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What counts as language#

Several tools read the same model file. An engine builds the model and hands it to a solver. A renderer prints it as equations. A checker reads it in CI with no data. This page says which decisions the language makes for all of them, and which each tool makes for itself.

The test is one question:

If two tools answered this differently, would the file have two meanings?

Suppose the engine sums p over generator and the renderer prints a sum over snapshot. The file now means two things, and that is a bug. So the language decides what sum(p, over=generator) means, and both tools read the answer instead of working it out.

Suppose instead that the engine writes the model in one solver's file format and the renderer sets the page width to 80 characters. They disagree, and nothing is wrong. Each tool decides those things for itself.

Four rules follow from the test:

  • A name means one thing. p cannot be a variable in the engine and a parameter in the renderer.
  • The set of operators is fixed. A tool cannot add a roll that the others do not know.
  • Each operator has one rule for the dimensions it produces. sum(p, by=gen_bus, over=generator, into=bus) lands on bus for every program.
  • Degree is decided when the file loads. Whether x * y is allowed does not depend on which engine builds the model.

A piecewise: block and a sos: block each state ordinary variables and constraints, so the language decides what they state, and spec.expand() writes it out the same way for every tool.

What each tool decides for itself#

A tool can refuse a model for a reason of its own. One engine can only take a literal offset in shift. Another has no concept of a special-ordered set. A file format has no way to write a quadratic constraint. None of these is a disagreement about what the file means, so none of them is the language's to settle. If the language refused everything one tool cannot build, every other tool would inherit that limit.

So the boundary runs both ways:

  • A tool must not invent a rule about what the file means. If it needs one, the rule goes into the language, once.
  • The language must not state a rule about what one tool can build.

A file that every tool accepts can still be a file that one engine cannot build. Accepting and building are different steps.

How this differs from the limits#

The limits answer a different question: which operators and blocks may be added to the language at all. This page answers who decides a rule once the operator or block exists.