A new YAML based format for "conda-build" files

cep-13.md

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+# A new Recipe format (Part 1)
+
+<table>
+<tr><td> Title </td><td> A new recipe format </td>
+<tr><td> Status </td><td> Accepted </td></tr>
+<tr><td> Author(s) </td><td> Wolf Vollprecht &lt;wolf@prefix.dev&gt;</td></tr>
+<tr><td> Created </td><td> May 23, 2023</td></tr>
+<tr><td> Updated </td><td> October 20, 2023</td></tr>
+<tr><td> Discussion </td><td>https://github.com/conda-incubator/ceps/pull/54</td></tr>
+<tr><td> Implementation </td><td>https://github.com/prefix-dev/rattler-build</td></tr>
+</table>
+
+## Abstract
+
+We propose a new recipe format that is heavily inspired by conda-build. The main
+change is a pure YAML format without arbitrary Jinja or comments with semantic
+meaning.
+
+## Motivation
+
+The conda-build format has grown over the years to become quite complex.
+Unfortunately it has never been formally "specified" and it has grown some
+features over time that make it hard to parse as straightforward YAML.
+
+The CEP attempts to introduce a subset of the conda build format that allows for
+fast parsing and building of recipes.
+
+### History
+
+A discussion was started on what a new recipe spec could or should look like.
+The fragments of this discussion can be found here:
+https://github.com/mamba-org/conda-specs/blob/master/proposed_specs/recipe.md
+The reason for a new spec are:
+
+- Make it easier to parse ("pure yaml"). conda-build uses a mix of comments and
+  jinja to achieve a great deal of flexibility, but it's hard to parse the
+  recipe with a computer
+- iron out some inconsistencies around multiple outputs (build vs. build/script
+  and more)
+- remove any need for recursive parsing & solving
+- cater to needs for automation and dependency tree analysis via a determinstic
+  format
+
+## Major differences with conda-build
+
+- no full Jinja2 support: no block support `{% set ...` support, only string
+  interpolation. Variables can be set in the toplevel "context" which is valid
+  YAML (all new features should be native to YAML specs)
+- Jinja variable syntax is changed to begin with `${{` so that it becomes valid
+  YAML, e.g. `- ${{ version }}`
+- Selectors use a YAML dictionary with `if / then / else` (vs. comments in
+  conda-build) and are only allowed in lists (dependencies, scripts, ...). The
+  syntax looks like:
+  ```yaml
+  - if: win
+    then: this
+    else: that  # optional
+  ```
+- for inline values, the Jinja ternary operator can be used, e.g. `number: ${{ 0
+  if linux else 100 }}`
+
+## Selectors
+
+Selectors in the new spec are only allowed in lists and take an explicit `if /
+then / else` syntax.
+
+For example the following `script` section:
+
+```yaml
+script:
+  - if: unix
+    then: |
+      # this is the unix script
+  - if: win
+    then: |
+      @rem a script for batch
+```
+
+The same could have been expressed with an `else`:
+
+```yaml
+script:
+  - if: unix
+    then: |
+      # this is the unix script
+    else: |
+      @rem a script for batch
+```
+
+This is a valid YAML dictionary. Selector if statements are simple boolean
+expressions and follow Python syntax. The following selectors are all valid:
+
+```
+win and arm64
+(osx or linux) and aarch64
+something == "test"
+```
+
+If the value of a selector statement is a list, it extends the "outer" list. For
+example:
+
+```yaml
+build:
+  - ${{ compiler('cxx') }}
+  - if: unix
+    then:
+      - make
+      - cmake
+      - pkg-config
+```
+
+evaluates for `unix == true` to a list with elements `[${{ compiler('cxx') }},
+make, cmake, pkg-config]`.
+
+### Preprocessing selectors
+
+You can add selectors to any item, and the selector is evaluated in a
+preprocessing stage. If a selector evaluates to `true`, the item is flattened
+into the parent element. If a selector evaluates to `false`, the item is
+removed.
