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Version: 7.0

Brief: metaprogramming

META and calling metacode (@)​

Metacode is a block of lsFusion code with parameters that produces other code when used. It is declared by the META statement — the name, the parameter list, a sequence of statements, the closing END — and used by the @ statement, which names the metacode and passes the arguments: every metacode parameter is replaced with the argument passed in all the places it is used.

Besides substitution, the metacode has operations on tokens: ## concatenates two adjacent tokens into one, and ### does the same while converting the first character of the second token to uppercase — except when everything to its left came out empty and the token itself does not start with ###, where the capitalization is skipped, so prefix###name with an empty prefix yields name, not Name. They are what the names of the generated elements are built from. The mechanism is described in metaprogramming.

Analogy: a macro with textual substitution of parameters.

META objectProperties(object, caption)
    object##Name 'Name of '##caption = DATA BPSTRING[100](object);
    object##Value 'Value of '##caption = DATA INTEGER (object);
END

@objectProperties(Document, 'the document');   // DocumentName 'Name of the document' = ...

Generated code and call arguments​

The body of a metacode consists of module-level statements, and the @ statement itself is written at module level: a metacode produces declarations of system elements, not operators inside an action body.

Hence the two typical uses: a family of same-shaped declarations for a class or a prefix passed in, and a set of elements added to an arbitrary form through EXTEND FORM with the form name as a parameter (Brief: extensions).

An argument of the call can be a composite ID, a class identifier, a literal, or an empty parameter — but not the value of a property: choosing behaviour by data is done not with a metacode but with abstract properties and actions.

The IDE writes the produced code after the @ call in braces so that navigation and analysis work on it; the platform ignores that block and generates the code anew, so it must not be edited by hand.