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GSLgen GSLgen
Version 2.0

The General Schema Language (GSL)


GSL is a scripting language developed by iMatix Corporation. It was first designed as a schema language for code generation and has grown into a powerful tool for manipulating XML data.

GSL is related to a reporting language such as is used to generate reports from a relational database, in that it provides a mechanism for iterating through the data, performing calculations and outputing text based on the data. Unlike a reporting language it can also manipulate, create, load and save data.

Many GSL concepts are borrowed directly from database terminology, to which it is closely related.

Important Concepts

Data Types

GSL recognises two data types: numeric and string. It makes no formal distinction between them; if a value looks numeric then it is treated as such, otherwise it is treated as a string.


A string constant is specified with either single- or double-quotes as delimiters, for example: "ABC". String constants may continue over several source lines. The line break is considered part of the string constant, unless the last character in the line is a single backslash (`\') in which case neither the backslash nor the line break is part of the string. A numeric constant is a simple number with an optional sign and optional decimal characters, for example 123 and -0.3.


A scope corresponds to an XML item or, more precisely, it is the presentation of an XML item to a piece of GSL script. It typically, although not necessarily, has the same name as the XML item. A scope is created by the `for' and `new' instructions and closed by the corresponding `endfor' and `endnew'. In between these lines, the value and attributes of the XML item can be defined and accessed. Child items (henceforth children) of the XML item can also be made available by introducing another scope with another `for' instruction.

At the start of processing of a script, two scopes are implicitly defined by GSLgen. The first corresponds to the top-level item of the XML file. We refer to this scope as the root scope. The second is named `global' and holds the predefined identifiers (see below) and command-line switches. Because these scopes are opened in this order, command-line switches take precedence over attributes of the root XML item when no scope name is specified.

Scopes may be referred to by name or by number. A positive number refers to the index of the scope starting from the first opened scope. A negative number or zero refers to the index of the scope relative to the most recently opened scope. One more complication:

Extended Scopes

An extended scope also corresponds to an XML item. In its simplest form it is just a scope specification. It may also contain any number of `member' specifications (`->'); these refer to children of the XML item, their children, and so on. This allows you to avoid introducing a new scope to access an only child or when you are only interested in the first child of that name.

Some examples of extended scopes:


The simplest form - this refers to a scope named `world'.


Shorthand for the most recently opened scope.

  1-> field

The first child named `field' of the XML item corresponding to the first open scope.

  -> parent-> baby

The first child named `baby' of the first child named `parent' of the XML item corresponding to the most recently opened scope.


GSL identifiers refer to XML attribute or item values. (An item value is the text between its open and close tags.) It generally consists of an extended scope specification and an attribute name. If the attribute name is missing then the identifier refers to the item value. There are also some short-hand forms.

There are a total of five different forms of identifier specification:

A full attribute specification.
An attribute specification with no scope specified. In this case GSL will search for an attribute with the given name in all the open scopes, starting with the most recently opened.
An attribute specification with an empty scope specification. This refers to the specified attribute in the most recently opened scope and is equivalent to `0.<attr>'.
An item value specification. This refers to the value of the XML item referred to by the extended scope specification.
An item value specification with no scope specified. This refers to the value of the item corresponding to the most recently opened scope.

To avoid clashes with GSL reserved words, the names of scopes, attributes and items may be enclosed in square brackets, for example: [for].

Some examples:


Case Sensitivity

GSLgen has two modes of handling the case of XML item and attribute names. In the default mode, GSLgen matches names without regard to the case (upper or lower) used to specify them. In certain substitutions GSLgen modifies the case of the value of the identifier to match the case used to specify the attribute name. In case-sensitive mode, GSLgen matches names taking into account the case, and does not modify the case of the result. See the description of subsitutions for details.

To change modes, set the value of the identifier `ignorecase' in the global scope to 0 or 1. Eg: `global.ignorecase = 0'


GSL expressions are much the same as expressions in other high-level programming languages. They include the following operators:

*, /
+, -
=, <>, >, >=, <, <=
Safe comparative
?=, ?<>, ?>, ?>=, ?<, ?<=
|, &, !

Operator precedence is standard (multiplicative, additive, default, comparative, logical) and brackets are treated as you would expect.

Logical operators treat zero as FALSE and non-zero as TRUE.

GSLgen optimises expression evaluation to the extent that the second operand of a binary logical operator (`|', `&') is not evaluated if the result of the expression is determined by the first operand. This allows you to use expressions such as

  defined (X) & X

since the second operator is not evaluated when X is undefined.

