The idea for DAFFODIL-2277 is that we want to create a warning if there is a discriminator on a sequence AND that sequence is non-empty. This is because a discriminator on a non-empty sequence is evaluated AFTER the contents are parsed, which is often unintuitive since the discriminator appears BEFORE the contents in the DFDL schema. For empty sequences with a discriminator, there's no confusion about when the discriminator is evaluated so we don't need a warning in that case.
Also note that the same goes for asserts as well (asserts are essentially the same as discriminators, except a true discriminator affects how Daffodil backtracks while a true assert does not). So this is a schema compile time check, which means the logic goes somewhere in the daffodil-core subproject. Inside this subproject, Daffodil has what we call dsom (DFDL Schema Object Model), which is essentially a representation of a DFDL schema in Scala classes with various members to access information about those schema objects (such as children of a model group/sequence, or if a object has discriminators/assert). The DSOM base class for sequences is SequenceGroupTermBase in daffodil-core/src/main/scala/org/apache/daffodil/dsom/SequenceGroup.scala The DSOM mixin that has variables such as discriminatorStatments and assertStatments, etc. that can be used to determine if such annotations exist is ProvidesDFDLStatementMixin in daffodil-core/src/main/scala/org/apache/daffodil/dsom/DFDLStatementMixin.scala Through various traits, SequenceGroupTermBase mixes in ProvidesDFDLStatementMixin, for reference SequenceGroupTermBase mixes in SequenceDefMixin which mixes in GroupDefLike which mixes in ProvidesDFDLStatementMixin Thus, the SequenceGroupTermBase has access to the discriminatorStatements and assertStatements variables. Additionally, the ModelGroup class provides variables about model groups (sequences are an example of a model group), including the groupMembers variable, which provides access to the children of the model group. Through various traits, SequenceGroupTermBase also mixes in ModelGroup, for refrence, SequenceGroupTermBase mixes in SequenceTermBase which mixes in ModelGroup So the SequenceGroupTermBase DSOM object has information about both the members of a sequence (via groupMembers variable) and the discriminators/asserts (via discriminatorStatements and assertStatements variables). So we just need to add a check to produce a warning if the length of groupMembers is > 0 and either the length of the discriminatorStatements or assertStatements are also > 0. Note that this class already has various checks (e.g. checkIfValidUnorderedSequence). You'll probably want to create something similar (e.g. checkIfNonEmptyAndDiscrimsOrAsserts), and that lazy val can create a SDW rather than an SDE if the check fails. Note that that although this is specific to this issue, the idea is the same throughout Daffodil. DSOM is just a bunch of classes that mixin traits to provide helpful member variables to easily inspect the DFDL schema. Understanding that DSOM hierarchy can take some time since it's pretty large but ultimately it's just a bunch of the same thing. - Steve On 8/25/20 2:59 PM, Steinberger, Rick wrote: > https://issues.apache.org/jira/browse/DAFFODIL-2277 > > Warning needed for misplaced discriminators > > Hey guys, > > Any pointers on where to look in the code to address this would be much appreciated. > > Thanks, > > Rick > On 8/26/20 1:38 PM, Steinberger, Rick wrote: > I'm new to Daffodil, and trying to learn my way around the code. > I have picked up Daffodil-2277 Warning needed for misplaced discriminators. > > Can you give me a hook into the code where schema elements are evaluated so > that I can learn how to detect the position of specific elements in the > schema like dfdl:discriminator. > > I have evaluated multipleDiscriminators.tdml and stepped through the parse > using the debugger. https://daffodil.apache.org/debugger/ > > Any pointers would be appreciated. > > >
