61 lines
3.4 KiB
Markdown
61 lines
3.4 KiB
Markdown
# Ranges
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## Summary
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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.
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## Detailed Design
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#### System.Index
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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.
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```csharp
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var thirdItem = list[2]; // list[2]
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var lastItem = list[^1]; // list[Index.CreateFromEnd(1)]
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var multiDimensional = list[3, ^2] // list[3, Index.CreateFromEnd(2)]
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```
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#### System.Range
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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.
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The language will introduce a new range operator `x..y`. It is a binary infix operator that accepts two expressions. Either operands 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.
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```csharp
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var slice1 = list[2..^3]; // list[Range.Create(2, Index.CreateFromEnd(3))]
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var slice2 = list[..^3]; // list[Range.ToEnd(Index.CreateFromEnd(3))]
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var slice3 = list[2..]; // list[Range.FromStart(2)]
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var slice4 = list[..]; // list[Range.All]
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var multiDimensional = list[1..2, ..] // list[Range.Create(1, 2), Range.All]
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```
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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.
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## Workarounds
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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:
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* `op_Indexer_Extension(this TCollection<TItem> collection, ...arguments supplied to the indexer)`
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This workaround will be removed once contract with runtime/framework is finalized, and before the feature is declared complete.
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## Alternatives
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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.
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## IL Representation
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These two operators will be lowered to regular indexer/method calls, with no change in subsequent compiler layers.
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## Runtime behaviour
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* Compiler can optimize indexers for built-in types like arrays and strings, and lower the indexing to the appropriate existing methods.
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* `System.Index` will throw if constructed with a negative value.
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* `^0` does not throw, but it translates to the length of the collection/enumerable it is supplied to.
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* `Range.All` is semantically equivalent to `0..^0`, and can be deconstructed to these indices.
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## Questions
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