Pattern match Span<char> on a constant string

Note

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Summary

Permit pattern matching a Span<char> and a ReadOnlySpan<char> on a constant string.

Motivation

For perfomance, usage of Span<char> and ReadOnlySpan<char> is preferred over string in many scenarios. The framework has added many new APIs to allow you to use ReadOnlySpan<char> in place of a string.

A common operation on strings is to use a switch to test if it is a particular value, and the compiler optimizes such a switch. However there is currently no way to do the same on a ReadOnlySpan<char> efficiently, other than implementing the switch and the optimization manually.

In order to encourage adoption of ReadOnlySpan<char> we allow pattern matching a ReadOnlySpan<char>, on a constant string, thus also allowing it to be used in a switch.

static bool Is123(ReadOnlySpan<char> s)
{
    return s is "123";
}

static bool IsABC(Span<char> s)
{
    return s switch { "ABC" => true, _ => false };
}

Detailed design

We alter the spec for constant patterns as follows (the proposed addition is shown in bold):

A constant pattern tests the value of an expression against a constant value. The constant may be any constant expression, such as a literal, the name of a declared const variable, or an enumeration constant, or a typeof expression etc.

If both e and c are of integral types, the pattern is considered matched if the result of the expression e == c is true.

If e is of type System.Span<char> or System.ReadOnlySpan<char>, and c is a constant string, and c does not have a constant value of null, then the pattern is considered matching if System.MemoryExtensions.SequenceEqual<char>(e, System.MemoryExtensions.AsSpan(c)) returns true.

Otherwise the pattern is considered matching if object.Equals(e, c) returns true. In this case it is a compile-time error if the static type of e is not pattern compatible with the type of the constant.

Well-known members

System.Span<T> and System.ReadOnlySpan<T> are matched by name, must be ref structs, and can be defined outside corlib.

System.MemoryExtensions is matched by name and can be defined outside corlib.

The signature of System.MemoryExtensions.SequenceEqual overloads must match:

• public static bool SequenceEqual<T>(System.Span<T>, System.ReadOnlySpan<T>)
• public static bool SequenceEqual<T>(System.ReadOnlySpan<T>, System.ReadOnlySpan<T>)

The signature of System.MemoryExtensions.AsSpan must match:

• public static System.ReadOnlySpan<char> AsSpan(string)

Methods with optional parameters are excluded from consideration.

Drawbacks

None

Alternatives

None

Unresolved questions

1. Should matching be defined independently from MemoryExtensions.SequenceEqual() etc.?

   ... the pattern is considered matching if e.Length == c.Length and e[i] == c[i] for all characters in e.

   Recommendation: Define in terms of MemoryExtensions.SequenceEqual() for performance. If MemoryExtensions is missing, report compile error.

2. Should matching against (string)null be allowed?

   If so, should (string)null subsume "" since MemoryExtensions.AsSpan(null) == MemoryExtensions.AsSpan("")?

   static bool IsEmpty(ReadOnlySpan<char> span)
   {
       return span switch
       {
           (string)null => true, // ok?
           "" => true,           // error: unreachable?
           _ => false,
       };
   }
   

   Recommendation: Constant pattern (string)null should be reported as an error.

3. Should the constant pattern match include a runtime type test of the expression value for Span<char> or ReadOnlySpan<char>?

   static bool Is123<T>(Span<T> s)
   {
       return s is "123"; // test for Span<char>?
   }
   
   static bool IsABC<T>(Span<T> s)
   {
       return s is Span<char> and "ABC"; // ok?
   }
   
   static bool IsEmptyString<T>(T t) where T : ref struct
   {
       return t is ""; // test for ReadOnlySpan<char>, Span<char>, string?
   }
   

   Recommendation: No implicit runtime type test for constant pattern. (IsABC<T>() example is allowed because the type test is explicit.)

4. Should subsumption consider constant string patterns, list patterns, and Length property pattern?

   static int ToNum(ReadOnlySpan<char> s)
   {
       return s switch
       {
           { Length: 0 } => 0,
           "" => 1,        // error: unreachable?
           ['A',..] => 2,
           "ABC" => 3,     // error: unreachable?
           _ => 4,
       };
   }
   

   Recommendation: Same subsumption behavior as used when the expression value is string. (Does that mean no subsumption between constant strings, list patterns, and Length, other than treating [..] as matching any?)

Design meetings

https://github.com/dotnet/csharplang/blob/master/meetings/2020/LDM-2020-10-07.md#readonlyspanchar-patterns