The default operator allows undefined expressions to be replaced by another expression. The value of

  <expr1> ? [<expr2>]

is equal to the value of <expr1>, if defined; otherwise it is equal to the value of <expr2>, whether or not the latter is defined. If the second operand <expr2> is omitted then the evaluation of the expression is `safe', that is, GSLgen does not object (when this is feasible) to the result of the expression being undefined. This feature can be used in symbol definitions and substitutions to make GSLgen accept an undefined expression. See the description of these instructions for details.

The safe comparative operators return the same result as their equivalent comparative operators when both operands are defined. If one or both operator is undefined, the safe operators return FALSE while the normal operators produce an error. Notice that `a ?<> b' returns TRUE if both a and b are defined and they are not equal and FALSE otherwise.

If an operand is not a constant then its type depends its value; if it looks like a number then it is treated as a number, otherwise it is treated as a string.

Generally, additive, multiplicative and logical operators only apply to numeric operands. There are two cases where an arithmetic operator can apply to string values:

"ABC" + "DEF" results in "ABCDEF"
"AB" * 3 results in "ABABAB"

Built-In Functions

count ([<scope> .] <child>, <expr>)
counts the number of children of the supplied or most recently opened scope of the given name. If an expression is specified, it is treated as a condition to determine which children are counted. In this case, a new scope `COUNT' is implicitly defined while the condition is evaluated. For example: count (ITEM, COUNT.NAME = "ABC") returns the number of children of the most recently opened scope whose attribute NAME has the value `ABC'.
index ([<scope>])
returns the index of the item associated with the supplied or most recently opened scope. This differs from the item function in that the index number is assigned after filtering (`by') and sorting (`where'), and is undefined withing `by' and `where' clauses. See the description of the `for' command.
item ([<scope>])
returns the item number corresponding to the supplied or most recently opened scope. This differs from the index function in that the item number is unaffected by sorting or filtering, and is available for use within a `where' or `by' clause. scope. See the description of the `for' command.
name ([<scope>])
returns the name of the XML item currently associated with the supplied or most recently opened scope. See the description of the `for' command.
exists (<FileName>)
returns a logical value (1 or 0) depending on whether a file of the given name exists of not.
timestamp (<FileName>)
returns the modification time of the file as a sixteen-character string CCYYMMDDHHMMSS00; undefined if the file does not exist.
defined ([Ident .] Ident)
returns a logical value (1 or 0) depending on whether the specified identifier is defined or not.
env (var)
returns the value of an environment variable.
length (string)
returns the length of a string (or number considered as a string).
substr (string, start, end, length)
returns a substring of the given string. This may be used in a number of ways: substr (string, start, end, ) - specify start and end offsets substr (string, start, , length) - specify start offset and length substr (string, , end, length) - specify end offset and length substr (string, start, , ) - end of string from start offset substr (string, , end, ) - same as substr (0, end, ) substr (string, , , length) - end of string of given length
trim (string)
trims a string by removing leading and trailing white space.
justify (string, width, prefix)
justifies the string according to the specified width, prefixing each line by the string `prefix'.
macro (name)
returns a logical value (1 or 0) depending on whether a macro or function with the given name is defined or not.
deleted ([<scope>])
returns a logical value (1 or 0) depending on whether the XML item associated with the specified scope has been deleted.

Template and Script Modes

GSL is useful as both a template and a scripting language. In template mode, the default mode when GSLgen starts with an XML file, script lines begin with a point (`.') and all other lines are template lines. In script mode, the default mode when GSLgen starts with a GSL script, template lines begin with a `>' (not necessarily in the first column) and all other lines are script lines. You can perform exactly the same operations in template and script modes - the only difference is convenience for the type of application.

You can change between template and script mode with the `template' and `endtemplate' commands. See the description of these commands below for details.

Template Lines

The simplest template line is just text, which is copied verbatim to the current output file. If no output file has been opened, or if the last output file has been closed, the output is copied to the standard output.

If the last character of an template line is a backslash (`\') then the line is output with no line terminator; otherwise a line terminator follows the template line. A backslash followed by another character is generally replaced by that character; this allows characters which would normally be interpreted as script commands to be output literally. There is one exception: `\n' is replaced by a line break. To output a backslash, use a double-backslash (`\\').

In template mode an template line is any line which does not begin with a point (`.'). If an template line must begin with a point, use a backslash immediately before the point.

In script mode, an template line begins with a greater-than sign (`>'), which is dropped before the line in output.

Script Lines

In template mode, these are introduced by a point (`.') as the first non-space character in the line. In script mode, any line that does not begin with a greater-than sign (`>') is a command line. In script mode, a script line may also begin with a point; this allows script commands to work in case the current mode is unknown.

