Navigating the Parse Tree

Fundamentally, the StateMachine presents an interface for iterating the syntax tree as if it were merely a linear sequence of segments. Purely syntactical values like LoopVals and TableVals are hidden from the programmer. While the interface only exposes segments and elements, the information hidden by StateMachine provides an efficient means to search the parse tree for specific segments.

For information on how to construct a parse tree programmatically, see the document on Generating X12. The StateMachine can be accessed via the BuilderDsl#machine method. For information about how to construct a parse tree from an input stream, see Parsing X12.

Iterating Segments

Current Segment

When you want to access the current SegmentVal, use the #segment method. It returns an AbstractCursor, which is a read-only pointer to the current segment within the parse tree, wrapped by Either. When the parse tree is empty, a failure will be returned.

# Success
machine.segment  #=> Either.success(#<Zipper::AbstractCursor ...>)

# Failure
machine.segment  #=> Either.failure("not a segment")

You may be wondering why the SegmentVal is wrapped by two wrappers. The first, Either, provides a manner to distinguish error return values from normal return values. It is more sophisticated and less error-prone than using conventions like returning nil on failure, because it supports chaining, detailed error information can be returned, and the risk of neglecting to test for an error is mitigated.

This Either value wraps an AbstractCursor, which points to the SegmentVal via its #node method. Because SegmentVal does not have information about its parent or siblings (it only is aware of its #children), returning only a SegmentVal does not always provide enough information. The AbstractCursor, on the other hand, allows access to any related AbstractVal node.

# Success  #=> Either.success(SegmentVal[IEA](...))  #=> Either.success(InterchangeVal[00501](...))

# Failure  #=> Either.failure("not a segment")

For another example, the current segment identifier can be accessed like any other method of SegmentVal.

# When machine.segment fails, nothing is printed
machine.segment.tap do |s|
  puts "Hello, #{}"

Going Forward

The #next method returns a StateMachine positioned at the segment immediately following the current segment. Optionally, you may specify how many segments to advance, which defaults to 1. You can check if the current segment is the last segment using the #last? method.

# Success
machine.last?  #=> false  #=> Either.success(StateMachine[1](SegmentVal[GS](...)))

# Success  #=> Either.success(StateMachine[1](SegmentVal[BHT](...)))

# Failure
machine.last?  #=> true  #=> Either.failure("cannot move to next after last segment")

Going Backward

The #prev method returns a StateMachine positioned at the segment immediately preceeding the current segment. Optionally, you may specify how many segments to rewind, which defaults to 1. You can check if the current segment is the first segment using the #first? method.

# Success
machine.first?  #=> false

machine.prev  #=> Either.success(StateMachine[1](SegmentVal[GS](...)))

# Success
machine.prev(2)  #=> Either.success(StateMachine[1](SegmentVal[ISA](...)))

# Failure
machine.first?  #=> true

machine.prev  #=> Either.failure("cannot move to prev before first segment")

Accessing Elements

Elements can be accessed using the #element method, which accepts up to three numeric arguments and requires at least one. Like #segment, the return value is a AbstractCursor wrapped by an Either.

Simple Elements

To access the first element of the current segment, call #element(1). Notice elements are counted starting at 1, not 0. Beware that #element will raise an ArgumentError if you attempt to access the fifth element of a segment whose declaration only indicates four elements, for instance.

# Success
machine.element(1).map(&:node)  #=> Either.success(Nn.value[  I16: Number of Included Functional Groups](1))

# Error
machine.element(3).map(&:node)  #=> IEA has only 2 elements (ArgumentError)

Composite Elements

If the first element of the current segment is a CompositeElementVal, calling #element(1) will return the entire composite value. To access a specific component, call #element(1, n) which will return the nth SimpleElementVal. Beware that #element will raise an ArgumentError if you attempt, for instance, to access the third component of a composite whose declaration only indicates two components.

