Composing structs

README.md

@@ -42,6 +42,7 @@ language.
 * TODO composition of structs to please the borrow checker
 * TODO `Error` traits and `Result` forwarding
 * TODO graphs
+* [Compose structs together for better borrowing](patterns/compose-structs.md)
 
 
 

patterns/compose-structs.md

@@ -0,0 +1,101 @@
+# Compose structs together for better borrowing
+
+TODO - this is not a very snappy name
+
+## Description
+
+Sometimes a large struct will cause issues with the borrow checker - although
+fields can be borrowed independently, sometimes the whole struct ends up being
+used at once, preventing other uses. A solution might be to decompose the struct
+into several smaller structs. Then compose these together into the original
+struct. Then each struct can be borrowed separately and have more flexible
+behaviour.
+
+This will often lead to a better design in other ways: applying this design
+pattern often reveals smaller units of functionality.
+
+
+## Example
+
+Here is a contrived example of where the borrow checker foils us in our plan to
+use a struct:
+
+```rust
+struct A {
+    f1: u32,
+    f2: u32,
+    f3: u32,
+}
+
+fn foo(a: &mut A) -> &u32 { &a.f2 }
+fn bar(a: &mut A) -> u32 { a.f1 + a.f3 }
+
+fn main(a: &mut A) {
+    // x causes a to be borrowed for the rest of the function.
+    let x = foo(a);
+    // Borrow check error
+    let y = bar(a); //~ ERROR: cannot borrow `*a` as mutable more than once at a time
+}
+```
+
+We can apply this design pattern and refactor `A` into two smaller structs, thus
+solving the borrow checking issue:
+
+```
+// A is now composed of two structs - B and C.
+struct A {
+    b: B,
+    c: C,
+}
+struct B {
+    f2: u32,
+}
+struct C {
+    f1: u32,
+    f3: u32,
+}
+
+// These functions take a B or C, rather than A.
+fn foo(b: &mut B) -> &u32 { &b.f2 }
+fn bar(c: &mut C) -> u32 { c.f1 + c.f3 }
+
+fn main(a: &mut A) {
+    let x = foo(&mut a.b);
+    // Now it's OK!
+    let y = bar(&mut a.c);
+}
+```
+
+
+## Motivation
+
+Why and where you should use the pattern
+
+
+## Advantages
+
+Lets you work around limitations in the borrow checker.
+
+Often produces a better design.
+
+
+## Disadvantages
+
+Leads to more verbose code.
+
+Sometimes, the smaller structs are not good abstractions, and so we end up with
+a worse design. That is probably a 'code smell', indicating that the program
+should be refactored in some way.
+
+
+## Discussion
+
+This pattern is not required in languages that don't have a borrow checker, so
+in that sense is unique to Rust. However, making smaller units of functionality
+often leads to cleaner code: a widely acknowledged principle of software
+engineering, independent of the language.
+
+This pattern relies on Rust's borrow checker to be able to borrow fields
+independently of each other. In the example, the borrow checker knows that `a.b`
+and `a.c` are distinct and can be borrowed independently, it does not try to
+borrow all of `a`, which would make this pattern useless.