Generic closures

text/0000-generic-closures.md

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+- Feature Name: generic_closure
+- Start Date: 2015-06-15
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+[summary]: #summary
+
+This RFC adds the ability to define closures that are generic over types.
+
+# Motivation
+[motivation]: #motivation
+
+Generic closures can be used to support compound operations on tuple types:
+
+```rust
+#[derive(Copy, Clone, Eq, PartialEq, Debug)]
+struct Tuple<A, B, C>(pub A, pub B, pub C);
+
+impl<A, B, C> Tuple<A, B, C> {
+    fn map<A2, B2, C2, F>(self, mut f: F) -> Tuple<A2, B2, C2>
+        where F: FnMut(A) -> A2 + FnMut(B) -> B2 + FnMut(C) -> C2
+    {
+        Tuple(f(self.0), f(self.1), f(self.2))
+    }
+
+    fn fold<T, F>(self, val: T, mut f: F) -> T
+        where F: FnMut(T, A) -> T + FnMut(T, B) -> T + FnMut(T, C) -> T
+    {
+        let val = f(val, self.0);
+        let val = f(val, self.1);
+        let val = f(val, self.2);
+        val
+    }
+}
+
+let a = Tuple(1u8, 2u32, 3.5f32).map(<T: Into<f64>>|x: T| x.into() + 1.0);
+assert_eq!(a, (2f64, 3f64, 4.5f64));
+
+let b = Tuple(1u8, 2u32, 3.5f32).fold(10.0, <T: Into<f64>>|x, y: T| x + y.into());
+assert_eq!(b, 16.5f64);
+```
+
+A fully working example of this code (with manually implemented closures) can be found [here](https://play.rust-lang.org/?gist=ea867336945253752e31873fc752ec06&version=nightly&backtrace=0).
+
+# Detailed design
+[design]: #detailed-design
+
+## Syntax
+
+There are two ways to specify generic bounds on closures:
+
+```rust
+<T: Debug>|x: T| println!("{:?}", x);
+
+<T>|x: T| where T: Debug {
+    println!("{:?}", x);
+}
+```
+
+When using the `where` syntax, the braces around the closure body are mandatory.
+
+If the `move` keyword is used then it must appear before the generic parameter list:
+
+```rust
+move <T: Debug>|x: T| println!("{:?}", x);
+
+move <T>|x: T| where T: Debug {
+    println!("{:?}", x);
+}
+```
+
+All generic parameters must be used in the closure argument list. This is necessary to ensure that the closure can implement all the required `Fn` traits.
+
+## Implementation
+
+The generated closure type will have generic implementations of `Fn`, `FnMut` and `FnOnce` with the provided type bounds. This is similar to the way closures currently have generic implementations over lifetimes.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+Increased language complexity.
+
+# Alternatives
+[alternatives]: #alternatives
+
+If the given syntax is determined to be ambiguous, this one can be used instead:
+
+```rust
+for<T: Debug>|x: T| println!("{:?}", x);
+
+for<T>|x: T| where T: Debug {
+    println!("{:?}", x);
+}
+```
+
+We could just not add this, however it would make generic operations on tuples less ergonomic. This feature is going to be even more useful when variadic generics are added in the future.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+What are the syntax interactions of `move` generic closures with the proposed `&move` reference type?
+
+Is the syntax in this RFC ambiguous for the parser?