# Success
machine.element(5).map(&:node)  #=> Either.success(CompositeElementVal[C023: HEALTH CARE SERVICE LOCATION INFORMATION](...))

# Success
machine.element(5, 1).map(&:node)  #=> Either.success(AN.value[E1331: Place of Service Code](11))

# Error
machine.element(5, 4).map(&:node)  #=> CLM05 has only 3 components (ArgumentError)

Repeated Elements

When the first element of the current segment is a RepeatedElementVal, calling #element(1) will return the entire sequence of element vals. To access a specific occurrence of the element, call #element(1, n) which will return the nth occurrence if it exists. If the element is a repeating composite element, an optional third argument can be given to select a specific component. Beware that #element will raise an ArgumentError if, for instance, you try to access the sixth occurrence when the element definition declares the element can occur a maximum of five times.

Taking Bigger Steps

First Segment

You can position the StateMachine at the first segment in the parse tree by calling #first. When there are no segments in the parse tree, this method returns a failure. This will typically position the StateMachine at the first ISA segment.  #=> Either.success(true)

Last Segment

Likewise, the #last method will position the StateMachine at the first segment in the parse tree if there is one. When the parse tree is empty, a failure is returned instead. This will typically position the StateMachine at the last IEA segment.  #=> Either.success(true)

Searching for Segments

The #find method performs an efficient context-sensitive search based on the current position. Being context-sensitive places restrictions on which segments are reachable from the current state, unlike iterating segments one at a time. These restrictions prevent complicated problems, like mistakenly finding an NM1 segment from the next interchange because there were no more NM1 segments in the current interchange.

The next matching segment is returned, or a failure if no segments matched. Searching for a segment that, according to the definition tree, cannot exist or is not reachable will cause #find to raise an exception. To be clear, #find only searches forward for segments, not backward.

Sibling Segments

View diagram

Segments connected directly by a horizontal dashed black line are siblings and are reachable using #find. For instance, from the third NM1, the N3, N4, and REF segments are reachable.

# From 2010AA NM1 right one segment to N3
machine.find(:N3)  #=> Either.success(StateMachine[1](N3(...)))

# Right two segments to N4
machine.find(:N4)  #=> Either.success(StateMachine[1](N4(...)))

# Right three segments to REF
machine.find(:REF)  #=> Either.success(StateMachine[1](REF(...)))

Likewise, N4 and REF are reachable from N3; however, the third NM1 is not reachable from N3 because it preceeds N3.

Uncle Segments

Segments that occur as siblings of an ancestor node are uncles (remember that #find only proceeds forward). Common uncle segments are GE and IEA, which are analogous to "closing tags" for envelope structures. Other examples include the IK5 and AK9 segments from the 999 Functional Acknowledgement transaction set.

# From BHT ascend twice and move left to GE

# From PRV ascend four times and move left to IEA

Uncles are relatively rare in X12 transaction sets, because most child structures, like loops, are not wrapped by segments at both ends. For instance there are no segments in Loop 2000A that follow the child loops.

Nephew Segments

View diagram

Segments that occur as the first direct child of a sibling node are nephews. The siblings that follow the first child are not directly reachable, but they can be reached indirectly by chaining two calls to #find. For example, GS is a nephew of ISA, and ST has two nephews named NM1; but BHT is not a nephew of GS because it is not the first child of its parent node.

# From ST move left twice and down to NM1

# From 2000A PRV move left and down to NM1

# From 2300 CLM move left three times and down to NM1

The first-child restriction prevents potential problems of ambiguity with certain grammars. Consider the possibility that Table 1 could be defined to have a 1100 PER loop in addition to its two NM1 loops. From the ST segment, without the restriction, there would be two possibly reachable PER segments. One is the sibling of 1000A NM1, and the other is the 1100 PER that we made up. Because #find returns the first matching segment, it would return 1000A PER if it occurred, otherwise it would return the 1100 PER. The caller would have to check which one it was -- the necessity of this test is not apparent without studying the grammar. Changing the grammar to add Loop 1100 would break existing code. The first-child restriction solves these problems.

