do sgml-parse

control structure

Syntax
do sgml-parse document (with id-checking switch-expression)? 
                       (with utf-8 switch-expression)?
                       (creating sgml-dtds{string-expression})? scan string-source-expression
   local-declaration*
   action*
done


do sgml-parse subdocument (with id-checking switch-expression)? 
                          (with utf-8 switch-expression)?
                          (creating sgml-dtds{string-expression})? scan string-source-expression
   local-declaration*
   action*
done


do sgml-parse instance (with document-element string-expression)? 
                       with (sgml-dtds{string-expression} | current sgml-dtd) 
                       (with id-checking switch-expression)? scan string-source-expression
   local-declaration*
   action*
done


Purpose

Basic usage

do sgml-parse is used to invoke the SGML parser. A number of activities must occur within a do sgml-parse block.

  1. Specify the type of data to be processed: document, subdocument, or instance.
  2. Provide a string source that will be used for input.
  3. Use either #content to consume the parsed markup, or a parse continuation operator (%c or suppress) to initiate processing of the data by markup rules.

The simplest use of do sgml-parse is to process a complete SGML document: 

  do sgml-parse document scan file "my-sgml.sgml"
     output "%c"
  done

This assumes that the file mysgml.sgml contains an SGML document. If the DTD and the instance are in different files, they can be joined: 

  do sgml-parse document scan file "my-dtd.dtd" || file "my-sgml.sgml"
     output "%c"
  done

Validating of multiple documents

If the same DTD is to be used to parse several input instances, it is best to pre-compile the DTD and store it on the built-in sgml-dtds shelf: 

  do sgml-parse document creating sgml-dtds{"my-dtd"} scan file "my-dtd.dtd" 
     suppress
  done
If the instance file names are stored on a shelf my-instances, then each instance can then be processed in turn: 
  repeat over my-instances
     do sgml-parse instance with sgml-dtds{"my-dtd"} scan file my-instances 
        output "%c"
     done
  again

Validating against an outer DTD

A nested SGML parse can use the same DTD as an outer SGML parse to validate its own input: for instance, 

  process
     using group "one"
     do sgml-parse document scan "<!doctype a ["
                              || "<!element a - - (b | #pcdata)*>"
                              || "<!element b - - (#pcdata)>]>"
                              || "<a><b>Hello, World!</b></a>"
        output "%c"
     done
  
  
  group "one"
     element "a"
        using group "b"
        do sgml-parse instance with current sgml-dtd scan "<a>Salut, Monde!</a>"
           output "%c"
        done
        output "%c"
  
  
     element "b"
        output "%c"
  
  
  group "b"
     element "a"
        output "%c"

In this program, the SGML parse launched in the element rule for a inside group one uses the same DTD as the parse launched in the process rule.

Validating a partial instance

It is possible to parse a partial instance: a piece of data comprising an element from a DTD which is not the doctype element of that DTD. In this case, the element to be used as the effective doctype for parsing the data is specified using the document-element argument: 

  do sgml-parse instance with document-element "lamb" with sgml-dtds{"my-dtd"} scan file "partinst.sgml" 
     output "%c"
  done
The element's start and end tags can be present, or they can be omitted if the element allows. SGML comments, processing instructions and even marked sections can precede and follow the element's start and end tags, but anything else (particularly other elements, data, entity references or usemap declarations) is an error.

Validating a subdocument

do sgml-parse can be used to parse an SGML subdocument. Subdocument processing can only occur in the middle of parsing another SGML document that includes the subdocument reference. The concrete syntax defined by the document currently being processed is used to parse the subdocument. In accordance with the SGML standard, the subdocument's text must not contain an SGML declaration.

This is an example of how to make references to SGML subdocument entities trigger parsing of the subdocument entities. The source of the subdocument entity text in the example is assumed to be a file whose name is either the system identifier (provided by a library rule), the public text description portion of the public identifier, or the name of the entity (uppercased and with .ent file extension appended). 

  external-data-entity #implied when entity is subdoc-entity
     local stream file-name
  
     output "subdoc depth exceeded!%n"
        when number of current subdocuments > 100
  
     do when entity is system
        set file-name to "%eq"
  
     else when entity is in-library
        set file-name to "%epq"
  
     else when entity is public
        do scan "%pq"
        match (["+-"] "//")? ((lookahead ! "//") any)* "//"
              [ \ " "]* " " "-//"?
              ((lookahead ! "//") any)* => public-text-description
           set file-name to public-text-description
        done
  
     else
        set file-name to "%uq.ent"
     done
  
     do sgml-parse subdocument scan file file-name
        output "%c"
     done

Processing a subdocument increments the integer value returned by the number of current subdocuments (and decrements it when the action has finished), but OmniMark does not issue an error message when the subdocument nesting level exceeds that allowed by the concrete syntax or when subdoc no is specified by the concrete syntax.

Controlling ID/IDREF checking

By default, OmniMark checks all SGML idref attributes to make sure they reference valid IDs. This checking may not be appropriate in processing a partial instance. It also takes time. It can be disabled using with id-checking followed by a switch expression. The following code will parse the specified document without checking IDREFs: 

  do sgml-parse document with id-checking false scan file "my-sgml.sgml"
     output "%c"
  done

Parsing documents with different character set encodings

SGML is an ASCII-based language. This means that character references greater than 127 (for example �) have no predefined encoding method appropriate to them. The OmniMark parser outputs character references between 128 and 255 as equivalent binary byte values. Character references greater than 255 cause a markup error.

If the document being processed contains numerical character references greater than 127, the parser can be instructed to output them as UTF-8 byte sequences. This will allow character references above 255 to be output as UTF-8 byte encodings. This is appropriate if, and only if, your output will be encoded and interpreted as a UTF-8 document.

If the document being processed is encoded in something other than UTF-8, it can be pre-processed into UTF-8: see Character set encoding for details.

To turn on UTF-8 output of character references, use the with utf-8 modifier with a switch expression that evaluates to true: 

  process
      do sgml-parse document with utf-8 true scan file "myfile.sgml"
          output "%c"
      done

Note that actual UTF-8 encoded characters in your input data are unaffected by this setting.

Note that with utf-8 can only be used with a full document or with a subdocument, but not with an instance parse.

Rules fired by SGML parsing

When parsing a document, markup rules are fired as follows (if specified in your code):

The same rules fire when parsing a subdocument, except for sgml-declaration-end.

When compiling a DTD, markup rules are fired as follows (if specified in your code):

When parsing an instance, the following markup rules fire:

As with subdocument, instance saves and resets the integer value returned by the number of current subdocuments and restores the saved value when the action is finished.

do sgml-parse saves the current setting of sgml-in and sgml-out and restores them at the end of the parse.

If there are errors in the SGML declaration or prolog (DTD), then prolog-in-error rule will be fired instead of prolog-end and the processing of the remaining input of do sgml-parse will terminate. Execution resumes in the actions following the most recently executed %c or suppress. However, the amount of input read is undefined in this situation. That is, OmniMark may choose to consume the entire input source, it may stop reading the input immediately, or it may do something in between.

Related Syntax
Related Concepts