The script commands are described below.

If a script command line ends with a backslash (`\') then the following script line is treated as a continuation of the current line.


There are three ways to include comments in GSL scripts. The first is to place a hyphen (`-') immediately after the point (`.') in a template mode or as the first character in script mode. The second is to place a hash (`#') after a GSL command. Any characters following the hyphen are ignored by GSLGen. The third way (pace Tony Blair) is to enclose comment text (which may continue over more than one line) inside comment markers (`/*' and `*/') just as in C. However if GSLGen finds these characters in a template line (but not inside a substitution) it assumes that they are destined for output, so does not treat them as a comment.


  .- This entire line is a comment
  .output "file"  # This is a trailing comment
  .output /* This is an embedded
  multi-line comment */ "file"
  If this is a template line then /* this is not a comment */
  $("but "/* this is */)

Substituting Symbols and Expressions

At any point within a template line, and in many places (described below) in a script line, a substitute construct may be used instead of literal text. The format for expression subsitution is:

  $( <expr> [% format] [: pretty-print] )

The order of the format and pretty-print modifiers is not important.

If the expression has a default operator with no second operand, and its result is undefined then the substitution resolves to an empty string.

If a format string is provided, it is used to format the result before continuing. The format string is similar to that used by the printf function in C. It must contain exactly one conversion specification, consisting of zero or more of the flags `#', `0', `-', ` ' and `+', an optional minimem field width, an optional precision consisting of a point (`.') followed by an optional number, and a mandatory conversion specifier among the following: `d', `i', `o', `u', `x', `X', `e', `E', `f', `g', `c' and `s'. The data are always converted to the appropriate type (one of long int, double, char or char *) for the conversion string. Note that not all legal C format strings are allowed in GSL.

The pretty-print modifier specifies how case modification and replacement of certain characters takes place. The valid pretty-print modifiers (not case-sensitive) are:

lower case
Neat Case Modification

More than one pretty-print modifier may be specified; they should be separated by commas.

If GSLgen is in ignore case mode, and a substition expression consists of a single identifier and no case-modifier is specified (c or cobol may still be specified), the case in which the identifier name is specified is used as an example to determine whether the case of the result should be modified to UPPER, lower or Neat. A final exception is that if an empty pretty-print string is provided, no case modification is performed.

Some examples: Assume the identifier IDENT has the value `IDENT value' and identifer XXX is undefined.

produces a run-time GSLgen error: Undefined expression.
`Ident Value'
`IDENT value'
$(1 + 1)

What You Can Substitute

A substitution can appear at any place inside straight text (template line or string constant) or an operand in an expression. It can also replace a single name in an identifier specification, but not a point (`.') or member (`->').

Some examples: Assume the identifier IDENT has the value `NUM' and identifer NUM has the value `1'.

`1.NAME' This may used in another expression as an identifer.


GSLgen can help to keep code neat by enlarging or shrinking white space so that column numbers match as far as possible between the script and the output file. For instance, in the value of the identifier X is ABCDEF then:

  $(X)   .

evaluates to



  $(X?"Undefined") .

evaluates to


The shuffle algorithm uses a parameter `shuffle' (actually an attribute of the global scope) whose numeric value influences the operation. It expands a block of white space longer than `shuffle' as much as necessary so that the text following the white space is output in the same column. It also shrinks white space down to a minimum of `shuffle' to make space for text preceeding the white space. If `shuffle' is zero, then shuffle is disabled. The default value of `shuffle' is 1; this is the value which produces the results shown above.

If the current output ends with a backslash, then the shuffle continues on the following line. Thus


evaluates to

  ABCDEF   .

Shuffle can cause problems in some cases, for example when outputting literal text where the size of white space is important. In this case shuffle should be disabled with

  .shuffle = 0


GSLgen helps you make neat COBOL code by automatically filling the first six characters of each line with the four-digit line number followed by two zeroes. To enable this function, define an attribute `cobol' of the root item either using

  .cobol = 1


  gslgen -cobol etc.

when you invoke GSLgen, or even (yuk) define an attribute COBOL right in your XML file.

Predefined Identifiers

There are some identifiers whose value is maintained by GSLgen. They are defined as attributes of the global item.

The name of GSL or template script file currently being processed.
The name of the XML file being processed.
The name of the current output file; undefined if there is none.
The line number of the line currently being output to the output file.
The name of the current application: gslgen.
The version of the current application.
The current date in the format YYYY/MM/DD
The current time in the format hh:mm:ss

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