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

[Michael Snoyman]

Nice repro Dan! I hope you don't mind, I took your repro and opened a GHC
Trac ticket about this:

https://ghc.haskell.org/trac/ghc/ticket/14717#ticket

On Thu, Jan 25, 2018 at 9:29 AM, Dan Burton <danburton.email at gmail.com>
wrote:

> 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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