+
+```yaml
+source:
+  - if: not win
+    then:
+      # note that we omit the `-`, both is valid
+      url: http://path/to/unix/source
+      sha256: 01ba4719c80b6fe911b091a7c05124b64eeece964e09c058ef8f9805daca546b
+    else:
+      - url: http://path/to/windows/source
+        sha256: 06f961b802bc46ee168555f066d28f4f0e9afdf3f88174c1ee6f9de004fc30a0
+```
+
+Because the selector is a valid Jinja expression, complicated logic is possible:
+
+```yaml
+source:
+  - if: win
+    then:
+      url: http://path/to/windows/source
+  - if: (unix and cmp(python, "2"))
+    then: 
+      url: http://path/to/python2/unix/source
+  - if: unix and cmp(python, "3")
+    then:
+      url: http://path/to/python3/unix/source
+```
+
+Lists are automatically "merged" upwards, so it is possible to group multiple
+items under a single selector:
+
+```yaml
+test:
+  commands:
+    - if: unix
+      then:
+        - test -d ${PREFIX}/include/xtensor
+        - test -f ${PREFIX}/include/xtensor/xarray.hpp
+        - test -f ${PREFIX}/lib/cmake/xtensor/xtensorConfig.cmake
+        - test -f ${PREFIX}/lib/cmake/xtensor/xtensorConfigVersion.cmake
+    - if: win
+      then:
+        - if not exist %LIBRARY_PREFIX%\include\xtensor\xarray.hpp (exit 1)
+        - if not exist %LIBRARY_PREFIX%\lib\cmake\xtensor\xtensorConfig.cmake (exit 1)
+        - if not exist %LIBRARY_PREFIX%\lib\cmake\xtensor\xtensorConfigVersion.cmake (exit 1)
+
+# On unix this is rendered to:
+test:
+  commands:
+    - test -d ${PREFIX}/include/xtensor
+    - test -f ${PREFIX}/include/xtensor/xarray.hpp
+    - test -f ${PREFIX}/lib/cmake/xtensor/xtensorConfig.cmake
+    - test -f ${PREFIX}/lib/cmake/xtensor/xtensorConfigVersion.cmake
+```
+
+## Templating with Jinja
+
+The spec supports simple Jinja templating in the `recipe.yaml` file.
+
+You can set up Jinja variables in the context YAML section:
+
+```yaml
+context:
+  name: "test"
+  version: "5.1.2"
+  major_version: ${{ version.split('.')[0] }}
+```
+
+Later in your `recipe.yaml` you can use these values in string interpolation
+with Jinja. For example:
+
+```yaml
+source:
+  url: https://github.com/mamba-org/${{ name }}/v${{ version }}.tar.gz
+```
+
+Jinja has built-in support for some common string manipulations.
+
+In the new spec, complex Jinja is completely disallowed as we try to produce
+YAML that is valid at all times. So you should not use any `{% if ... %}` or
+similar Jinja constructs that produce invalid YAML. We also do not use the
+standard Jinja delimiters (`{{ .. }}`) because that is confused by the YAML
+parser as a dictionary. We follow Github Actions and others and use `${{ ... }}`
+instead:
+
+```yaml
+package:
+  name: {{ name }}   # WRONG: invalid yaml
+  name: ${{ name }} # correct
+```
+
+Jinja functions work as usual. As an example, the `compiler` Jinja function will
+look like this:
+
+```yaml
+requirements:
+  build:
+    - ${{ compiler('cxx') }}
+```
+
+## Shortcomings
+
+Since we deliberately limit the amount of "Jinja" that is allowed in recipes
+there will be several shortcomings.
+
+For example, using a `{% for ... %}` loop is prohibited. After searching through
+the conda-forge recipes with the Github search, we found for loops mostly used
+in tests. 
