[Haskell-cafe] LTS-10.* missing build-depends suggestion

[Rick Owens]

Dear Haskell Café,

I am a long-time Haskell developer with a proliferation of private
projects. I typically make very heavy use of stack and LTS. Today, I tried
to bootstrap my first project using LTS-10.* (with GHC-8.2.2) and I ran
into an interesting problem (for me).

In the past, with lts-9.* and below, when I started cranking on some code
and writing a bunch of imports:

> import Control.Monad.Trans.Class (MonadTrans)
> import Data.Map (Map)

I would try to build first without updating the cabal file build-depends,
and I would get something like the following error messages:

>   /path/to/Module.hs:48:1: error:
>       Failed to load interface for ‘Data.Map’
>       It is a member of the hidden package ‘containers-0.5.7.1’.
>       Perhaps you need to add ‘containers’ to the build-depends in your
.cabal file.
>       Use -v to see a list of the files searched for.
>
>   /path/to/Module.hs:21:1: error:
>       Failed to load interface for ‘Control.Monad.Trans.Class’
>       It is a member of the hidden package ‘transformers-0.5.2.0’.
>       Perhaps you need to add ‘transformers’ to the build-depends in your
.cabal file.
>       Use -v to see a list of the files searched for.


This was good because the error contained a suggested solution which
contained the missing package, and I could sort of lazily add the
build-depends I needed without having to memorize a (Module -> Package)
mapping. I even had developer tooling to do it automatically.

With lts-10.*, however, the analogous error message looks like this:

> /path/to/Module.hs:9:1: error:
>     Could not find module ‘Data.DoubleWord’
>     Use -v to see a list of the files searched for.
>   |
> 9 | import Data.DoubleWord (Word256(Word256), Word128(Word128))
>   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

I am wondering, is there a way to bring back the solution suggestion? Not
having access to it has made me realize how heavily I was using that
feature. What do others think about this missing bit of supplemental error
information?

Thanks,

-Rick Owens
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