gen-constraints.pl

#!/usr/bin/perl

# A script to generate constraints based on the package versions installed
# in a cabal sandbox.
#
# Like cabal freeze / cabal-constraints, but is not limited to a single
# package or build tree.
#
# For this to work, you have either to be in a directory with
# cabal.sandbox.config, or have the environment set up
# (see http://ro-che.info/articles/2014-03-05-cabal-sandbox-tips.html)
#
# Then the usage is simply
#
#   gen-constraints >> cabal.config

use warnings;
use strict;

my $pkg_list;
open $pkg_list, "-|", "cabal sandbox hc-pkg list";

# state machine
# 0 = skipping
# 1 = interpreting
my $state = 0;

my @constrs;

while (<$pkg_list>) {
  chomp;
  if ( $state == 1 ) {
    my ($pkg_name, $pkg_ver) =
      /\s+(.*?)-([\d.]+)/ or next;
    push @constrs, "  $pkg_name == $pkg_ver";
  } else
  {
    if ( /\.cabal-sandbox.*packages\.conf\.d/ ) {
      $state = 1;
    }
  }
}

print
  "constraints:\n",
  join(",\n", @constrs),
  "\n";