Create typestate.md

SUMMARY.md

@@ -32,6 +32,7 @@
   - [RAII Guards](./patterns/RAII.md)
   - [Prefer Small Crates](./patterns/small-crates.md)
   - [Strategy](./patterns/strategy.md)
+  - [Typestate/Session Types](./patterns/typestate.md)
   - [Contain unsafety in small modules](./patterns/unsafe-mods.md)
   - [Visitor](./patterns/visitor.md)
 

patterns/typestate.md

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+# The Typestate Pattern
+
+## Description
+
+
+The typestate pattern is an API design pattern that encodes information about an object's run-time state in its compile-time type. In particular, an API using the typestate pattern will have:
+
+* Operations on an object (such as methods or functions) that are only available when the object is in certain states,
+
+* A way of encoding these states at the type level, such that attempts to use the operations in the wrong state fail to compile,
+
+* State transition operations (methods or functions) that change the type-level state of objects in addition to, or instead of, changing run-time dynamic state, such that the operations in the previous state are no longer possible.
+
+## Example
+
+This is one example of how to implement the pattern for a simple type that needs to be initialized or otherwise prepared before use 
+(that is, it has two states: (), and Ready). More states and operations may be added to implement a more complex state machine.
+
+
+```rust
+
+use std::marker::PhantomData;
+
+pub struct Ready;
+
+pub struct Thing<S = ()> {
+     // tracks state type info at compile time, optimized out for runtime.
+    marker: PhantomData<S>
+}
+
+// Private constructor to internally control what state the struct is in.
+fn state_constructor<S>() -> Thing<S> {
+    Thing { marker: PhantomData }
+}
+
+// Operations in our default state ()
+impl Thing {
+    pub fn new() -> Self {
+        Self { marker: PhantomData }
+    }
+
+    // Consumes the struct to return one with a new type state
+    pub fn get_ready(self) -> Thing<Ready> {
+        state_constructor::<Ready>()
+    }
+}
+
+// Operations available in any state
+impl<S> Thing<S> {
+    pub fn do_any_time(&self) {
+        println!("We can do this function whenever");
+    }
+}
+
+// We can only use this function when ready
+pub fn do_only_when_ready(_: Thing<Ready>) {
+    println!("We can only do this when we are Ready")
+}
+
+fn main() {
+    let thing = Thing::new();
+
+    // Not ready yet
+    thing.do_any_time();
+    // do_only_when_ready(thing); // this won't compile
+
+    // Transition to Ready
+    let ready = thing.get_ready();
+
+    // Now we're ready
+    ready.do_any_time();
+    do_only_when_ready(ready);
+}
+```
+
+
+## Motivation
+
+You are modelling a system that functions as a state machine, and want to ensure at compile-time that invalid states never occur in any runtime scenario.
+
+## Advantages
+
+
+* It moves certain types of errors from run-time to compile-time, giving programmers faster feedback.
+* It interacts nicely with IDEs, which can avoid suggesting operations that are illegal in a certain state.
+* It can eliminate run-time checks, making code faster/smaller.
+
+## Disadvantages
+
+* It can add some verbosity and complexity to the code.
+* Implementing it for complex structs can be difficult.
+* It can make compiler error messages very hard to understand.
+
+## See also
+
+  - [The Typestate Pattern in Rust, Cliff L. Biffle (2019)](https://web.archive.org/web/20210103081241/https://cliffle.com/blog/rust-typestate/)