Cousin Segments

View diagram

Segments that occurr as the first child of a sibling of the parent node are cousins of the current segment. Similar to the restriction on nephew segments, siblings that follow the first child are not directly reachable. For example, the second NM1 is a cousin of the first NM1 and PER segments, the 2000AB NM1 is a cousin of all segments in Loop 2010AA, and each HL is a cousin of all segments in Table 1.

# From 1000A PER to 2000A HL

# From 2000BA NM1 to 2000BB NM1

You may have noticed, in some cases there are more than one cousin with the same segment identifier -- there are three cousins of BHT named HL, for instance. See Element Constraints for information on how to find a specific occurrence of HL segment based on its qualifier elements, or Chaining Method Calls for details on iterating each HL segment, one-at-a-time.

Parent Segments

View diagram

Internal knowledge of the underlying tree structure makes it possible to rewind to the first segment of a parent structure, using the #parent method. The parent may be the first segment of a loop, table, functional group, or interchange, but never a transaction set, because they do not parent segments directly.

# From PRV up two nodes, left one node, and down to ST

# From PER left one node to NM1

Traversing to the parent segment, unlike #find, always traverses backwards in the sequence of segments. The #parent method can only rewind to the segment defined by the grammar, so it will always find the same segment from a given starting position.

Element Constraints

Finding the next segment by identifier alone is often not specific enough. For example, there are several different NM1 segments that each have a different meaning -- one is a provider, another is the insurance company, another is the patient, etc. In the case of NM1, the first element is a qualifier that indicates the meaning. To find NM1*QC "Patient Name", you can iterate the reachable NM1 segments and stop when you find the occurrence whose first element equals 41 or when there are no more NM1 occurrences,

# Find the NM1 occurrence with "41" in the first element
position  = Either.success(machine)
searching = true

while searching and position.defined?  # Move position to the next NM1 segment

  position = position.flatmap{|m| m.find(:NM1) }

  # Check the constraint
  position.tap do |nm1|
    nm1.element(1).tap do |element|
      # Stop the while loop if we found the match
      searching = element.node != "QC"

# Success
searching  #=> false

position  #=> Either.success(StateMachine[1](SegmentVal[NM1](...)))

# Failure
searching  #=> true

position  #=> Either.failure("NM1 segment does not occur")

There is a much simpler and less error-prone way, however. The #find method accepts a variable number of filter arguments. For instance, the above filter can be accomplished by calling,

machine.find(:NM1, "QC")

Multiple constraints can be specified and #blank or nil should be used to indicate a wildcard, if needed. For instance to find the NM1*PR "Payer Name" that has a certain organization name in element NM103,

machine.find(:NM1, "PR", nil, "MEDICARE")

Syntactic Constraints

In addition to improving readability, #find checks the validity of your element constraints and raises an exception when you ask for something that is guaranteed never to occur in a valid parse tree. For instance, if you called #find(:NM1, "XX") from a position where there are no reachable NM1 segments,

machine.find(:NM1, "XX")  #=> NM1 segment cannot be reached from the current state (ArgumentError)

Or from a position where every reachable NM1 segment is defined such that "XX" is not allowed,

machine.find(:NM1, "XX")  #=> "XX" is not allowed in NM101 (ArgumentError)

You can get a list of potentially reachable segments from the current position by calling #successors, which returns one InstructionTable per active state. That is, when the machine is in a deterministic state, a single InstructionTable will be returned. See the section on Non-determinism for more information.

