap (list (` (let [(~ g!output) (~ expr)]
+                     (exec (assert! (~ (ast;text (format "Post-condition failed: " (%ast test))))
+                                    ((~ test) (~ g!output)))
+                       (~ g!output))))))))