Read/write issues on package config variables

[LasseBlaauwbroek]

The following is tested on Opam 2.1.0~beta2: Create an opam package my-package, with the following config file:

opam-version: "2.0"
variables {
          myvar: ["hello" "hello world"]
}

• [Read issue] Running opam var my-package:myvar results in hello hello world. This is problematic because I can not distinguish separate list items from spaces in a string. One possible solution would be to return the array verbatim, or at least retain the quotes.
• [Write issue] The help for opam var advertises the following syntax:

opam var [OPTION]... [VAR[=[VALUE]]]

However, running opam var my-package:myvar=test returns an error:

[ERROR] Variable my-package:myvar=test not found in in switch

(Note also the double "in" in the error message)

• [Feature request] In addition, it would be useful if opam var supports += and -= like opam option does.

[LasseBlaauwbroek]

Additional feature request: It would be nice if configuration variables could contain variables themselves. These should then be resolved when being read.

[LasseBlaauwbroek]

Additional bug: During installation, options with an empty array myvar: [] are filtered out. Then running opam var my-package:myvar gives an error. However, if I manually modify the installed my-package.config file, and re-add the variable, I do not get an error from opam var my-package:myvar.

[rjbou]

opam var documentation is not clear on package variables. This commands is designed to act (read/write) on opam defined variables : global and per switch ; and to read packages variable; It can't modify package defined variables as for that it's a package (opam or config file) modification.

For append, as opam variables can be string, string list or bool, it doesn't suit for such operation.

On variables display, we could display the same way than opam variables, ie verbatim. You'll have i this case ["hello" "hello world"] as my-var is a list.

[LasseBlaauwbroek]

It can't modify package defined variables as for that it's a package (opam or config file) modification.

Opam copies the config files of packages to <prefix>/.opam-switch/config/<pkg-name>.config. It seems to me that opam var should be able to safely modify this file. The modification would not change the package, but just the local config file of that package. The manual does not make this 100% clear, but I cannot find anywhere that this config file is meant to be immutable.

For append, as opam variables can be string, string list or bool, it doesn't suit for such operation.

In case of a string list, it would be a suitable operation right?

On variables display, we could display the same way than opam variables, ie verbatim. You'll have i this case ["hello" "hello world"] as my-var is a list.

That's what I'd propose as well.

[dra27]

Updates from a developer meeting this morning: control of package variables is a likely feature for opam 2.3. In the meantime, there are two changes which we will slate for 2.2:

• The documentation should be explicit that package variables are read-only
• The syntax opam var my-package:myvar=test should be recognised by opam in order to display a more meaningful error. Depending on whether my-package is installed, opam should display my-package is not installed (package variables are also read-only) or Package variables are read-only and cannot be updated using `opam var`.

[rjbou]

Documentation still need to be updated