How to set up the Haskell library for access through GHCi using Nixos?

Question

I managed to install ghc with nix-env -i ghc .

Now I want to install the Haskell library, how to do this? For example, turtle library ( https://hackage.haskell.org/package/turtle ).

I ran nix-env -f "<nixpkgs>" -iA haskellPackages.turtle , however ghci and import Turtle are ghci :

 Prelude> import Turtle <no location info>: error: Could not find module 'Turtle' It is not a module in the current program, or in any known package. 

The output of ghc-pkg list :

 /nix/store/fvf278s3lqsjv488ahhdi8jx6i0qzsr9-ghc-8.0.2/lib/ghc-8.0.2/package.conf.d Cabal-1.24.2.0 array-0.5.1.1 base-4.9.1.0 binary-0.8.3.0 bytestring-0.10.8.1 containers-0.5.7.1 deepseq-1.4.2.0 directory-1.3.0.0 filepath-1.4.1.1 ghc-8.0.2 ghc-boot-8.0.2 ghc-boot-th-8.0.2 ghc-prim-0.5.0.0 ghci-8.0.2 haskeline-0.7.3.0 hoopl-3.10.2.1 hpc-0.6.0.3 integer-gmp-1.0.0.1 pretty-1.1.3.3 process-1.4.3.0 rts-1.0 template-haskell-2.11.1.0 terminfo-0.4.0.2 time-1.6.0.1 transformers-0.5.2.0 unix-2.7.2.1 xhtml-3000.2.1 

Answer 1

This works differently on NixOS due to cleanliness. NixOS 'GHC will only look at its own immutable installation directory and packages that were installed by the user using cabal install .

What you can do is install a GHC wrapper in your user profile that delivers a good set of packages when ghci starts ghci .

Create the my-ghc.nix :

 (import <nixpkgs> {}).haskellPackages.ghcWithPackages (hpkgs: with hpkgs; [ lens aeson turtle ]) 

Delete the previous attempt to avoid name conflicts:

 nix-env -e ghc turtle 

Install wrapped GHC:

 nix-env -if my-ghc.nix 

You can edit the file in the future and re-run this command.

When I work on a project, I prefer to use cabal2nix and nix-shell . (For an introduction, see Oliver Charles's blog post )

Answer 2

As an alternative answer to Robert, you can use the nix-shell environment by creating a shell.nix file with the contents:

 { pkgs ? import <nixpkgs> {} }: let myGhc = pkgs.haskellPackages.ghcWithPackages (hpkgs: with hpkgs; [ turtle ]); in pkgs.mkShell { buildInputs = [ myGhc ]; } 

And enter this environment with nix-shell .