Method checkSelectField

checker/checker.go:215–257

(e, operand ast.Expr, field string, optional bool)

}

func (c *checker) checkSelectField(e, operand ast.Expr, field string, optional bool) *types.Type {
	// Interpret as field selection, first traversing down the operand.
	c.check(operand)
	operandType := substitute(c.mappings, c.getType(operand), false)

	// If the target type is 'optional', unwrap it for the sake of this check.
	targetType, isOpt := maybeUnwrapOptional(operandType)

	// Assume error type by default as most types do not support field selection.
	resultType := types.ErrorType
	switch targetType.Kind() {
	case types.MapKind:
		// Maps yield their value type as the selection result type.
		resultType = targetType.Parameters()[1]
	case types.StructKind:
		// Objects yield their field type declaration as the selection result type, but only if
		// the field is defined.
		messageType := targetType
		if fieldType, found := c.lookupFieldType(e.ID(), messageType.TypeName(), field); found {
			resultType = fieldType
		}
	case types.TypeParamKind:
		// Set the operand type to DYN to prevent assignment to a potentially incorrect type
		// at a later point in type-checking. The isAssignable call will update the type
		// substitutions for the type param under the covers.
		c.isAssignable(types.DynType, targetType)
		// Also, set the result type to DYN.
		resultType = types.DynType
	default:
		// Dynamic / error values are treated as DYN type. Errors are handled this way as well
		// in order to allow forward progress on the check.
		if !isDynOrError(targetType) {
			c.errors.typeDoesNotSupportFieldSelection(e.ID(), c.location(e), targetType)
		}
		resultType = types.DynType
	}

	// If the target type was optional coming in, then the result must be optional going out.
	if isOpt || optional {
		return types.NewOptionalType(resultType)
	}
	return resultType
}

func (c *checker) checkCall(e ast.Expr) {
	// Note: similar logic exists within the `interpreter/planner.go`. If making changes here