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ternary-expression-support-spec.md

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ProtoScript Ternary Expression Support Spec

Goal

Add support for compiling and executing ternary expressions in ProtoScript:

condition ? whenTrue : whenFalse

Current behavior throws Unexpected during parse when ? is encountered.

Scope

  • Parser support in ProtoScript.Parsers/Expressions.cs.
  • Compiler support in ProtoScript.Interpretter/Compiler.cs.
  • Runtime evaluation support in ProtoScript.Interpretter/NativeInterpretter.cs.
  • Automated coverage in Ontology.Tests.

Out of Scope

  • New syntax forms beyond standard ternary ? :.
  • Changes to unrelated operator semantics.

Parsing Design

  • Parse ternary after initial left-side expression term is resolved.
  • Represent ternary as:
    • BinaryOperator("?")
    • Left = condition expression
    • Right = BinaryOperator(":")
    • ":" .Left = true branch
    • ":" .Right = false branch
  • Require a : token after parsing the true branch.
  • Support nesting by recursively parsing branch expressions.
  • Preserve precedence by assigning ternary lower precedence than existing binary operators in GetPrecedence.

Compilation Design

  • Add compiled expression type ConditionalOperator with:
    • Condition
    • TrueExpression
    • FalseExpression
  • In compiler binary-operator dispatch:
    • Route BinaryOperator("?") to CompileConditionalOperator.
  • CompileConditionalOperator behavior:
    • Validate Right is a BinaryOperator(":").
    • Compile condition/true/false subexpressions.
    • Infer result type from branch compatibility:
      • If one branch type can be assigned to the other, use the broader compatible target.
      • If incompatible, emit compiler diagnostic and keep a deterministic inferred type.

Runtime Design

  • Extend expression dispatch in NativeInterpretter.Evaluate(Compiled.Expression exp) for ConditionalOperator.
  • Evaluate condition via existing boolean coercion (EvaluateAsBool).
  • Short-circuit semantics:
    • If condition is true, evaluate only true branch.
    • If condition is false, evaluate only false branch.

Test Plan

Create Ontology.Tests/TernaryOperator_Tests.cs with the following coverage:

  1. Ternary_BasicTrueBranch_ReturnsTrueExpression
  • Script returns true ? "A" : "B".
  • Assert result is "A".
  1. Ternary_BasicFalseBranch_ReturnsFalseExpression
  • Script returns false ? "A" : "B".
  • Assert result is "B".
  1. Ternary_Nested_RightAssociative_BehavesCorrectly
  • Script uses nested ternary, e.g. false ? "A" : true ? "B" : "C".
  • Assert result is "B".
  1. Ternary_Precedence_WithComparison_BehavesCorrectly
  • Script uses 1 < 2 ? "A" : "B".
  • Assert result is "A".
  1. Ternary_ShortCircuit_DoesNotEvaluateUnselectedBranch
  • Script uses branch with invalid operation in non-selected side.
  • Example: true ? "OK" : missingSymbol.
  • Assert call returns "OK" with no runtime failure.

Acceptance Criteria

  • Parsing no longer throws Unexpected for valid ternary expressions.
  • Compiler produces a compiled conditional expression for ? :.
  • Runtime returns correct branch result and short-circuits correctly.
  • New ternary tests pass.
  • Existing tests continue to pass for touched areas.