[Haskell-cafe] Running ghci in a Cabal sandbox

[Brandon Allbery]

ghc including ghci does not know about sandboxes. At all. That is what
cabal/stack exec is for; it runs a command inside the sandbox.

There are sandboxing mechanisms that don't require this, but they require
your shell dotfiles to be set up in a way almost nobody does these days
(proper separation of environment variables; otherwise you get things like
the sandbox's package database, but the wrong $PATH). hsenv used to work
that way.

On Friday, August 4, 2017, Emil Axelsson <78emil at gmail.com
<javascript:_e(%7B%7D,'cvml','78emil at gmail.com');>> wrote:

> Thanks for your reply! But I actually don't want `cabal repl`. The bigger
> picture is explained here:
>
> http://fun-discoveries.blogspot.com/2017/08/building-haskell
> -projects-with-ghc.html
>
> Towards the end of that post I suggest using `cabal exec` to integrate
> with a Cabal sandbox; however, as Daniel Trstenjak pointed out, that
> shouldn't be needed since `cabal-cargs` already sets the `-package-db` flag
> for GHC.
>
> So my question is why `cabal exec` is needed (in this particular case)
> even though `-package-db` is given?
>
> I know `cabal exec` sets a few environment variables
> (`CABAL_SANDBOX_CONFIG`, `CABAL_SANDBOX_PACKAGE_PATH` and
> `GHC_PACKAGE_PATH`). I've checked that these are all set correctly, so
> Cabal seems to be doing its job. But it's not clear why ghci gets confused
> when these variables are not set (and `-package-db` is given), but not when
> the variables are set.
>
> Cheers
>
> / Emil
>
> Den 2017-08-04 kl. 15:31, skrev nek0:
>
>> Hi Emil,
>>
>> What you want is hidden behind the `cabal repl` command, which starts
>> ghci with the package-db of the sandbox.
>>
>> Greetings,
>>
>> nek0
>>
>> On 4.8.2017 13:41, Emil Axelsson wrote:
>>
>>> Hi!
>>>
>>> I have a small file Test.hs alone in a directory:
>>>
>>>      {-# LANGUAGE DeriveGeneric #-}
>>>
>>>      module Test where
>>>
>>>      import Data.Hashable
>>>      import Data.Scientific
>>>      import GHC.Generics
>>>
>>>      data Sc = Sc Scientific deriving (Generic)
>>>
>>>      instance Hashable Sc
>>>
>>> To be able to load this file, I set up a Cabal sandbox:
>>>
>>>      $ ghc --numeric-version
>>>      8.0.2
>>>
>>>      $ cabal --numeric-version
>>>      1.24.0.2
>>>
>>>      $ cabal sandbox init
>>>      ...
>>>
>>>      $ cabal install hashable-1.2.6.0 scientific
>>>      ...
>>>
>>> (Note: not the latest version of hashable.)
>>>
>>> Now, if I try to run GHCi and point it to the sandbox' package database
>>> I get this error:
>>>
>>>      $ ghci
>>> -package-db=.cabal-sandbox/x86_64-linux-ghc-8.0.2-packages.conf.d
>>> Test.hs GHCi, version 8.0.2: http://www.haskell.org/ghc/  :? for help
>>>      [1 of 1] Compiling Test             ( Test.hs, interpreted )
>>>
>>>      Test.hs:12:14: error:
>>>          • No instance for (Hashable Scientific)
>>>              arising from a use of
>>> ‘hashable-1.2.6.1:Data.Hashable.Class.$dmhashWithSalt’
>>>          • In the expression:
>>>              hashable-1.2.6.1:Data.Hashable.Class.$dmhashWithSalt @Sc
>>>            In an equation for ‘hashWithSalt’:
>>>                hashWithSalt
>>>                  = hashable-1.2.6.1:Data.Hashable.Class.$dmhashWithSalt
>>> @Sc
>>>            In the instance declaration for ‘Hashable Sc’
>>>
>>> Somehow it mixes in version 1.2.6.1 of hashable, even though this
>>> package isn't installed (neither in the sandbox nor the global database).
>>>
>>> It turns out that wrapping the command in `cabal exec` fixes the problem:
>>>
>>>      $ cabal exec -- ghci
>>> -package-db=.cabal-sandbox/x86_64-linux-ghc-8.0.2-packages.conf.d
>>> Test.hs
>>>      GHCi, version 8.0.2: http://www.haskell.org/ghc/  :? for help
>>>      [1 of 1] Compiling Test             ( Test.hs, interpreted )
>>>      Ok, modules loaded: Test.
>>>      *Test>
>>>
>>> Any idea what's going on?
>>>
>>> / Emil
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-- 
brandon s allbery kf8nh                               sine nomine associates
allbery.b at gmail.com                                  ballbery at sinenomine.net
unix, openafs, kerberos, infrastructure, xmonad        http://sinenomine.net
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