From cc07c82fefb2affc1772e12b8357471cccc8d578 Mon Sep 17 00:00:00 2001
From: Aleksey Kladov <aleksey.kladov@gmail.com>
Date: Mon, 8 Jun 2020 12:54:48 +0200
Subject: Preconditions style

---
 docs/dev/README.md | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

(limited to 'docs/dev/README.md')

diff --git a/docs/dev/README.md b/docs/dev/README.md
index 64d595b68..5a9c0a148 100644
--- a/docs/dev/README.md
+++ b/docs/dev/README.md
@@ -241,6 +241,26 @@ struct Foo {
 For `.md` and `.adoc` files, prefer a sentence-per-line format, don't wrap lines.
 If the line is too long, you want to split the sentence in two :-)
 
+## Preconditions
+
+Function preconditions should generally be expressed in types and provided by the caller (rather than checked by callee):
+
+```rust
+// Good
+fn frbonicate(walrus: Walrus) {
+  ...
+}
+
+// Not as good
+fn frobnicate(walrus: Option<Walrus>) {
+  let walrus = match walrus {
+    Some(it) => it,
+    None => return,
+  };
+  ...
+}
+```
+
 # Architecture Invariants
 
 This section tries to document high-level design constraints, which are not
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