https://buildd.debian.org/status/package.php?p=haskell-harp&suite=sid
...
hlibrary.setup: Encountered missing or private dependencies:
base <4.15
at /usr/share/perl5/Debian/Debhelper/Buildsystem/Haskell/Recipes.pm line 107.
Debian::Debhelper::Buildsystem::Haskell::Recipes::run_quiet("debian/hlibrary.setup", "configure", "--ghc", "-v2", "--package-db=/var/lib/ghc/package.conf.d", "--prefix=/usr", "--libdir=/usr/lib/haskell-packages/ghc/lib", "--libexecdir=/usr/lib", ...) called at /usr/share/perl5/Debian/Debhelper/Buildsystem/Haskell/Recipes.pm line 131
Debian::Debhelper::Buildsystem::Haskell::Recipes::run("debian/hlibrary.setup", "configure", "--ghc", "-v2", "--package-db=/var/lib/ghc/package.conf.d", "--prefix=/usr", "--libdir=/usr/lib/haskell-packages/ghc/lib", "--libexecdir=/usr/lib", ...) called at /usr/share/perl5/Debian/Debhelper/Buildsystem/Haskell/Recipes.pm line 620
Debian::Debhelper::Buildsystem::Haskell::Recipes::configure_recipe() called at -e line 1
make: *** [/usr/share/cdbs/1/class/hlibrary.mk:153: build-ghc-stamp] Error 25