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

Rick Owens rick at owensmurray.com
Thu Jan 25 20:21:37 UTC 2018 

FWIW, I discovered how to easily find the missing package using ghc-pkg:

$ stack exec ghc-pkg -- find-module Data.DoubleWord
/home/rowens/.stack/programs/x86_64-linux/ghc-8.2.2/lib/ghc-8.2.2/package.conf.d
    (no packages)
/home/rowens/.stack/snapshots/x86_64-linux/lts-10.3/8.2.2/pkgdb
    data-dword-0.3.1.1
/home/rowens/projects/om-legion-objects/.stack-work/install/x86_64-linux/lts-10.3/8.2.2/pkgdb
    (no packages)

Thanks to Dan for bringing up ghc-pkg, which I had not previously had
occasion to learn much about.

-Rick

On Thu, Jan 25, 2018 at 11:32 AM, Rick Owens <rick at owensmurray.com> wrote:

> Thanks for the help/info guys!
>
> On Thu, Jan 25, 2018 at 3:51 AM, Michael Snoyman <michael at snoyman.com>
> wrote:
>
>> 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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