pkg: defer evaluating platform vars in commands #11475
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Package build and install commands are expressions that generate the command to run by evaluating a series of conditional statements. This allows the commands to be tailored to the current machine where the command is being run. Previously dune would evaluate platform-specific variables (e.g. os, arch) in these expressions at solve-time, producing lockfiles specialized to the machine where they are solved. This change defers evaluation of these variables until build-time. The consequence of deferring evaluation of platform variables to build time is that lockfiles will preserve more of the conditional logic found in the corresponding opam file. Note that some variables are still evaluated, such as `with-test`, as these variables are not based on the platform. The benefit of deferring evaluation of platform variables is that users of computers that differ from the computer where the lockfile was generated will be able to run the build/install commands specialized for their computer, which is more likely to work than attempting to run the build/install commands specialized for the computer where the lockfile was generated. Note that this change alone is not sufficient for making lockfiles portable, as it doesn't address the fact that the dependencies of a package can also vary depending on platform-specific variables. Signed-off-by: Stephen Sherratt <[email protected]>
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@rgrinberg let me know if you'd like this change to be behind a feature flag. |
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The last time we tried doing this, we weren't able to correctly model the semantics of opam's formula evaluation in the dune language. Did we solve those problems? |
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Package build and install commands are expressions that generate the command to run by evaluating a series of conditional statements. This allows the commands to be tailored to the current machine where the command is being run. Previously dune would evaluate platform-specific variables (e.g. os, arch) in these expressions at solve-time, producing lockfiles specialized to the machine where they are solved. This change defers evaluation of these variables until build-time.
The consequence of deferring evaluation of platform variables to build time is that lockfiles will preserve more of the conditional logic found in the corresponding opam file. Note that some variables are still evaluated, such as
with-test, as these variables are not based on the platform. The benefit of deferring evaluation of platform variables is that users of computers that differ from the computer where the lockfile was generated will be able to run the build/install commands specialized for their computer, which is more likely to work than attempting to run the build/install commands specialized for the computer where the lockfile was generated.Note that this change alone is not sufficient for making lockfiles portable, as it doesn't address the fact that the dependencies of a package can also vary depending on platform-specific variables.