`stack query` generates a `.cabal` file as side effect

[andreasabel]

Running stack query, e.g. stack query compiler will create a .cabal file from package.yaml if such .cabal file does not exist yet.

Reproducer: create a directory with e.g. these two files:
stack.yaml

resolver: lts-22.44

package.yaml

name: stack-query-created-me
library:
  source-dirs: .

Then ask stack for the GHC version of the project:

$ stack query compiler actual                                                           
ghc-9.6.7

And, oops!, you have given birth to a new .cabal file:

$ ls *.cabal
stack-query-created-me.cabal

I am using the latest Stack (3.7.1):

$ stack --version                                                                      1 х │ ghc-9.6.7 hs │ 16:32:09 
Version 3.7.1, Git revision 331f4880f1a33f1d39e14317dc9ecc6d3d6d617d aarch64 hpack-0.38.1

I got here because I am using the zsh Powerlevel 10k prompt, and I was wondering why .cabal files were mysteriously appearing on my file system:

• haskell_stack is slow and has unwanted side effects romkatv/powerlevel10k#2890

[mpilgrem]

Yes, that is correct. stack query runs within a withBuildConfig withEnvConfig and like other similar Stack commands (for example stack build --dry-run) that involves configuring the build environment.

Stepping back, what information is being sought? If it is not build information, then there may be a better 'home' for that functionality than stack query.

[andreasabel]

Stepping back, what information is being sought?

Yeah, I don't really know. This Powerlevel 10k prompt displays all kinds of useful information on the environment of the current directory (git branch name etc.), so I guess they thought, why not print the GHC version if there is a stack.yaml present in the project?

That a command named query writes new files to the local directory must be very surprising to anyone who is not an implementer of stack.
So I think there should be prominent alerts in the documentation and also in the help text:

$ stack query --help                                                       
Usage: stack query [SELECTOR...] [--setup-info-yaml URL] 
                   [--snapshot-location-base URL]

  Query general build information (experimental).

Available options: ...

The expected behavior of stack query compiler would be that it extracts the resolver/snapshot from the stack.yaml file, computes the ghc version from this and prints it.
Apparently just a stack.yaml is not enough:

$ stack query compiler                                                                                         

Error: [S-636]
       Stack looks for packages in the directories configured in the packages and extra-deps fields defined in its project-level configuration file (usually
       stack.yaml). The current entry points to /Users/abela/tmp/stack-query-compiler-no-package-yaml/ but no Cabal file or package.yaml could be found there.

I think at this point it might be enough to add the alert to the documentation, to make potential users of the stack query command aware of the consequences.
I suppose the authors of the shell prompt would have refrained from adding a hook for stack if they had known about the consequences of running stack query.

[mpilgrem]

@andreasabel, there is a nuance to 'compiler' information: 'wanted' versus 'actual'. 'Wanted' is what is specified in the project-level configuration file and at the command line. 'Actual' is what is used. They can differ because of compiler-check options.

A BuildConfig value has information about 'wanted' (via its smWanted field).

A EnvConfig value also has information about 'actual' (via its sourceMap field).

I think it would be straightforward to add a stack ls compiler command that output the 'wanted' compiler but did not configure the build environment.

[andreasabel]

I think it would be straightforward to add a stack ls compiler command that output the 'wanted' compiler but did not configure the build environment.

Good.
I am not in a position to request this feature, though, as I do not have an application for it right now, and the "Powerlevel 10k" folks might not be actively developing their product anymore, given the disclaimer on their github repo.