Pattern Matching

[gafter]

An implementation of pattern matching as specified in #206. You can follow the progress here. Much of this work will be pulling pieces of the implementation from https://github.com/semihokur/pattern-matching-csharp

Related features possibly to be added at the same time as pattern-matching:

• Proposal: expression-based switch ("match") for pattern matching #5154 expression-based switch ("match")
• Proposal: Make "throw expression" an expression form #5143 make "throw expression" an expression form
• Proposal: out var declaration #6183 "out var" declarations
• Proposal: Add completeness checking to pattern matching draft specification #188 Completeness checking for "match" and Algebraic Data Types
• Proposal: Destructuring (assignment?) statement #6400 destructuring assignment
• Proposal: Pattern matching and record types #206 Record types, Proposal: "with" expressions for record types #5172 "with" expressions, Notes on Records and Pattern Matching for 2015-10-07 design review #5757 Algebraic Data Types

Progress checklist:

• Scoping for variables introduced in patterns (binders)
• SemanticModel.GetDeclaredSymbol for pattern variable declarations.
• Simple pattern matching expressions expression is Type Identifier in most statements.
• Extend the parser to handle all of the other specified pattern-matching operations.
  • Add tests for the parser, including precedence tests for the edge cases.
  • Augment TestResource.AllInOneCSharpCode to handle all pattern forms.
• Check for feature availability in the parser (error if feature not supported).
• Error pattern matching to a nullable value type
• Implement pattern matching to a type that is an unconstrained type variable (requires a double type test)
• Implement and test scoping in remaining "odd" contexts (where the scope is not the enclosing statement)
  • pattern matching in ctor-initializers
  • pattern matching in catch filters
  • pattern matching in field initializers
  • pattern matching in expression-bodied methods and properties
  • pattern matching in an expression-bodied lambda
  • pattern matching in an expression-bodied local function
  • pattern matching in attributes and parameter defaults (lookup and error recovery)
  • test these "odd" contexts in SemanticModel.
• Semantics and code-gen for all pattern forms
  • Type ID
  • *
  • 3
    • matching with exact type for integral constants (as a short-term hack)
    • matching with appropriate integral conversions
  • var ID
  • Type { ID is Pattern ... }
  • Type ( Pattern ... ) This will be done when Records are integrated.
• Extend the switch statement to handle patterns
  • Parser
  • Syntax Tests
  • Binding
  • Binding (failure cases) tests
  • Flow analysis
  • Lowering
  • Code-gen tests
• Allow declaration of operator is This will be done when Records are integrated.
• An expression form for mutli-armed pattern-matching (match?)
• Extend the scope of a pattern variable declared in a catch filter to the catch block.
  • Test for error on reusing a variable name, and lambda-capturing.
• Add a decision tree to enable
  • completeness checking: a mutli-armed pattern-matching expression is required to be complete
  • subsumption checking: a branch of a switch statement or match expression may not be subsumed by the totality of previous branches
• Generate efficient code like switch does in corresponding situations.
• Test for name conflicts with locals in enclosing scopes for normal and "odd" contexts.
• Lots more Tests!

Records will be added in a separate feature branch.

[gafter]

/cc @semihokur

[gafter]

This should be called BoundIsPatternExpression

[gafter]

FYI @AlekseyTs @VSadov @jaredpar @agocke @MadsTorgersen @ljw1004 @AnthonyDGreen