135 lines
4.7 KiB
Markdown
135 lines
4.7 KiB
Markdown
Notes on Records and Pattern Matching for 2015-10-07 design review
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==================================================================
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Discussion for these notes can be found at https://github.com/dotnet/roslyn/issues/5757.
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### Records and Pattern Matching (https://github.com/dotnet/roslyn/issues/206)
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Prototyped together last year by @semihokur, but that prototype is
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based on very out-of-date Roslyn code. We also have some design changes
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since that time and we want to separate a pattern-matching prototype from
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records/ADTs so we can make independent decisions about whether
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and when to include them in the language(s).
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First step is to port pattern matching to the latest sources.
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In-progress port at https://github.com/dotnet/roslyn/pull/4882
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### Spec changes since the 2014 prototype
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For pattern matching:
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1. Scoping of pattern-introduced variables (with "funny" rule for `if`)
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2. Rules for `switch` statement that make it a compatible extension of the existing construct (https://github.com/dotnet/roslyn/issues/4944)
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3. An expression form of multi-arm pattern-matching (https://github.com/dotnet/roslyn/issues/5154)
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4. A `when` clause added to `switch` cases.
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And, for records:
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1. No `record` keyword necessary
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2. `with` expressions (https://github.com/dotnet/roslyn/issues/5172)
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3. Approach for for algebraic data types
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### Implementation status of prototype port
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1. For pattern matching, checklist at https://github.com/dotnet/roslyn/pull/4882 tracking the progress
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2. For records, port not started
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### Making the extension of `switch` backward-compatible
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- We say that the cases are matched in order, except `default` which is always the last
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resort.
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- Integral-typed case labels match any integral-valued control expression with the same value.
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- One issue around user-defined conversions to switchable types is
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resolved (https://github.com/dotnet/roslyn/issues/4944). In the draft spec,
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a conversion will be applied on the `case`s, not on the control-expression unilaterally.
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Instead of converting only to `swithable` types, each
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`case` arm will consider any conversions that allow the `case` to be applied.
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Any given conversion would be applied at most once.
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```cs
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Foo foo = ...; // has a conversion to int
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switch (foo)
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{
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case 1: // uses the converted value
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case Foo(2): // uses the original value
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case 3: // uses the converted value
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}
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```
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- The `goto case` statement is extended to allow any expression as its argument.
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### Expression form of multi-arm pattern matching (https://github.com/dotnet/roslyn/issues/5154)
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```cs
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var areas =
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from primitive in primitives
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let area = primitive match (
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case Line l: 0
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case Rectangle r: r.Width * r.Height
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case Circle c: Math.PI * c.Radius * c.Radius
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case *: throw new ApplicationException()
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)
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select new { Primitive = primitive, Area = area };
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```
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There is no `default` here, so cases are handled strictly in order.
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I propose the spec require that the compiler "prove" that all cases are handled
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in a `match` expression using not-yet-specified rules. Writing those rules
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is an open work item, but I imagine it will require the compiler to build
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a decision tree and check it for completeness. That will also be needed to
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implement checks that no case is subsumed by a previous case, which will
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cause a warning (for `switch`) or error (for `match`).
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### With-expressions (https://github.com/dotnet/roslyn/issues/5172)
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```cs
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class Point(int X, int Y, int Z);
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...
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Point p = ...;
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Point q = p with { Y = 2 };
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```
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The latter is translated into
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```cs
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Point q = new Point(X: p.X, Y: 2, Z: p.Z);
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```
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We know how to do this for record types (because the language specifies the
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mapping between constructor parameters and properties). We're examining how
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to extend it to more general types.
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To support inheritance, rather than directly using the constructor (as above) the generated code will
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invoke a compiler-generated (but user-overridable) factory method.
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```cs
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Point q = p.With(X: p.X, Y: 2, Z: p.Z);
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```
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### Draft approach for algebraic data types
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```cs
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abstract sealed class Expression
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{
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class Binary(Operator Operator, Expression Left, Expression Right) : Expression;
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class Constant(int Value) : Expression;
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}
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```
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None of these classes would be permitted to be extended elsewhere.
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a `match` expression that handles both `Binary` and `Constant` cases
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would not need a `*` (default) case, as the compiler can prove it
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is complete.
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### Remaining major issues
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1. We need to specify the rules for checking
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- If the set of cases in a `match` is complete
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- If a `case` is subsumed by a previous `case`
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2. We need more experience with algebraic data types and active patterns.
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3. Can we extend `with` expressions to non-record types?
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