Add new schema definition

.github/workflows/check_python_package.yaml

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+name: Check
+
+on:
+  push:
+  pull_request:
+    branches: ["main"]
+
+jobs:
+  test:
+    strategy:
+      fail-fast: false
+      matrix:
+        runs-on: [ubuntu-latest, macos-latest, windows-latest]
+        python-version: ["3.10", "3.11", "3.12", "3.13"]
+    runs-on: ${{matrix.runs-on}}
+    steps:
+      - uses: actions/checkout@v6
+        with:
+          fetch-depth: 0
+
+      - name: Install uv
+        uses: astral-sh/setup-uv@v7
+        with:
+          python-version: ${{ matrix.python-version }}
+
+      - name: Install the project
+        run: uv sync --all-extras --dev
+
+      - name: Lint with ruff
+        run: uv run ruff check --output-format=github .
+
+      - name: Run tests
+        run: uv run pytest --cov=opltools --cov-report=term-missing

SCHEMA.md

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+# OPL Schema
+
+The OPL schema catalogs optimization **problems**, **suites**, **generators**, and their **implementations** in a single, machine-readable format. 
+
+Three design choices shape everything below:
+
+1. **One flat library, keyed by ID.** 
+   Every entity lives in a `Library` dict. 
+   Suites reference problems, problems reference implementations using their respective ID.
+   There is no embedding of problems or implementations within suites to facilitate reuse. 
+   F.e. an implementation might be referenced by multiple problems or suites.
+2. **Numeric fields accept a scalar, a set, or a range.** 
+   A problem may have exactly `2` objectives, one of `{2, 3, 5}`, or any value in `{min: 2, max: 50}`. 
+   The same union type is used for variable dimensions and constraint counts.
+3. **Three-valued logic for yes/no fields.** 
+   Many boolean fields (f.e. `hard`, `allows_partial_evaluation`, ...) [`YesNoSome`](#yesnosome) as their value.
+   We lose some expressive power but simplify the data entry.
+   If we force authors to decide on yes or no, then we would need more complex structures for variables, constraints etc. and that would make the usual case unnecessarily complex.
+
+## Contents
+
+- [Library](#library) 
+- [Thing types](#thing-types)
+  - [Implementation](#implementation)
+  - [ProblemLike](#problemlike) (shared fields)
+    - [Problem](#problem)
+    - [Suite](#suite)
+    - [Generator](#generator)
+- [Shared building blocks](#shared-building-blocks)
+  - [Variable](#variable) / [VariableType](#variabletype)
+  - [Constraint](#constraint) / [ConstraintType](#constrainttype)
+  - [Reference](#reference) / [Link](#link)
+  - [ValueRange](#valuerange)
+  - [YesNoSome](#yesnosome)
+
+---
+
+## Library
+
+A `Library` is a dict from ID to a [Thing](#thing-types).
+IDs are free-form but must be unique and the convention is to add a prefix marking the type to avoid collisions:
+
+| Prefix  | Type             |
+|---------|------------------|
+| `impl_` | Implementation   |
+| `fn_`   | Problem          |
+| `suite_`| Suite            |
+| `gen_`  | Generator        |
+
+On load the library validates that every ID referenced by a suite (`problems`) or problem (`implementations`) exists and has the correct type. Suites also have their `fidelity_levels` auto-populated from their problems.
+
+```yaml
+impl_coco:
+  type: implementation
+  name: COCO
+  description: Comparing Continuous Optimisers
+fn_sphere:
+  type: problem
+  name: Sphere
+  objectives: [1]
+  implementations: [impl_coco]
+suite_bbob:
+  type: suite
+  name: BBOB
+  problems: [fn_sphere]
+```
+
+---
+
+## Thing types
+
+All entities inherit from `Thing`, which only carries a discriminator:
+
+```yaml
+type: problem  # or: suite | generator | implementation
+```
+
+We want to have as flat a structure as possible to make exploring and searching OPL as easy as possible. 
+That's one of the reasons the top level object is a dictionary of dissimilar things.
+But we need to be able to tell them apart so we have a `type` field to discriminate between them.
+
+### Implementation
+
+A pointer to code that implements one or more problems. 
+Intentionally minimal so that the schema describes *what* a problem is, not how to run it.
+There are separate files which contain curated usage examples for problems or suites keyed by their respective IDs.
