# Pinning versions

The previous re-packing of .tar.gz to the .nar (Nix Archive) format, to be able to extract the SRI, can be done by using the following shell script and a Hackage Downloads .tar.gz link:

#!/bin/sh

url="$1"

if [ "${url}" == "" ] 
then
    echo "You must provide a URL: ./nixos_hash_sha256_nar.sh https://…/….tar.gz"
else
    nar=$(nix-prefetch-url --type sha256 --unpack ${1})
    
    echo "# Nixos SHA-256 SRI (Sub-Resource Integrity) for '${url}' is:"
    nix hash convert \
        --extra-experimental-features nix-command \
        --hash-algo sha256 --to sri ${nar}
    echo
fi

You will now be able to use the information from above, on the header script files as such:

#! /usr/bin/env nix-shell
#! nix-shell --keep --pure -i runghc
#! nix-shell --keep --pure -p haskell.compiler.ghc9103 cacert curl git
#! nix-shell --keep --pure -p '(((haskellPackages.override{ overrides = self: super: { containers = haskell.lib.dontCheck(self.callHackage "containers" "0.8" {});};}).extend (self: super: {pin = self.callHackageDirect { pkg = "A-gent"; ver = "0.11.0.5"; sha256 = "1f+EBe7HLUtvre7j17hF81J4KRdWPM71wl7TcUCQJHU=";} {};})).ghcWithPackages (ps: with ps; [pin]))'

Due to the limitations of nix scripting, we sadly need to pass the package pinning as a one-liner (horizontal scroll code block from above). However, for the sake of transparency, we are going to explain in detail:

• haskellPackages.override: As there are always some kind of issues with the containers package as a dependency, we therefore override the current packages default with a specific version from Hackage:

  haskellPackages.override{ 
    overrides = self: super: { 
      containers = haskell.lib.dontCheck(self.callHackage "containers" "0.8" {});
    };
  }

• haskellPackages.extend: If the latest package hasn’t been made part of the nix Haskell packages, we can retrieve it directly from Hackage:

  haskellPackages.extend (
    self: super: { 
      pin = self.callHackageDirect { 
        pkg = "A-gent"; 
        ver = "0.11.0.5"; 
        sha256 = "1f+EBe7HLUtvre7j17hF81J4KRdWPM71wl7TcUCQJHU=";
      } {};
    }
  )

• haskellPackages.ghcWithPackages: Once we have updated the package definitions, we can now use the packages:

  haskellPackages.ghcWithPackages (ps: with ps; [pin])