Here's a first tweak: Tighten recursion so there's fewer lines to read that contain recursion.

go :: String
go = let vind = Prelude.map toInd [(4,4),(2,3),(4,3),(1,1),(1,3),(2,1),(3,1),(1,1)]
         pool = V.fromList vind
         cmp x y = indCmpFit x y 0
         pool_s = poolSort pool cmp
     in show $ rankSort pool_s

rankSort :: Pool -> Fronts
rankSort = foldl new_fronts start_front where
  start_front = V.fromList [V.fromList [(pool V.! 0){ rank = 0 }]] -- first ind. always has rank 0
  new_fronts :: Ind -> Fronts -> Fronts
  new_fronts ind fronts = if rank' == V.length fronts
    then V.snoc fronts (V.fromList [upd_ind]) -- add a new front with ind as member
    else V.update fronts (V.fromList [(rank', upd_front)]) -- add individual to existing front
    where
      upd_ind = rankIndex ind fronts
      rank' = rank upd_ind
      upd_front = V.snoc (fronts V.! rank') upd_ind -- add individual to already existing front

Edit: Second: Shuffle arguments around so eta reduction is feasible. Also inline most stuff that's only used once.

indCmpFit :: Int -> Ind -> Ind -> Ordering
indCmpFit fitIdx x y

go :: String
go = let vind = Prelude.map toInd [(4,4),(2,3),(4,3),(1,1),(1,3),(2,1),(3,1),(1,1)]
     in show $ rankSort $ poolSort (V.fromList vind) (indCmpFit 0)

Here's a first tweak: Tighten recursion so there's fewer lines to read that contain recursion.

go :: String
go = let vind = Prelude.map toInd [(4,4),(2,3),(4,3),(1,1),(1,3),(2,1),(3,1),(1,1)]
         pool = V.fromList vind
         cmp x y = indCmpFit x y 0
         pool_s = poolSort pool cmp
     in show $ rankSort pool_s

rankSort :: Pool -> Fronts
rankSort = foldl new_fronts start_front where
  start_front = V.fromList [V.fromList [(pool V.! 0){ rank = 0 }]] -- first ind. always has rank 0
  new_fronts :: Fronts -> Ind -> Fronts
  new_fronts fronts ind = if rank' == V.length fronts
    then V.snoc fronts (V.fromList [upd_ind]) -- add a new front with ind as member
    else V.update fronts (V.fromList [(rank', upd_front)]) -- add individual to existing front
    where
      upd_ind = rankIndex ind fronts
      rank' = rank upd_ind
      upd_front = V.snoc (fronts V.! rank') upd_ind -- add individual to already existing front

Edit: Second: Shuffle arguments around so eta reduction is feasible. Also inline most stuff that's only used once.

indCmpFit :: Int -> Ind -> Ind -> Ordering
indCmpFit fitIdx x y

go :: String
go = let vind = Prelude.map toInd [(4,4),(2,3),(4,3),(1,1),(1,3),(2,1),(3,1),(1,1)]
     in show $ rankSort $ poolSort (V.fromList vind) (indCmpFit 0)

Here's a first tweak: Tighten recursion so there's fewer lines to read that contain recursion.

go :: String
go = let vind = Prelude.map toInd [(4,4),(2,3),(4,3),(1,1),(1,3),(2,1),(3,1),(1,1)]
         pool = V.fromList vind
         cmp x y = indCmpFit x y 0
         pool_s = poolSort pool cmp
     in show $ rankSort pool_s

rankSort :: Pool -> Fronts
rankSort = foldl new_fronts start_front where
  start_front = V.fromList [V.fromList [(pool V.! 0){ rank = 0 }]] -- first ind. always has rank 0
  new_fronts :: Ind -> Fronts -> Fronts
  new_fronts ind fronts = if rank' == V.length fronts
    then V.snoc fronts (V.fromList [upd_ind]) -- add a new front with ind as member
    else V.update fronts (V.fromList [(rank', upd_front)]) -- add individual to existing front
    where
      upd_ind = rankIndex ind fronts
      rank' = rank upd_ind
      upd_front = V.snoc (fronts V.! rank') upd_ind -- add individual to already existing front

Here's a first tweak: Tighten recursion so there's fewer lines to read that contain recursion.

go :: String
go = let vind = Prelude.map toInd [(4,4),(2,3),(4,3),(1,1),(1,3),(2,1),(3,1),(1,1)]
         pool = V.fromList vind
         cmp x y = indCmpFit x y 0
         pool_s = poolSort pool cmp
     in show $ rankSort pool_s

rankSort :: Pool -> Fronts
rankSort = foldl new_fronts start_front where
  start_front = V.fromList [V.fromList [(pool V.! 0){ rank = 0 }]] -- first ind. always has rank 0
  new_fronts :: Ind -> Fronts -> Fronts
  new_fronts ind fronts = if rank' == V.length fronts
    then V.snoc fronts (V.fromList [upd_ind]) -- add a new front with ind as member
    else V.update fronts (V.fromList [(rank', upd_front)]) -- add individual to existing front
    where
      upd_ind = rankIndex ind fronts
      rank' = rank upd_ind
      upd_front = V.snoc (fronts V.! rank') upd_ind -- add individual to already existing front

Edit: Second: Shuffle arguments around so eta reduction is feasible. Also inline most stuff that's only used once.

indCmpFit :: Int -> Ind -> Ind -> Ordering
indCmpFit fitIdx x y

go :: String
go = let vind = Prelude.map toInd [(4,4),(2,3),(4,3),(1,1),(1,3),(2,1),(3,1),(1,1)]
     in show $ rankSort $ poolSort (V.fromList vind) (indCmpFit 0)