Ranges

Summary

This feature is about delivering two new operators that allow constructing System.Index and System.Range objects, and using them to index/slice collections at runtime.

Overview

System.Index

C# has no way of indexing a collection from the end, but rather most indexers use the "from start" notion, or do a "length - i" expression. We introduce a new Index expression that means "from the end". The feature will introduce a new unary prefix "hat" operator. Its single operand must be convertible to System.Int32. It will be lowered into the appropriate System.Index factory method call.

We augment the grammar for unary_expression with the following additional syntax form:

unary_expression
    : '^' unary_expression
    ;

We call this the index from end operator. The predefined index from end operators are as follows:

    System.Index operator ^(int fromEnd);

The behavior of this operator is only defined for input values greater than or equal to zero.

Examples:

var thirdItem = list[2];                // list[2]
var lastItem = list[^1];                // list[Index.CreateFromEnd(1)]

var multiDimensional = list[3, ^2]      // list[3, Index.CreateFromEnd(2)]

System.Range

C# has no syntactic way to access "ranges" or "slices" of collections. Usually users are forced to implement complex structures to filter/operate on slices of memory, or resort to LINQ methods like list.Skip(5).Take(2). With the addition of System.Span<T> and other similar types, it becomes more important to have this kind of operation supported on a deeper level in the language/runtime, and have the interface unified.

The language will introduce a new range operator x..y. It is a binary infix operator that accepts two expressions. Either operand can be omitted (examples below), and they have to be convertible to System.Index. It will be lowered to the appropriate System.Range factory method call.

We replace the C# grammar rules for shift_expression with the following (in order to introduce a new precedence level):

shift_expression
    : range_expression
    | shift_expression '<<' range_expression
    | shift_expression right_shift range_expression
    ;

range_expression
    : additive_expression
    | range_expression? '..' additive_expression?
    ;

We call the .. operator the range operator. The built-in range operator can roughly be understood to correspond to the invocation of a built-in operator of this form:

    System.Range operator ..(Index start = 0, Index end = ^0);

Examples:

var slice1 = list[2..^3];               // list[Range.Create(2, Index.CreateFromEnd(3))]
var slice2 = list[..^3];                // list[Range.ToEnd(Index.CreateFromEnd(3))]
var slice3 = list[2..];                 // list[Range.FromStart(2)]
var slice4 = list[..];                  // list[Range.All]

var multiDimensional = list[1..2, ..]   // list[Range.Create(1, 2), Range.All]

Moreover, System.Index should have an implicit conversion from System.Int32, in order not to need to overload for mixing integers and indexes over multi-dimensional signatures.

Workarounds

For prototyping reasons, and since runtime/framework collections will not have support for such indexers, the compiler will finally look for the following extension method when doing overload resolution:

op_Indexer_Extension(this TCollection<TItem> collection, ...arguments supplied to the indexer)

This workaround will be removed once contract with runtime/framework is finalized, and before the feature is declared complete.

Alternatives

The new operators (^ and ..) are syntactic sugar. The functionality can be implemented by explicit calls to System.Index and System.Range factory methods, but it will result in a lot more boilerplate code, and the experience will be unintuitive.

IL Representation

These two operators will be lowered to regular indexer/method calls, with no change in subsequent compiler layers.

Runtime behaviour

Compiler can optimize indexers for built-in types like arrays and strings, and lower the indexing to the appropriate existing methods.
System.Index will throw if constructed with a negative value.
^0 does not throw, but it translates to the length of the collection/enumerable it is supplied to.
Range.All is semantically equivalent to 0..^0, and can be deconstructed to these indices.

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