9.6 KiB
Index and Range Changes
Summary
This proposes to make several changes to the index and range design based on customer feedback: particularly from the CoreFX team and their experiences adding Index / Range support to .NET Core. The change is not to the syntax but rather how the langauge maps the syntax to APIs.
Motivation
The level of API churn necessary to adopt Index
and Range
is quite high today. Every collection type must have at
least two new indexers added for Index
and Range
respectively. The implementation of Index
is exactly the same
for virtually every collection in .NET. The implementation for Range
is likewise often similar by simply deferring to
the underlying collection type.
This work must be done by the collection author. The reliance on indexers as the primary use case means developers can't update existing collections using extension methods. This means the adoption of the feature will be slowed by waiting for library authors to update their types.
The use of Index
does come with a small performance hit. Typically a consumer of Index
will pay an extra branch
check (IsFromEnd
and bounds) as they would with a int
parameter (bounds only). Moving to Range
doubles this cost
as it occurs on each Index
.
This penalty is small and insignificant in the vast majority of code. For highly optimized code though this extra
branching can be unacceptable. It simply can't be thought of as a syntactic transformation from a call to
span.Slice(start, end)
to span[start..end]
. Instead the performance implications must be evaluated on each
usage. This will likely lead to the feature being banned in certain portions of code bases.
This proposals seeks to remove the abstraction penalties around Index
and Range
: both at an API and performance
layer. The goal being that most collection types just work today with minimal change and the feature can be adopted
in existing code without fear of lost performance.
This proposal specifically does not want to change how Index
and Range
type binding occurs. The index and range
expressions continue to have the same syntax and types.
Detailed Design
Indexable types
Any type which has an accessible getter property named Length
or Count
with a return type of int
is considered
Indexable. The language can make use of this property to convert an index expression into an int
at the point of
the expression without the need to use the type Index
at all.
Note: For simplicity going forward the proposal will use the name Length
to represent Count
or Length
.
For example it allows the following simplication:
Span<char> span = ...;
char c = span[^1];
// Can be translated to
Span<char> span = ...;
char c = span[span.Length - 1];
Transforming an index expression to an int
at the call site significantly reduces the burden of frameworks to adopt
Index
. Vitrually any collection type will automatically work with Index
now as the compiler can translate it to
int
in all cases.
Further this can improve performance by eliminating extra branching. The callee when accepting an Index
parameter must
do both test to see if the value is from the end, Index.IsFromEnd
, and if the value is inside the bounds of the
collection. While a small check this can be important in performance sensitive areas.
Doing the translation to int
at the call site means the IsFromEnd
check can often be eliminated. For example when
dealing with an int
the compiler can pass the value through. Or in the cases where ^
is used the computation from
end can be done directly without the additional branching.
Index and Range implementations are known
The implementations of Index
and Range
are considered to be known and side effect free. Much like
ValueTuple<T1, T2>
the language can assume a standard implementation and emit code inline which represents the
implementation of methods on Index
and Range
. This implementation includes methods like GetOffset
or conversions
like the implicit conversion from int
to Index
.
All arithemtic operations which are emitted will be done so using an unchecked
context. That matches the context
in which Index
and Range
are compiled in.
Index target type conversion
Whenever an expression with type Index
is used as an argument to an instance member invocation and the receiver is
Indexable then the expression will have a target type conversion to int
. The member invocations applicable for this
conversion include methods, indexers, properties, extension methods, etc ... Only constructors are excluded as they
have no receiver.
The target type conversion will be implemented as follows on the index expression:
- When the expression is
^expr
and the type isint
it will be translated toreceiver.Length - expr
. - Else it will be translated as
expr.GetOffset(receiver.Length)
whereexpr
is the expression typed asIndex
.
The receiver will be spilled as appropriate to ensure he side effects of obtaining the receiver are only executed once. For example:
class SideEffect {
int[] Get() {
Console.Write("Get ");
return new [] { 1, 2 , 3};
}
void Use() {
int i = Get()[^1]
Console.WriteLine(i);
}
}
This code will print "Get 3".
When a range expression is used as an argument to an instance member invocation then the target type conversion to
int
extends to both Index
operands. In the case either of the Index
members are omitted then the the appropriate
start or end value will be inserted using 0
or receiver.Length
as appropriate.
class RangeTargettype {
void Example() {
var array = new[] { 1, 2, 3 };
Console.WriteLine(array[1..]);
// Becomes
Console.WriteLine(array[new Range(1, array.Length));
}
}
Indexing on Range
When binding an index member on a Indexable type where the single argument is of type Range
the language will
attempt to translate it to a Slice
call. The arguments to slice will be both Index
values of the range converted
to Index
using their target typed conversion described in the previous section. If this translation is not succesful
then normal index binding will occur.
class Collection {
public int Length { get; }
public int[] this[Range range] => ...;
}
class Slice {
void Example(Span<int> span, Collection collection) {
Span<int> slicedSpan = span[2..]
int[] slicedCollection = collection[2..];
// Translated to
Span<int> slicedSpan = span.Slice(2, span.Length);
int[] slicedCollection = collection[new Range(2, collection.Count)v;
}
}
The Slice
method can instance or extension so long as it is accessible has types that are convertible from int
.
The compiler will special case the following receiver types binding to Slice
:
string
: instead ofSlice
the methodSubstring
will be used.- array: the runtime helper for array slicing will be used.
Open Issues
Considerations
Detect Indexable based on ICollection
The inspiration for this proposal was collection initializers. Using the structure of a type to convey that it had
opted into a feature. In the case of collection initializers types can opt into the feature by implementing the
interface IEnumerable
(non generic).
Initially this proposal required that types implement ICollection
in order to qualify as Indexable. That though
required a number of special cases:
ref struct
: these cannot implement interfaces yet types likeSpan<T>
are ideal for index / range support.string
: does not implementICollection
and adding thatinterface
has a large cost.
This means to support key types special casing is already needed. The special casing of string
is less interesting
as the language does this in other areas (foreach
lowering, constants, etc ...). The special casing of ref struct
is more concerning as it's special casing an entire class of types. The get labeled as Indexable if they simply have
a property named Count
with a return type of int
.
After consideration the design was normalized to say that any type which has a property Count
/ Length
with a
return type of int
is Indexable. That removes all special casing, even for string
and arrays.
Detect just Count
Detecting on the property names Count
or Length
does complicate the design a bit. Picking just one to standardize
though is not sufficient as it ends up excluding a large number of types:
- Use
Length
: excludes pretty much every collection in System.Collections and sub-namespaces. Those tend to derive fromICollection
and hence preferCount
over length. - Use
Count
: excludesstring
, arrays,Span<T>
and mostref struct
based types
The extra complication on the initial detection of Indexable types is outweighed by it's simplification in other aspects.
Choice of Slice as a anme
The name Slice
was chosen as it's the de-facto standard name for slice style operations in .NET. Starting with
netcoreapp2.1 all span style types use the name Slice
for slicing operations. Prior to netcoreapp2.1 there really
aren't any examples of slicing to look to for an example. Types like List<T>
, ArraySegment<T>
, SortedList<T>
would've been ideal for slicing but the concept didn't exist when types were added.
Thus Slice
being the sole example it was chosen as the name.
Related Issues
- https://github.com/dotnet/csharplang/blob/master/proposals/csharp-8.0/ranges.cs
- https://github.com/dotnet/csharplang/blob/master/proposals/csharp-8.0/ranges.md