+
+In our view, for loops are a nice helper, but not necessary for many tasks: the
+same functionality could be achieved in a testing script, for example. At the
+same time we also plan to formalize a more powerful testing harness ([prototyped
+in
+boa](https://github.com/mamba-org/boa/blob/main/docs/source/recipe_spec.md#check-for-file-existence-in-the-final-package)).
+
+This could be used instead of a for loop to check the existence of shared
+libraries or header files cross-platform (instead of relying on Jinja templates
+as done
+[here](https://github.com/conda-forge/opencv-feedstock/blob/2fc7848655ca65419050336fe38fcfd87bec0649/recipe/meta.yaml#L131)
+or
+[here](https://github.com/conda-forge/boost-cpp-feedstock/blob/699cfb6ec3488da8586833b1500b69133f052b6f/recipe/meta.yaml#L53)).
+
+Other uses of `for` loops should be relatively easy to refactor, such as
+[here](https://github.com/conda-forge/libiio-feedstock/blob/1351e5846b753e4ee85624acf3a14aee4bcf321d/recipe/meta.yaml#L45-L51).
+
+However, since the new recipe format is "pure YAML" it is very easy to create
+and pre-process these files using a script, or even generating them with Python
+or any other scripting language. That means, many of the features that are
+currently done with Jinja could be done with a simple pre-processing step in the
+future.
+
+Another option would be to allow "full" Jinja inside the test script text blocks
+(as long as it doesn't change the structure of the YAML).
+
+## Examples
+
+### xtensor
+
+Original recipe [found
+here](https://github.com/conda-forge/xtensor-feedstock/blob/feaa4fd8015f96038168a9d67d69eaf06a36d63f/recipe/meta.yaml).
+
+```yaml
+context:
+  name: xtensor
+  version: 0.24.6
+  sha256: f87259b51aabafdd1183947747edfff4cff75d55375334f2e81cee6dc68ef655
+
+package:
+  name: ${{ name|lower }}
+  version: ${{ version }}
+
+source:
+  fn: ${{ name }}-${{ version }}.tar.gz
+  url: https://github.com/xtensor-stack/xtensor/archive/${{ version }}.tar.gz
+  sha256: ${{ sha256 }}
+
+build:
+  number: 0
+  # note: in the new recipe format, `skip` is a list of conditional expressions
+  #       but for the "YAML format" discussion we pretend that we still use the 
+  #       `skip: bool` syntax
+  skip: ${{ true if (win and vc14) }}
+
+requirements:
+  build:
+    - ${{ compiler('cxx') }}
+    - cmake
+    - if: unix
+      then: make
+  host:
+    - xtl >=0.7,<0.8
+  run:
+    - xtl >=0.7,<0.8
+  run_constrained:
+    - xsimd >=8.0.3,<10
+
+test:
+  commands:
+  - if: unix
+    then:
+      - test -d ${PREFIX}/include/xtensor
+      - test -f ${PREFIX}/include/xtensor/xarray.hpp
+      - test -f ${PREFIX}/share/cmake/xtensor/xtensorConfig.cmake
+      - test -f ${PREFIX}/share/cmake/xtensor/xtensorConfigVersion.cmake
+  - if: win
+    then:
+      - if not exist %LIBRARY_PREFIX%\include\xtensor\xarray.hpp (exit 1)
+      - if not exist %LIBRARY_PREFIX%\share\cmake\xtensor\xtensorConfig.cmake (exit 1)
+      - if not exist %LIBRARY_PREFIX%\share\cmake\xtensor\xtensorConfigVersion.cmake (exit 1)
+
+about:
+  home: https://github.com/xtensor-stack/xtensor
+  license: BSD-3-Clause
+  license_family: BSD
+  license_file: LICENSE
+  summary: The C++ tensor algebra library
+  description: Multi dimensional arrays with broadcasting and lazy computing
+  doc_url: https://xtensor.readthedocs.io
+  dev_url: https://github.com/xtensor-stack/xtensor
+
+extra:
+  recipe-maintainers:
+    - some-maintainer
+```
+