Using the Cabal API with sandboxes

[nh2]

I have a question about the Cabal API and sandboxes.

When I'm using cabal configure in my project (which has a sandbox configured), everything is fine and I get no warnings.

However, if I use the Cabal API to configure the project like this: https://gist.github.com/nh2/cabe35a058f10ff5313d/8653affaf7ceb5190ce03f44333e9d45af174726 then I get:

% runhaskell cabal-packagedb-test.hs
Configuring ganeti-2.12...
Warning: This package indirectly depends on multiple versions of the same
package. This is highly likely to cause a compile failure.
package snap-server-0.9.4.5 requires mtl-2.1.3.1
package snap-core-0.9.6.3 requires mtl-2.1.3.1
package parsec-3.1.5 requires mtl-2.1.3.1
package lens-4.0.5 requires mtl-2.1.3.1
package json-0.7 requires mtl-2.1.3.1
package hslogger-1.2.4 requires mtl-2.1.3.1
package exceptions-0.6.1 requires mtl-2.1.3.1
package aeson-0.7.0.4 requires mtl-2.1.3.1
package regex-base-0.93.2 requires mtl-2.2.1
package ganeti-2.12 requires mtl-2.2.1
package zlib-enum-0.2.3.1 requires transformers-0.3.0.0
package transformers-compat-0.3.3.3 requires transformers-0.3.0.0
package transformers-base-0.4.2 requires transformers-0.3.0.0
package temporary-1.2.0.3 requires transformers-0.3.0.0
package semigroupoids-4.0.4 requires transformers-0.3.0.0
package profunctors-4.2.0.1 requires transformers-0.3.0.0
package mtl-2.1.3.1 requires transformers-0.3.0.0
package monads-tf-0.1.0.2 requires transformers-0.3.0.0
package monad-control-0.3.3.0 requires transformers-0.3.0.0
package lens-4.0.5 requires transformers-0.3.0.0
package exceptions-0.6.1 requires transformers-0.3.0.0
package enumerator-0.4.20 requires transformers-0.3.0.0
package distributive-0.4.4 requires transformers-0.3.0.0
package contravariant-0.6.1.1 requires transformers-0.3.0.0
package comonad-4.2.2 requires transformers-0.3.0.0
package blaze-builder-enumerator-0.2.0.6 requires transformers-0.3.0.0
package MonadCatchIO-transformers-0.3.1.2 requires transformers-0.3.0.0
package transformers-base-0.4.3 requires transformers-0.4.1.0
package mtl-2.2.1 requires transformers-0.4.1.0
package ganeti-2.12 requires transformers-0.4.1.0
package monad-control-0.3.3.0 requires transformers-base-0.4.2
package lifted-base-0.2.3.0 requires transformers-base-0.4.2
package ganeti-2.12 requires transformers-base-0.4.3

How does it work that cabal configure finds a way to configure the project without using multiple versions, but the Cabal API doesn't? Am I using the API correctly here?

My first guess is that somehow that I'm accidentally escaping the sandbox, because the output is talking about transformers-base-0.4.3 which I don't even have installed in my sandbox:

% cabal sandbox hc-pkg list transformers-base
/home/niklas/opt/ghc-7.8/lib/ghc-7.8.3/package.conf.d
/home/niklas/ganeti/.cabal-sandbox/x86_64-linux-ghc-7.8.3-packages.conf.d
   transformers-base-0.4.2

Same for transformers:

% cabal sandbox hc-pkg list transformers
/home/niklas/opt/ghc-7.8/lib/ghc-7.8.3/package.conf.d
   transformers-0.3.0.0
/home/niklas/ganeti/.cabal-sandbox/x86_64-linux-ghc-7.8.3-packages.conf.d

The only place from which Cabal can know about transformers-0.4.1.0 is from outside the sandbox:

% ghc-pkg list transformers
/home/niklas/opt/ghc-7.8/lib/ghc-7.8.3/package.conf.d
   transformers-0.3.0.0
/home/niklas/.ghc/x86_64-linux-7.8.3/package.conf.d
   transformers-0.4.1.0

Or is setting configPackageDBs = [Just "path/to/sandbox/packagedb"] as I do here not enough, and allows cabal to use the normal home directory package db?

[nh2]

I just noticed that in cabal.sandbox.config, there's a user-install: False. So I commented out the configUserInstall = toFlag True I used so far, and it seems to work now.

Is this the reason? Do I have to set configUserInstall to False if I want to refer to the sandbox / does the sandbox count as a global package db? If so, could we mention that in the haddocks of configUserInstall?

[23Skidoo]

Yes, we use user-install: False with sandboxes. There's a reason for this, which I don't remember off hand. This is really a hack, and we should be distinguishing between sandbox, global and user install types instead.

Second, there is currently no easily-accessible API for working with sandboxes since the sandbox-related code mostly resides inside cabal-install. So making code that uses Cabal API work with sandboxes can be a bit challenging, as you seem to have already noticed ;-)

[ezyang]

It's also worth noting that the Cabal API has a very simple constraint solver, and is not the one used by cabal configure; that constraint solver lives in cabal-install and is not easily accessible.

[nh2]

These are interesting points. It would be great if somebody could make a small summary of those and add them to a section in the Cabal API: "What does one have to take into account when working with sandboxes?"

[ttuegel]

Related to #1848.

[phadej]

sandbox functionality is removed.