+
+| Field             | Type                              | Notes                                        |
+|-------------------|-----------------------------------|----------------------------------------------|
+| `name`            | str                               | required                                     |
+| `description`     | str                               | required                                     |
+| `language`        | str (e.g. `Python`, `C`)          |                                              |
+| `links`           | list of [Link](#link)             | repo, release, docs…                         |
+| `evaluation_time` | set of str                        | free-form list ("8 minutes", "fast")         |
+| `requirements`    | str or list of str                | URL to requirements file or list of packages |
+
+```yaml
+impl_coco:
+  type: implementation
+  name: COCO
+  description: Comparing Continuous Optimisers benchmarking platform
+  language: c
+  links:
+    - {type: repository, url: https://github.com/numbbo/coco-experiment}
+impl_py_cocoex:
+  type: implementation
+  name: Python bindings for COCO
+  description: The Python bindings for the experimental part of the COCO framework
+  language: Python
+  links:
+    - {type: source, url: https://github.com/numbbo/coco-experiment/tree/main/build/python}
+    - {type: package, url: https://pypi.org/project/coco-experiment/}
+```
+
+### ProblemLike
+
+Fields shared by [Problem](#problem), [Suite](#suite), and [Generator](#generator). 
+The schema deliberately puts most descriptive fields here so suites can be characterised without explicitly having to add all problems in the suite.
+
+| Field                                    | Type                                           | Notes                                              |
+|------------------------------------------|------------------------------------------------|----------------------------------------------------|
+| `name`                                   | str                                            | required                                           |
+| `long_name`                              | str                                            |                                                    |
+| `description`                            | str  (markdown)                                | longer prose                                       |
+| `tags`                                   | set of str                                     | free-form keywords                                 |
+| `references`                             | set of [Reference](#reference)                 |                                                    |
+| `implementations`                        | set of IDs?                                    | must resolve to [Implementation](#implementation)s |
+| `objectives`                             | set of int?                                    | e.g. `{1}`, `{2, 3}` — **not** a ValueRange        |
+| `variables`                              | set of [Variable](#variable)                   |                                                    |
+| `constraints`                            | set of [Constraint](#constraint)               | omit entirely for unconstrained                    |
+| `dynamic_type`                           | set of str                                     | `{"no"}`, `{"time-varying"}`…                      |
+| `noise_type`                             | set of str                                     | `{"none"}`, `{"gaussian"}`…                        |
+| `allows_partial_evaluation`              | [YesNoSome](#yesnosome)                        |                                                    |
+| `can_evaluate_objectives_independently`  | [YesNoSome](#yesnosome)                        |                                                    |
+| `modality`                               | set of str                                     | `{"unimodal"}`, `{"multimodal"}`                   |
+| `fidelity_levels`                        | set of int                                     | `{1}` = single-fidelity, `{1,2}` = multi-fidelity  |
+| `code_examples`                          | set of str                                     | paths to example scripts                           |
+| `evaluation_time`                        | set of str                                     | free-form list ("8 minutes", "fast")         |
+| `source`                                 | set of str                                     | `{"artificial"}`, `{"real-world"}`                 |
+
+> `objectives` is a set of integers because we don't assume extreme scalability in this property so explicit enumeration is fine.
+> Dimensions of variables on the other hand are ranges because here problems often are scalable over wide ranges.
+
+When no `evaluation_time` is set, it percolates up from any referenced implementations.
+The same is true for the `variables` and `constraints` properties of a suite that has references to problems.
+
+### Problem
+
+One optimization problem (possibly parameterised by instances).
+
+Adds:
+
+| Field       | Type                                       | Notes                                      |
+|-------------|--------------------------------------------|--------------------------------------------|
+| `instances` | [ValueRange](#valuerange) or list of str?  | e.g. `{min: 1, max: 15}` or named variants |
+
+```yaml
+fn_sphere:
+  type: problem
+  name: Sphere
+  objectives: [1]
+  variables: [{type: continuous, dim: {min: 2, max: 40}}]
+  modality: [unimodal]
+  source: [artificial]
+  instances: {min: 1, max: 15}
+  implementations: [impl_coco]
+```
+
+### Suite
+
+A curated, fixed collection of problems.
+
+Adds:
+
+| Field      | Type         | Notes                                         |
+|------------|--------------|-----------------------------------------------|
+| `problems` | set of IDs?  | must resolve to [Problem](#problem)s          |
+
+`fidelity_levels` is auto-unioned from member problems at validation time.
