How do I add the “containers” package to my .cabal file (without getting overwritten by stack at compile time)?

Question

I am working on the "roman-numerals" task from the exercism Haskell track and followed their instructions to installing stack. I am working on a Fedora 24 box.

As long as I was working with Haskell modules from base, I didn't have a problem. Now I am trying to import the Data.Map module. It works fine using the ghci command line:

$ ghci
GHCi, version 7.8.4: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Prelude> import Data.Map
Prelude Data.Map> 

However, when I try to import it from inside my src file with the command:

import qualified Data.Map as M (foldlWithKey, fromList)

I am running into problems when I try to run the test:

$ stack test
roman-numerals-0.0.0: build (lib + test)
Preprocessing library roman-numerals-0.0.0...
[2 of 2] Compiling Roman            (...)
(...) /roman-numerals/src/Roman.hs:3:1: 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.
Progress: 1/2
(...)

I googled the problem and found a straightforward solution at the Cabal FAQ at haskell.org:

What you need to do is to add containers to the build-depends in your .cabal file.

I am assuming they mean the file roman-numerals.cabal that is in my working directory. The contents are:

-- This file has been generated from package.yaml by hpack version 0.14.0.
--
-- see: https://github.com/sol/hpack

name:           roman-numerals
version:        0.0.0
build-type:     Simple
cabal-version:  >= 1.10

library
  hs-source-dirs:
      src
  build-depends:
      base
  exposed-modules:
      Roman
  other-modules:
      Paths_roman_numerals
  default-language: Haskell2010

test-suite test
  type: exitcode-stdio-1.0
  main-is: Tests.hs
  hs-source-dirs:
      test
  build-depends:
      base
    , roman-numerals
    , hspec
  default-language: Haskell2010

I tried to add "containers" to the build-depends in either and both the "library" and "test-suite" sections, but when I run

$ stack test

the error persists, and the .cabal file is reverted to the same contents shown above.

Any pointers? Much appreciated!

Answer 1

This is hinting at the problem:

-- This file has been generated from package.yaml by hpack version 0.14.0.
--
-- see: https://github.com/sol/hpack

hpack is an alternative, YAML-based specification format for Haskell packages which can be used instead of the traditional cabal format. The hpack program can then be used to convert a specification from the hpack format to the cabal format to be able to integrate with the rest of the Haskell toolchain.

Some basic support for hpack was added to stack some time ago. It checks for a file called package.yaml in the current directory, which is the standard name for hpack format package specifications, and if it exists, it runs hpack to convert it to a cabal file and then proceeds building as normal. This is what's trampling over your .cabal file.

To solve this, either:

• Modify package.yaml instead of roman-numerals.cabal to achieve the same effect.
• Delete package.yaml and continue working directly with roman-numerals.cabal.

The syntax for adding dependencies in the hpack format is:

dependencies:
  - base
  - containers