ECL was created to be the programming language for all of our HPCC Systems technology. Therefore, it must be able to meet all the demands of a complete business solution: from data ingest, through querying and processing, and all the way to fulfillment and customer output.
In most every business solution that we create, the end-users will be using some kind of a custom Graphical User Interface (GUI) application specific to their business (typically created for them by us) to specify their queries into the data and set up processing jobs for the supercomputer. These custom GUI applications can generate for the user the ECL that will actually perform the query or process. The task of generating that ECL can be daunting if approached through a hard-coding perspective when you consider the exponential curve of all possible sets of choices the user could make in any moderately-complex system, and as the system grows more complex the problem becomes even worse. That means that a hard-coding solution is out of the question.
ECL's Template language provides the solution to this problem. The Template language is a Meta-language that takes standard XML input, typically generated from an end-user GUI application (thereby vastly simplifying the coding problem in the GUI) and in turn generating the appropriate ECL code to implement the user's choices.
Template Language statements all begin with # to clearly differentiate them from the ECL code that will be generated by the template. Most statements take parameters that determine their specific action in each instance.
The required statement terminator is the semi-colon (just as in ECL) and there are multi-line structures that terminate with the #END statement. These structures may be nested within each other.
Template Language uses user-defined symbols as variables. These symbols must be explicitly declared before use (see #DECLARE). The tag names in the XML text being processed are also treated like user-defined symbols.
A user-defined symbol or XML tag is referenced by surrounding the name of the symbol or tag with percent signs. An XML tag used as a template symbol may be a simple tag name, or an xpath to the XML data to retrieve (see the RECORD structure documentation for a description of the supported xpath syntax). If an xpath is used, then the symbol used must be the full xpath to the data expressed inside curly braces ({}). This syntax takes several forms:
| %symbol% | returns the value of the symbol |
| %'symbol'%. | returns value of the symbol as a string |
| %'' % | (an empty string) returns the contents of the current XML tag |
| %{xpath}% | returns the value of the data pointed to by the xpath |
| %'{xpath}'%. | returns value of the data pointed to by the xpath as a string |