Partition.hs

data Decl a
  = TypeSig String a
  | Law String a a
  | FunModel String a
  | TypeModel String a
  | Import a
  | Other a
  deriving (Show)

decls =
  [ TypeSig "Hello" 1,
    Law "Fred" 1 42,
    FunModel "Fun" 2,
    Import 10,
    Other 99,
    Law "Be good" 2 3,
    TypeModel "huh" 0
  ]

partitionThem :: [Decl a] -> ([Decl a], [Decl a], [Decl a], [Decl a])
partitionThem = foldMap pickSlot
  where
    pickSlot lw@Law {} = ([lw], mempty, mempty, mempty)
    pickSlot fm@FunModel {} = (mempty, [fm], mempty, mempty)
    pickSlot tm@TypeModel {} = (mempty, mempty, [tm], mempty)
    pickSlot other = (mempty, mempty, mempty, [other])