Support Setting Arrays/Fields

Signed-off-by: Will Hawkins <hawkinsw@obs.cr>

Sources/P4Runtime/Expressions.swift

@@ -93,6 +93,30 @@ extension TypedIdentifier: EvaluatableExpression {
   }
 }
 
+// Variables are evaluatable because they can be looked up by identifiers.
+extension TypedIdentifier: EvaluatableLValueExpression {
+  public func set(
+    to: any Common.P4Value, inScopes scopes: Common.ValueScopes, duringExecution execution: ProgramExecution
+  ) -> Common.Result<(Common.ValueScopes, P4Value)> {
+    if case .Error(let e) = scopes.lookup(identifier: self) {
+      return .Error(e)
+    }
+
+    return .Ok((scopes.set(identifier: self, withValue: to), to))
+  }
+
+  public func check(to: any Common.EvaluatableExpression, inScopes scopes: Common.TypeScopes) -> Result<()> {
+    guard case .Ok(let type) = scopes.lookup(identifier: self) else {
+      return .Error(Error(withMessage: "Cannot assign to identifier not in scope"))
+    }
+
+    if !type.eq(rhs: to.type()) {
+      return .Error(Error(withMessage: "Cannot assign value with type \(to.type()) to identifier \(self) with type \(type)"))
+    }
+    return .Ok(())
+  }
+}
+
 public func binary_equal_operator_evaluator(left: P4Value, right: P4Value) -> P4Value {
   if left.eq(rhs: right) {
     return P4BooleanValue(withValue: true)
@@ -141,7 +165,7 @@ extension ArrayAccessExpression: EvaluatableExpression {
     }
     let accessed = array.access(indexor_int.access())
 
-    return accessed.evaluate(execution: execution)
+    return .Ok(accessed)
   }
 
   public func type() -> any Common.P4Type {
@@ -149,6 +173,56 @@ extension ArrayAccessExpression: EvaluatableExpression {
   }
 }
 
+extension ArrayAccessExpression: EvaluatableLValueExpression {
+  public func set(
+    to: any Common.P4Value, inScopes scopes: Common.ValueScopes, duringExecution execution: ProgramExecution
+  ) -> Common.Result<(Common.ValueScopes, P4Value)> {
+    // For purposes of documentation, assume the field access expression we are evaluating is
+    // (strct_id)[indexor] = new_value
+    // where strct_id expands to
+    // (identifier.field_id1.field_id2...).field_id = new_field_value
+
+    // First, evaluate strct_id and make sure that it names a struct.
+    let maybe_value = self.name.evaluate(execution: execution)
+    guard case .Ok(let value) = maybe_value else {
+        return Result.Error(Error(withMessage: "\(self.name) cannot be evaluated: \(maybe_value.error()!)"))
+    }
+    guard let array_value = value as? P4ArrayValue else {
+        return Result.Error(Error(withMessage: "\(self.name) does not identify a struct"))
+    }
+
+    // Now, get the indexor!
+    let maybe_indexor_value = self.indexor.evaluate(execution: execution)
+    guard case .Ok(let indexor_value) = maybe_indexor_value else {
+        return Result.Error(Error(withMessage: "\(self.indexor) cannot be evaluated: \(maybe_indexor_value.error()!)"))
+    }
+    guard let indexor_int = indexor_value as? P4IntValue else {
+        return Result.Error(Error(withMessage: "\(self.indexor) cannot be used to index an array"))
+    }
+
+    // Now we have an array and an index!
+
+    // Update field_id of that structure and get the new structure value.
+    let set_result = array_value.set(index: indexor_int.access(), to: to)
+    guard case .Ok(let new_array_value) = set_result else {
+      return .Error(set_result.error()!)
+    }
+
+    let array_lvalue = self.name as! EvaluatableLValueExpression
+    return array_lvalue.set(to: new_array_value, inScopes: scopes, duringExecution: execution)
+  }
+
+  public func check(
+    to: any Common.EvaluatableExpression, inScopes scopes: Common.TypeScopes
+  ) -> Common.Result<()> {
+
+    if !self.type.value_type().eq(rhs: to.type()) {
+      return .Error(Error(withMessage: "Cannot assign value of type \(to.type()) to array with values of type \(self.name.type())"))
+    }
+    return .Ok(())
+  }
+}
+
 extension FieldAccessExpression: EvaluatableExpression {
   public func evaluate(execution: Common.ProgramExecution) -> Common.Result<any Common.P4Value> {
     let maybe_struct = self.strct.evaluate(execution: execution)
@@ -172,3 +246,48 @@ extension FieldAccessExpression: EvaluatableExpression {
     return self.field.type
   }
 }
+
+extension FieldAccessExpression: EvaluatableLValueExpression {
+  public func set(
+    to: any Common.P4Value, inScopes scopes: Common.ValueScopes, duringExecution execution: ProgramExecution
+  ) -> Common.Result<(Common.ValueScopes, P4Value)> {
+    // For purposes of documentation, assume the field access expression we are evaluating is
+    // (strct_id).field_id = new_field_value
+    // where strct_id expands to
+    // (identifier.field_id1.field_id2...).field_id = new_field_value
+
+    // First, evaluate strct_id and make sure that it names a struct.
+    let maybe_value = self.strct.evaluate(execution: execution)
+    guard case .Ok(let value) = maybe_value else {
+        return Result.Error(Error(withMessage: "\(self.strct) cannot be evaluated: \(maybe_value.error()!)"))
+    }
+
+    guard let struct_value = value as? P4StructValue else {
+        return Result.Error(Error(withMessage: "\(self.strct) does not identify a struct"))
+    }
+
+    // Now we know that struct_id identifies a structure value.
+
+    // Update field_id of that structure and get the new structure value.
+    let set_result = struct_value.set(field: self.field, to: to)
+    guard case .Ok(let new_struct_value) = set_result else {
+      return .Error(set_result.error()!)
+    }
+
+    // That new structure value should be assignable to the lvalue that is strct_id.
+    // We use recursion here -- ultimately finding our way to a TypedIdentifier that
+    // will update the scope. Pretty cool!
+    let struct_lvalue = self.strct as! EvaluatableLValueExpression
+    return struct_lvalue.set(to: new_struct_value, inScopes: scopes, duringExecution: execution)
+  }
+
+  public func check(
+    to: any Common.EvaluatableExpression, inScopes scopes: Common.TypeScopes
+  ) -> Common.Result<()> {
+
+    if !self.field.type.eq(rhs:to.type()) {
+      return .Error(Error(withMessage: "Cannot assign value of type \(to.type()) to field with type \(self.field.type)"))
+    }
+    return .Ok(())
+  }
+}