Introduction

The Validation and Transformation Language is aimed at defining Transformations of the artefacts of the VTL Information Model, as more extensively explained in the User Manual.

A Transformation consists of a statement which assigns the outcome of the evaluation of an expression to an Artefact of the IM. The operands of the expression are IM Artefacts as well. A Transformation is made of the following components:

  • A left-hand side, which specifies the Artefact which the outcome of the expression is assigned to (this is the result of the Transformation);

  • An assignment operator, which specifies also the persistency of the left hand side. The assignment operators are two, the first one for the persistent assignment (<-) and the other one for the non-persistent assignment (:=).

  • A right-hand side, which is the expression to be evaluated, whose inputs are the operands of the Transformation. An expression consists in the invocation of VTL Operators in a certain order. When an Operator is invoked, for each input Parameter, an actual argument (operand) is passed to the Operator, which returns an actual argument for the output Parameter. In the right hand side (the expression), the Operators can be nested (the output of an Operator invocation can be input of the invocation of another Operator). All the intermediate results in an expression are non-persistent.

Examples of Transformations are:

DS_np := (DS_1 - DS_2 ) * 2;
DS_p <- if DS_np >= 0 then DS_np else DS_1;

(DS_1 and DS_2 are input Data Sets, DS_np is a non persistent result, DS_p is a persistent result, the invoked operators (apart the mentioned assignments) are the subtraction (-), the multiplication (*), the choice (if…then…else), the greater or equal comparison (>=) and the parentheses that control the order of the operators’ invocations.

Like in the example above, Transformations can interact with one another through their operands and results; in fact the result of a Transformation can be operand of one or more other Transformations. The interacting Transformations form a graph that is oriented and must be acyclic to ensure the overall consistency, moreover a given Artefact cannot be result of more than one Transformation (the consistency rules are better explained in the User Manual, see VTL Information Model / Generic Model for Transformations / Transformations consistency). In this regard, VTL Transformations have a strict analogy with the formulas defined in the cells of the spreadsheets.

A set of more interacting Transformations is usually needed to perform a meaningful and self-consistent task like for example the validation of one or more Data Sets. The smaller set of Transformations to be executed in the same run is called Transformation Scheme and can be considered as a VTL program.

Transformations do not necessarily need to be written in sequence like a classical software program. In fact, they are associated to the Artefacts they calculate, like it happens in the spreadsheets (each spreadsheet’s formula is associated to the cell it calculates).

Nothing prevents, however, from writing the Transformations in sequence, taking into account that the Transformations are not necessarily performed in the same order as they are written, because the order of execution depends on their input-output relationships (a Transformation which calculates a result that is operand of other Transformations must be executed first). For example, if the two Transformations of the example above were written in the reverse order:

(i) DS_p <- if DS_np >= 0 then DS_np else DS_1;
(ii) DS_np := (DS_1 - DS_2 ) * 2;

All the same the Transformation (ii) would be executed first, because it calculates the Data Set DS_np which is an operand of the Transformation (i).

When Transformations are written in sequence, a semicolon (;) is used to denote the end of a Transformation and the beginning of the following one.