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

[Dan Burton]

Hm, you're right. Seems like a GHC thing. I've tested a simple file with
"import Data.Map" on both a stack project and a cabal project -- with
containers installed in ghc's main package-db, as verified via stack/cabal
exec ghc-pkg list containers -- and it doesn't give the hint in either case.

Taking it a step further and removing cabal/stack out of the equation...

$ cat Main.hs
module Main where
import Data.Map
main = putStrLn "It compiles"

$ ghc-pkg list containers
/usr/local/Cellar/ghc/8.2.2/lib/ghc-8.2.2/package.conf.d
    containers-0.5.10.2

$ ghc --make -hide-all-packages -package base Main.hs
[1 of 1] Compiling Main             ( Main.hs, Main.o )

Main.hs:2:1: error:
    Could not find module ‘Data.Map’
    Use -v to see a list of the files searched for.
  |
2 | import Data.Map
  | ^^^^^^^^^^^^^^^

$ ghc --make -hide-all-packages -package base -package containers Main.hs
[1 of 1] Compiling Main             ( Main.hs, Main.o )
Linking Main ...

-- Dan Burton

On Wed, Jan 24, 2018 at 8:38 PM, Michael Snoyman <michael at snoyman.com>
wrote:

> FWIW, I've also noticed that with LTS 10/GHC 8.2.2 I no longer get the
> package name suggestions, even when I know I have the package installed. I
> don't know if it's an issue with Stack or GHC, I haven't investigated. But
> I can't think of something on the Stack side which would make the behavior
> different between GHC 8.0.2 and GHC 8.2.2.
>
> On Thu, Jan 25, 2018 at 4:02 AM, Dan Burton <danburton.email at gmail.com>
> wrote:
>
>> I believe the particular feature you are talking about is a feature of
>> GHC. Whether a suggestion is show or not merely depends on whether GHC sees
>> the package in question in its package database(s).
>>
>> If you are using stack, then you can look at one of your LTS 9 projects,
>> and run
>>
>> stack exec -- ghc-pkg list
>>
>> In order to see the list of installed packages. (I think something like
>> `cabal exec ghc-pkg list` should do the same for a cabal project.)
>>
>> Identify the packages you tend to use, go into one of your LTS 10
>> projects, and `stack install pkg1 pkg2 ...`
>>
>> Note that with stack, each minor version of LTS has its own package
>> database, so packages locally installed into lts-10.1 will not necessarily
>> be available in your local instance of lts-10.2. Repeat stack install to
>> taste.
>>
>> tl;dr the trick is to set up your package database so that GHC knows what
>> to suggest for you
>>
>> -- Dan Burton
>>
>> On Wed, Jan 24, 2018 at 5:12 PM, Rick Owens <rick at owensmurray.com> wrote:
>>
>>> 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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>>
>>
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>
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