example.hs

-- in Haskell, you can do this:

data T =
    A
  | B

-- or this:

data T
  = A
  | B

-- but not this:

data T =
  | A -- note leading "|"
  | B

-- which makes sorting/diffs/merging a nightmare

-- this preprocessor fixes that. just invoke the Haskell
-- compiler with "-F -pgmF ./preprocessor.pl"

preprocessor.pl

#!/usr/bin/env perl
use strict;
use warnings;

shift @ARGV;
open my $in, '<', shift(@ARGV);
open my $out, '>', shift(@ARGV);

my $data_decl_start = 0;
while (<$in>) {
    if ($data_decl_start) {
        s/^\s+\|//;
        $data_decl_start = 0;
    } elsif (/^data .*?=$/) {
        $data_decl_start = 1;
    }
    print $out $_;
}