GHC找不到已安装的模块

Question

My haskell installation can not find bytestring module installed by operating system

$ ghci
GHCi, version 7.6.3: 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> :m +Data.ByteString.Lazy

<no location info>:
Could not find module `Data.ByteString.Lazy'
It is not a module in the current program, or in any known package.

But I have installed this module using yum:

$ rpm -ql ghc-bytestring
/usr/lib64/ghc-7.6.3/bytestring-0.10.0.2
/usr/lib64/ghc-7.6.3/bytestring-0.10.0.2/libHSbytestring-0.10.0.2-ghc7.6.3.so
/usr/share/doc/ghc-bytestring
/usr/share/doc/ghc-bytestring/LICENSE

What is wrong?

Answer 1

If this is happening, you should be able to figure out more via ghc-pkg list . This could happen, for example, if the binary package provided by your software repository was broken; ghc-pkg list would report that. In general, either GHC is not looking for packages in /usr/lib64/ghc-7.6.3/ or else that directory has a package.cache which was not updated to reflect the new package.

One thing that could cause GHC to look in the wrong place is if there are multiple GHCs on the machine: for example if which ghc reveals /usr/local/bin/ghc then you probably compiled GHC from source at some point and its packages are occupying some /usr/local/lib/ghc-7.6.3/package.conf.d/ folder, while your repository has installed /usr/bin/ghc which is looking in the folder you want.

Anyway, fixes: if the package.cache file exists and has a valid entry for the file, then you can run ghc -package-conf /path/to/package.cache ... to add those packages to your executable. If you have further problems, ghc -v ... is a great resource for debugging "which version of that package is being used here?" types of problems.

If the package.cache file does not exist then you've got a bigger problem, and probably the easiest way to move forward is to look for a directory under /home which appears on ghc-pkg list . Install the required package to that directory and GHC should pick up on it even though it doesn't understand these bigger contexts. You could also start working with a cabal sandbox of local packages to your project.

Answer 2

Situation here is similiar to C++ you have libraries used during dynamic linking stage and header used for compilation. In Fedora packages like ghc-bytestring are only libraries without headers. To install headers I had to install ghc-bytestring-devel package.

An example on Fedora 24:

server.hs:7:8:
    Could not find module ‘Data.Text’
    Perhaps you meant Data.Set (from containers-0.5.5.1)
    Locations searched:
      Data/Text.hs
      Data/Text.lhs

So change to user root , then:

What packages are there?

# dnf search ghc|grep text

ghc-text.x86_64 : An efficient packed Unicode text type
ghc-boxes.x86_64 : 2D text pretty-printing library
ghc-pango.x86_64 : Binding to the Pango text rendering engine
ghc-css-text.x86_64 : CSS parser and renderer
ghc-hgettext.x86_64 : Haskell binding to libintl
ghc-attoparsec.x86_64 : Fast combinator parsing for bytestrings and text
ghc-text-devel.x86_64 : Haskell text library development files
ghc-blaze-textual.x86_64 : Fast rendering of common datatypes
ghc-css-text-devel.x86_64 : Haskell css-text library development files
ghc-hgettext-devel.x86_64 : Haskell hgettext library development files
ghc-blaze-textual-devel.x86_64 : Haskell blaze-textual library development files

So what's installed?

# rpm --query ghc-text
ghc-text-1.1.1.3-3.fc24.x86_64

# rpm --query ghc-text-devel
package ghc-text-devel is not installed

So let's install the devel package.

# dnf install ghc-text-devel
Installed:
  ghc-text-devel.x86_64 1.1.1.3-3.fc24

...and compilation succeeds after that.