+
+```yaml
+suite_bbob:
+  type: suite
+  name: BBOB
+  problems: [fn_sphere, fn_rosenbrock, fn_rastrigin]
+  objectives: [1]
+  source: [artificial]
+  implementations: [impl_coco]
+```
+
+### Generator
+
+A parametric family of problems — unlike a [Suite](#suite), the member problems are not enumerated. Uses the same fields as [ProblemLike](#problemlike) with no additions; the distinction from [Problem](#problem) is that a generator produces instances on demand.
+
+```yaml
+gen_mpm2:
+  type: generator
+  name: MPM2
+  description: Multiple peaks model, second instantiation
+  objectives: [1]
+  variables: [{type: continuous, dim: {min: 1}}]
+  modality: [multimodal]
+```
+
+---
+
+## Shared building blocks
+
+### Variable
+
+A group of decision variables of the same type. 
+Multi-type problems list multiple entries.
+While you can have multiple entries of the same type, this should be justified in some way like when you can evaluate the problem on only one subset of variables.
+
+| Field  | Type                                          | Default              |
+|--------|-----------------------------------------------|----------------------|
+| `type` | [VariableType](#variabletype)                 | `unknown`            |
+| `dim`  | int, set of int, [ValueRange](#valuerange), or null | `0`            |
+
+```yaml
+variables:
+  - {type: continuous, dim: 10}
+  - {type: integer, dim: {min: 1, max: 5}}
+```
+
+### VariableType
+
+`continuous | integer | binary | categorical | unknown`. 
+Use `unknown` for permutation/combinatorial problems the schema doesn't yet distinguish **and** add an appropriate tag.
+We are actively watching for unknown variable types and are open to extending the above list if there is a critical mass of problems to justify it.
+
+### Constraint
+
+A group of constraints. 
+To indicate that the problem is unconstrained, you need an _empty_ `constraints` field.
+A missing `constraints` field or if it is set to `null` means it is not known if unconstrained.
+
+| Field      | Type                                          | Notes                              |
+|------------|-----------------------------------------------|------------------------------------|
+| `type`     | [ConstraintType](#constrainttype)             | default `unknown`                  |
+| `hard`     | [YesNoSome](#yesnosome)                       | hard vs. soft                      |
+| `equality` | [YesNoSome](#yesnosome)                       | equality vs. inequality            |
+| `number`   | int, set of int, [ValueRange](#valuerange), null |                                 |
+
+```yaml
+constraints:
+  - {type: box, hard: yes, number: 10}
+  - {type: linear, hard: some, equality: no, number: {min: 1}}
+```
+
+### ConstraintType
+
+`box | linear | function | unknown`. `function` covers non-linear/black-box constraints.
+
+### Reference
+
+Bibliographic pointer. 
+Requires either a `title` or a `link` and optionally a list of  `authors`.
+
+```yaml
+references:
+  - title: "Honey Badger Algorithm: New metaheuristic algorithm for solving optimization problems."
+    authors: 
+      - Fatma A. Hashim
+      - Essam H. Houssein
+      - Kashif Hussain
+      - Mai S. Mabrouk
+      - Walid Al-Atabany      
+    link: {type: doi, url: "https://doi.org/10.1016/j.matcom.2021.08.013"]
+```
+
+### Link
+
+`{type?: str, url: str}`. 
+`type` is free-form (`repository`, `arxiv`, `paper`, `doi`, ...).
+`url` is a URL to some resource. 
+
+If `type` is `doi`, please use the full URL (starting with `https://doi.org/...`) instead of the raw DOI.
+
+### ValueRange
+
+An inclusive numeric range type. 
+At least one of `min`/`max` must be given.
+If `min` is given and `max` is missing, it does not imply that there is no upper bound. 
+There might be one, it is just not known.
+The same applies for the case where `max` is given and `min` is missing.
+
+```yaml
+dim: {min: 2}            # 2 or more
+dim: {min: 2, max: 40}   # between 2 and 40
+dim: {max: 100}          # up to 100
+```
+
+Used by `Variable.dim`, `Constraint.number`, `Problem.instances`.
+
+### YesNoSome
+
+Three-valued flag: `yes | no | some | ?` (the last serialises as the literal `'?'` string, meaning unknown).
+`some` captures the common case where *part* of something has some property.
+For example only some constraints might hard but we don't know the exact number of hard and soft constraints, only the total number.
+
+```yaml
+constraints: [{type: box, hard: some}]
+allows_partial_evaluation: "?"
+```