mirror of https://github.com/rust-lang/rfcs.git
RFC 213 is Finalize defaulted type parameters
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Aaron Turon
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- Start Date: (fill me in with today's date, YYYY-MM-DD)
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- RFC PR: (leave this empty)
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- Rust Issue: (leave this empty)
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- Start Date: 2015-02-04
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- RFC PR: https://github.com/rust-lang/rfcs/pull/213
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- Rust Issue: https://github.com/rust-lang/rust/issues/21939
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# Summary
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@ -19,7 +19,7 @@ parameters:
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a suitable type variable will be used.
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- Outside of a fn body, only defaulted type parameters can be
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omitted, and the specified default is then used.
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Points 2 and 4 extend the current behavior of type parameter defaults,
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aiming to address some shortcomings of the current implementation.
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@ -34,7 +34,7 @@ Defaulted type parameters are very useful in two main scenarios:
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1. Extended a type without breaking existing clients.
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2. Allowing customization in ways that many or most users do not care
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about.
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Often, these two scenarios occur at the same time. A classic
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historical example is the `HashMap` type from Rust's standard
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library. This type now supports the ability to specify custom
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@ -58,7 +58,7 @@ a single `range()` function that is defined generically over all
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pub fn range<A:Enumerable>(start: A, stop: A) -> Range<A> {
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Range{state: start, stop: stop, one: One::one()}
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}
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This version is often more convenient to use, particularly in a
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generic context.
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@ -80,7 +80,7 @@ default (`uint`, for example):
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pub fn range<A:Enumerable=uint>(start: A, stop: A) -> Range<A> {
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Range{state: start, stop: stop, one: One::one()}
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}
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Using this definition, a call like `range(0, 10)` is perfectly legal.
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If it turns out that the type argument is not other constraint, `uint`
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will be used instead.
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@ -135,7 +135,7 @@ type parameters declared *before* `X` in the list of type parameters:
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// ERROR. Default value of `B` refers to `C`, which comes AFTER
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// `B` in the list of parameters.
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fn foo<A,B=C,C=uint>() { .. }
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## Instantiating defaults
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This section specifies how to interpret a reference to a generic
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@ -154,13 +154,13 @@ as follows:
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- *Motivation:* This is consistent with Rust tradition, which
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generally requires explicit types or a mechanical defaulting
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process outside of `fn` bodies.
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### References to generic types
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We begin with examples of references to the generic type `Foo`:
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struct Foo<A,B,C=DefaultHasher,D=C> { ... }
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`Foo` defines four type parameters, the final two of which are
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defaulted. First, let us consider what happens outside of a fn
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body. It is mandatory to supply explicit values for all non-defaulted
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@ -168,37 +168,37 @@ type parameters:
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// ERROR: 2 parameters required, 0 provided.
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fn f(_: &Foo) { ... }
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Defaulted type parameters are filled in based on the defaults given:
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// Legal: Equivalent to `Foo<int,uint,DefaultHasher,DefaultHasher>`
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fn f(_: &Foo<int,uint>) { ... }
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Naturally it is legal to specify explicit values for the defaulted
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type parameters if desired:
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// Legal: Equivalent to `Foo<int,uint,uint,char,u8>`
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fn f(_: &Foo<int,uint,char,u8>) { ... }
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It is also legal to provide just one of the defaulted type parameters
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and not the other:
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// Legal: Equivalent to `Foo<int,uint,char,char>`
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fn f(_: &Foo<int,uint,char>) { ... }
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If the user wishes to supply the value of the type parameter `D`
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explicitly, but not `C`, then `_` can be used to request the default:
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// Legal: Equivalent to `Foo<int,uint,DefaultHasher,uint>`
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fn f(_: &Foo<int,uint,_,uint>) { ... }
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Note that, outside of a fn body, `_` can *only* be used with
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defaulted type parameters:
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// ERROR: outside of a fn body, `_` cannot be
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// used for a non-defaulted type parameter
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fn f(_: &Foo<int,_>) { ... }
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Inside a fn body, the rules are much the same, except that `_` is
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legal everywhere. Every reference to `_` creates a fresh type
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variable `$n`. If the type parameter whose value is omitted has an
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@ -217,7 +217,7 @@ information). Here are some examples:
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let x: Foo<_,_> = ...;
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let x: Foo<_,_,_> = ...;
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let x: Foo<_,_,_,_> = ...;
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// Results in a type `Foo<int,uint,$0,char>` where `$0`
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// has a fallback of `DefaultHasher`.
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let x: Foo<int,uint,_,char> = ...;
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@ -248,7 +248,7 @@ except that it is legal to omit values for all type parameters if
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desired. In that case, the behavior is the same as it would be if `_`
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were used as the value for every type parameter. Note that functions
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can only be referenced from within a fn body.
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### References to generic impls
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Users never explicitly "reference" an impl. Rather, the trait matching
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@ -275,7 +275,7 @@ fn foo<A,B=A>(a: A, b: B) { ... }
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fn bar() {
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// Here, the values of the type parameters are given explicitly.
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let f: fn(uint, uint) = foo::<uint, uint>;
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// Here the value of the first type parameter is given explicitly,
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// but not the second. Because the second specifies a default, this
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// is permitted. The type checker will create a fresh variable `$0`
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@ -359,17 +359,17 @@ do not loop over each type variable, check whether it is unbound, and
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apply the default only if it is unbound. The reason for this is that
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it can happen that there are contradictory defaults and we want to
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ensure that this results in an error:
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fn foo<F:Default=uint>() -> F { }
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fn bar<B=int>(b: B) { }
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fn baz() {
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// Here, F is instantiated with $0=uint
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let x: $0 = foo();
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// Here, B is instantiated with $1=uint, and constraint $0 <: $1 is added.
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bar(x);
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}
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In this example, two type variables are created. `$0` is the value of
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`F` in the call to `foo()` and `$1` is the value of `B` in the call to
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`bar()`. The fact that `x`, which has type `$0`, is passed as an
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@ -395,17 +395,17 @@ defaults, and processing defaults created trait obligations:
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trait Foo { }
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trait Bar { }
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impl<T:Bar=uint> Foo for Vec<T> { } // Impl 1
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impl Bar for uint { } // Impl 2
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fn takes_foo<F:Foo>(f: F) { }
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fn main() {
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let x = Vec::new(); // x: Vec<$0>
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takes_foo(x); // adds oblig Vec<$0> : Foo
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}
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When we finish type checking `main`, we are left with a variable `$0`
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and a trait obligation `Vec<$0> : Foo`. Processing the trait
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obligation selects the impl 1 as the way to fulfill this trait
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@ -438,17 +438,17 @@ The next example does 2 full iterations of the loop.
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trait Foo { }
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trait Bar<U> { }
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trait Baz { }
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impl<U,T:Bar<U>=Vec<U>> Foo for Vec<T> { } // Impl 1
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impl<V=uint> Bar for Vec<V> { } // Impl 2
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fn takes_foo<F:Foo>(f: F) { }
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fn main() {
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let x = Vec::new(); // x: Vec<$0>
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takes_foo(x); // adds oblig Vec<$0> : Foo
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}
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Here the process is as follows:
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1. Trait resolution executed to resolve `Vec<$0> : Foo`. The result is
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