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List patterns

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Note

This article is a feature specification. The specification serves as the design document for the feature. It includes proposed specification changes, along with information needed during the design and development of the feature. These articles are published until the proposed spec changes are finalized and incorporated in the current ECMA specification.

There may be some discrepancies between the feature specification and the completed implementation. Those differences are captured in the pertinent language design meeting (LDM) notes.

You can learn more about the process for adopting feature speclets into the C# language standard in the article on the specifications.

Summary

Lets you to match an array or a list with a sequence of patterns e.g. array is [1, 2, 3] will match an integer array of the length three with 1, 2, 3 as its elements, respectively.

Detailed design

The pattern syntax is modified as follow:

list_pattern_clause
  : '[' (pattern (',' pattern)* ','?)? ']'
  ;

list_pattern
  : list_pattern_clause simple_designation?
  ;

slice_pattern
  : '..' pattern?
  ;

primary_pattern
  : list_pattern
  | slice_pattern
  | // all of the pattern forms previously defined
  ;

There are two new patterns:

  • The list_pattern is used to match elements.
  • A slice_pattern is only permitted once and only directly in a list_pattern_clause and discards zero or more elements.

Pattern compatibility

A list_pattern is compatible with any type that is countable as well as indexable — it has an accessible indexer that takes an Index as an argument or otherwise an accessible indexer with a single int parameter. If both indexers are present, the former is preferred.

A slice_pattern with a subpattern is compatible with any type that is countable as well as sliceable — it has an accessible indexer that takes a Range as an argument or otherwise an accessible Slice method with two int parameters. If both are present, the former is preferred.

A slice_pattern without a subpattern is compatible with any type that is compatible with a list_pattern.

This set of rules is derived from the range indexer pattern.

Subsumption checking

Subsumption checking works just like positional patterns with ITuple - corresponding subpatterns are matched by position plus an additional node for testing length.

For example, the following code produces an error because both patterns yield the same DAG:

case [_, .., 1]: // expr.Length is >= 2 && expr[^1] is 1
case [.., _, 1]: // expr.Length is >= 2 && expr[^1] is 1

Unlike:

case [_, 1, ..]: // expr.Length is >= 2 && expr[1] is 1
case [.., 1, _]: // expr.Length is >= 2 && expr[^2] is 1

The order in which subpatterns are matched at runtime is unspecified, and a failed match may not attempt to match all subpatterns.

Given a specific length, it's possible that two subpatterns refer to the same element, in which case a test for this value is inserted into the decision DAG.

  • For instance, [_, >0, ..] or [.., <=0, _] becomes length >= 2 && ([1] > 0 || length == 3 || [^2] <= 0) where the length value of 3 implies the other test.
  • Conversely, [_, >0, ..] and [.., <=0, _] becomes length >= 2 && [1] > 0 && length != 3 && [^2] <= 0 where the length value of 3 disallows the other test.

As a result, an error is produced for something like case [.., p]: case [p]: because at runtime, we're matching the same element in the second case.

If a slice subpattern matches a list or a length value, subpatterns are treated as if they were a direct subpattern of the containing list. For instance, [..[1, 2, 3]] subsumes a pattern of the form [1, 2, 3].

The following assumptions are made on the members being used:

  • The property that makes the type countable is assumed to always return a non-negative value, if and only if the type is indexable. For instance, the pattern { Length: -1 } can never match an array.
  • The member that makes the type sliceable is assumed to be well-behaved, that is, the return value is never null and that it is a proper subslice of the containing list.

The behavior of a pattern-matching operation is undefined if any of the above assumptions doesn't hold.

Lowering

A pattern of the form expr is [1, 2, 3] is equivalent to the following code:

expr.Length is 3
&& expr[new Index(0, fromEnd: false)] is 1
&& expr[new Index(1, fromEnd: false)] is 2
&& expr[new Index(2, fromEnd: false)] is 3

A slice_pattern acts like a proper discard i.e. no tests will be emitted for such pattern, rather it only affects other nodes, namely the length and indexer. For instance, a pattern of the form expr is [1, .. var s, 3] is equivalent to the following code (if compatible via explicit Index and Range support):

expr.Length is >= 2
&& expr[new Index(0, fromEnd: false)] is 1
&& expr[new Range(new Index(1, fromEnd: false), new Index(1, fromEnd: true))] is var s
&& expr[new Index(1, fromEnd: true)] is 3

The input type for the slice_pattern is the return type of the underlying this[Range] or Slice method with two exceptions: For string and arrays, string.Substring and RuntimeHelpers.GetSubArray will be used, respectively.

Unresolved questions

  1. Should we support multi-dimensional arrays? (answer [LDM 2021-05-26]: Not supported. If we want to make a general MD-array focused release, we would want to revisit all the areas they're currently lacking, not just list patterns.)
  2. Should we accept a general pattern following .. in a slice_pattern? (answer [LDM 2021-05-26]: Yes, any pattern is allowed after a slice.)
  3. By this definition, the pattern [..] tests for expr.Length >= 0. Should we omit such test, assuming Length is always non-negative? (answer [LDM 2021-05-26]: [..] will not emit a Length check)