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  "$type": "site.standard.document",
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  "path": "/t/is-it-okay-to-use-the-cabal-solver-to-conditionally-define-dependencies-and-instances-like-this/14103#post_1",
  "publishedAt": "2026-05-14T03:45:55.000Z",
  "site": "https://discourse.haskell.org",
  "textContent": "Let’s say I have a package `alice`, and a datatype that can be an instance of a class in package `bob`, but otherwise `alice` does not depend on `bob`\n\nThe common approach is to define a library `alice-bob-instance` and define an orphan instance inside this package.\n\nIf people need the `bob` instance for `alice` they can just import `alice-bob-instance`.\n\nThe problem is, yes, this is an orphan instance, so wherever one wants to use the instance you’ll have to explicitly import the module it comes from which is a bit annoying.\n\nBut let’s say I did something different. I actually release two versions of `alice-bob-instance`. A version `0`, which was a completely empty package with no dependencies. And a version `0.1` which actually has the implementation of the `alice-bob` instance, but not the instance declaration itself.\n\nNow anyone needing the instance will actually need to import `alice-bob-instance > 0`, but that’s not a big deal.\n\nThen, inside `alice.cabal`, I do the following:\n\n\n    Flag alice-bob-instance-flag {\n      Description: Enable Bob instance\n      Default: False\n      Manual: False\n    }\n\n    if flag(alice-bob-instance-flag)\n      build-depends: alice-bob-instance > 0\n    else\n      build-depends: alice-bob-instance = 0\n\n\nThen I can use CPP to conditionally import the `alice-bob-instance` module and also to implement the instance (which will just be a call to the code in that module).\n\nIt seems like this just avoids the orphan instances, whilst ensuring that instances (and hence dependencies) are only pulled in if they’re explicitly needed. Also whilst this could be done with manual flags, if a package that depends on `alice` and `bob` needs the instance whilst it can’t set a flag, but it can set an dependency on `alice-bob-instance > 0`.\n\nNow arguably this is a little naughty as a flag is changing the external interface of the module (i.e. we’re exporting a new instance) but surely lots of automatic flags do this, as they seem to be designed to allow compatibility with a larger set of dependent library versions. Unless a library is completely opaquely wrapping the types in that dependent library something is going to leak into the interface anyway.\n\nIs this a pattern that’s okay to use or is it a terrible idea?",
  "title": "Is it okay to use the cabal solver to conditionally define dependencies and instances like this?"
}