pp b.successors

  1: Instruction[REF: Subscriber Secon..](pop: 0, drop: 0),
  2: Instruction[REF: Property and Cas..](pop: 0, drop: 0),
  3: Instruction[PER: Property and Cas..](pop: 0, drop: 3),
  4: Instruction[NM1: Subscriber Name   ](pop: 1, drop: 0, push: LoopState),
  5: Instruction[NM1: Payer Name        ](pop: 1, drop: 0, push: LoopState),
  6: Instruction[CLM: Claim Informatio..](pop: 1, drop: 2, push: LoopState),
  7: Instruction[ HL: Subscriber Hiera..](pop: 2, drop: 0, push: LoopState),
  8: Instruction[ HL: Billing Provider..](pop: 3, drop: 0, push: TableState),
  9: Instruction[ HL: Subscriber Hiera..](pop: 3, drop: 0, push: TableState),
 10: Instruction[ HL: Patient Hierachi..](pop: 3, drop: 0, push: TableState),
 11: Instruction[ SE: Transaction Set ..](pop: 3, drop: 4, push: TableState),
 12: Instruction[ ST](pop: 4, drop: 0, push: TransactionSetState),
 13: Instruction[ GE: Functional Group..](pop: 4, drop: 2),
 14: Instruction[ GS](pop: 5, drop: 0, push: FunctionalGroupState),
 15: Instruction[IEA: Interchange Cont..](pop: 5, drop: 2),
 16: Instruction[ISA](pop: 6, drop: 0, push: InterchangeState))]

Chaining Method Calls

The Either datatype allows chaining via the #map, #or, #flatmap, and #tap methods. The use of each method is demonstrated in the following examples.


The #map method transforms one Either.success into another Either.success. It leaves Either.failure values unaltered, passing them through.

result = machine.find(:REF).map do |ref|
  # When a REF segment was found, this block is
  # executed. The return value of this block is
  # wrapped by Either.success. If a REF segment
  # was not found, this block is not executed and
  # the Either.failure is propogated by #map

# Briefer syntax, implementing Symbol#to_proc
machine.find(:REF).map(&:id)  #=> Either.success(:REF)

machine.find(:REF).map(&:id).map(&:to_s)  #=> Either.success("REF")

machine.find(:REF).map(&:id).map(&:to_s).map(&:length)  #=> Either.success(3)


To transform one Either.success into another Either, which may be a success or failure, use the #flatmap method. Like #map, it also leaves Either.failure values unaltered. This is an important technique to traverse the parse tree. For instance, the following shows how to locate the "Billing Provider Organization" of the first claim in the parse tree.

  flatmap{|x| x.find(:GS)                 }.
  flatmap{|x| x.find(:ST)                 }.
  flatmap{|x| x.find(:HL, nil, nil, "20") }.
  flatmap{|x| x.find(:NM1, "85")          }.
  flatmap{|x| x.element(3)                }.
  map(&:node)  #=> Either.success(AN.value[E1035: Billing Provider Last or Organizational Name](BEN KILDARE SERVICE))

You can use #flatmap to iterate a sequence of segments with the same identifier, or use #iterate which does the same thing.

# Find the first HL segment
position = machine.first.
  flatmap{|x| x.find(:GS) }.
  flatmap{|x| x.find(:ST) }.
  flatmap{|x| x.find(:HL) }

while position.defined?
  position = position.flatmap do |hl|
    # Process the HL segment

    # Find the next HL segment

Note that the value returned by the block given to #flatmap must return an instance of Either or a TypeError will be raised. The Object#try method is similar to #flatmap in many ways.


The #iterate method is a convenience method that calls #find repeatedly with the given arguments, yielding its result to a block.

# Process each ST in the first ISA envelope
machine.first.flatmap do |isa|
  isa.iterate(:GS) do |gs|
    gs.iterate(:ST) do |st|
      # ...

Note that the search starts ahead of the current position, so the following machine.first.flatmap{|m| m.iterate(:ISA) {|isa| ... }} will never yield the first ISA segment in the document, because it started searching after m, which is the first ISA.

Since #iterate calls #find, it enforces the same syntactic constraints.

