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Constructs

A generator produces a value. A construct decides what happens around that value: which one is used, whether it appears at all, how many of them go in a cell, whether it may repeat, and how many rows one record turns into.

That's the whole list: six constructs. Everything else in TDC is either a generator, a place to put text, or a way to compute something from values you already have.

ConstructThe question it answers
<mix>Which of several branches — in exact proportions?Choosing between values
<switch>Which value follows from another field?Lookup tables
ifShould this piece appear at all?Conditions
repeatHow many values go in one cell?Several values in a cell
eachHow many rows does one record produce?One row per element
uniq · <distinct>May a value repeat — in the row, or ever?Uniqueness

Three more things behave like constructs and are documented elsewhere, because they belong to a bigger topic:

  • <pool> — a row referencing a whole record instead of a value: thirty doctors, and a patient row that gets one of them entire. It grew into a topic of its own, with its own section.

  • parent — a value drawn from the subset its parent selected. It's part of how sequences relate to one another, so it lives with Sequences; the applied side is in Hierarchical dependencies.

  • <compute> — deriving a value by calculation instead of by drawing. It's a small language of its own, with its own section.

Four of them in one config​

Nothing here is contrived — this is what a config looks like once the constructs are doing the work. A plan drawn in fixed proportions, a price that follows from the plan, a variable number of tags in one cell, and a word that appears only on paid rows:

<env count="6" seed="tour">
  <mix name="Plan" percent="50,30,20">
    <case><gen type="text" value="free"/></case>
    <case><gen type="text" value="pro"/></case>
    <case><gen type="text" value="team"/></case>
  </mix>

  <switch name="Price" on="Plan">
    <map>free:0, pro:12, team:40</map>
  </switch>

  <sequence name="Tags">
    <gen type="text" value="api,web,cli,db" repeat="1..3" separator=";"/>
  </sequence>

  <sequence name="Id"><gen type="increment" value="1"/></sequence>
</env>

<block>
  <line>
    <data>${{Id}} ${{Plan}} $${{Price}} [${{Tags}}]</data>
    <data if="Price > 0"> paid</data>
  </line>
</block>
./run tour.tdc
1 free $0 [web;api]
2 free $0 [api;db;cli]
3 pro $12 [web] paid
4 free $0 [cli]
5 pro $12 [cli;api;db] paid
6 team $40 [db;web] paid

Read the output against the config and every construct is visible in it. Three of the six rows are free, which is the 50 in percent. No free row carries $12, because Price is derived from Plan rather than drawn on its own. The tag count changes from row to row. And paid is missing on exactly the rows where the price is zero.

Where to go next​

Each page in this section covers one construct in full. Read them in order if you're learning the language — they build on one another, and the examples get less trivial as they go.

Once the constructs are familiar, the Guides show what to build with them: coherent records, CSV and SQL output, statistical shapes, outliers, gaps, and datasets too big for memory.