Side Effects

In cases where you do not want to transform the Either value, but only need to execute a side effect on Either.success, the #tap method is suitable. On Either.success values, it passes the wrapped value to the block and returns the original value, and it passes Either.failure values along without calling the block.

# Record GS06 Group Control Number
machine.first.flatmap{|x| x.find(:GS) }.tap do |gs|
  # The return value of this block is discarded
  gs.element(6).tap do |control|
    @numbers << control.node.to_s
end #=> Either.success(StateMachine[1](SegmentVal[GS](...)))

# Contrived example
  flatmap{|x| x.find(:GS) }.tap{|x| puts "Hi, GS" }
  flatmap{|x| x.find(:ST) }.tap{|x| puts "Hi, ST" }  #=> Either.success(StateMachine[1](SegmentVal[ST](...)))

The Object#tap method is similar to Either#tap except #tap always calls the block, even on nil, while #tap does not call the block on Either.failure values.

Error Recovery

When a method call earlier in the chain returns a Either.failure, the error will be propogated through the chain unless it the #or method is used to recover from the error.

result = machine.find(:ST).map do |st|
end.or do |reason|
  # This block is executed if #find failed. The
  # block must return an instance of Either, or
  # a TypeError will be raised

# Result is guaranteed to be Either.success because
# of the block given to #or. The wrapped value then
# is StateMachine or FalseClass
result.defined?  #=> true

Word of Caution

Beware that the #find method only searches forward in the sequence of segments. In some cases, you will need to save the current position to let you restart another search from that position, rather than chaining successive searches together.

For instance, in the X222 837P transaction set, there are sixteen different consecutive DTP segments in Loop 2300. While the X222 implementation guide arranges them in what appears to be a sequence, there is no restriction on the order in which the DTP segments occur -- they all have the same position. Thus DTP*439 "Accident Date" can follow or preceed DTP*096 "Discharge Date", and one or both might not be present.

clm = machine.first.
  flatmap{|x| x.find(:GS) }.
  flatmap{|x| x.find(:ST) }.
  flatmap{|x| x.find(:HL, nil, nil, nil, "0") }.
  flatmap{|x| x.find(:CLM) }

clm. # Wrong: this assumes DTP*439 occurs before DTP*096
  flatmap{|x| x.find(:DTP, "439") }.tap{|x| puts "accident  ..." }
  flatmap{|x| x.find(:DTP, "096") }.tap{|x| puts "discharge ..." }

# Correct: no order is assumed among the DTP segments
clm.flatmap{|x| x.find(:DTP, "439") }.tap{|x| puts "accident  ..." }
clm.flatmap{|x| x.find(:DTP, "096") }.tap{|x| puts "accident  ..." }

In general, siblings following the first segment in a purely syntactic node, like a table, loop, or envelope structure cannot exist unless the syntactic node exists -- and the syntactic node cannot exist unless an entry segment from the definition of that node occurs. With few exceptions, the entry segment of a syntactic node is the first segment in its definition. Therefore, the 2300 DTP segments cannot exist unless the 2300 CLM segment occurs; that is why it is best to save the CLM position and use it as a starting point.


Certain sequences of input segments can be described by more than one parse tree. Often these sequences are malformed. For instance, in an X222 837P transaction, an HL segment that has an empty HL03 qualifier could potentially be the HL segment describing the "Billing Provider Detail", "Subscriber Detail", or "Patient Detail". In this case the parser will construct three parse trees: one for each possibility. The parser will respond to #deterministic? with false when it is in a non-deterministic state.

In a non-determistic state, methods that normally return a single node will return Either.failure("non-deterministic state"). These are #segment, #element, and #zipper. Traversal methods, however, like #next #first, #last, #find, and #parent, operate on each parse tree in parallel.

These traversal methods will position the parser on parallel segments within each parse tree. To use the HL example again, the parser would point to each tree's version of the HL segment, one named "Patient Detail", one named "Bililng Provider Detail", and another named "Subscriber Detail". These segments all have the same element values, but have a different meaning.