utils/ListSetOps.hs

    {-
    (c) The University of Glasgow 2006
    (c) The GRASP/AQUA Project, Glasgow University, 1992-1998

    \section[ListSetOps]{Set-like operations on lists}
    -}

    {-# LANGUAGE CPP #-}

    module ListSetOps (
            unionLists, minusList,

            -- Association lists
            Assoc, assoc, assocMaybe, assocUsing, assocDefault, assocDefaultUsing,

            -- Duplicate handling
            hasNoDups, runs, removeDups, findDupsEq,
            equivClasses,

            -- Indexing
            getNth
       ) where

    #include "HsVersions.h"

    import Outputable
    import Util

    import Data.List

    getNth :: Outputable a => [a] -> Int -> a
    getNth xs n = ASSERT2( xs `lengthExceeds` n, ppr n $$ ppr xs )
                 xs !! n

    {-
    ************************************************************************
    *                                                                      *
            Treating lists as sets
            Assumes the lists contain no duplicates, but are unordered
    *                                                                      *
    ************************************************************************
    -}

    unionLists :: (Outputable a, Eq a) => [a] -> [a] -> [a]
    -- Assumes that the arguments contain no duplicates
    unionLists xs ys
      = WARN(length xs > 100 || length ys > 100, ppr xs $$ ppr ys)
        [x | x <- xs, isn'tIn "unionLists" x ys] ++ ys

    minusList :: (Eq a) => [a] -> [a] -> [a]
    -- Everything in the first list that is not in the second list:
    minusList xs ys = [ x | x <- xs, isn'tIn "minusList" x ys]

    {-
    ************************************************************************
    *                                                                      *
    \subsection[Utils-assoc]{Association lists}
    *                                                                      *
    ************************************************************************

    Inefficient finite maps based on association lists and equality.
    -}

    -- A finite mapping based on equality and association lists
    type Assoc a b = [(a,b)]

    assoc             :: (Eq a) => String -> Assoc a b -> a -> b
    assocDefault      :: (Eq a) => b -> Assoc a b -> a -> b
    assocUsing        :: (a -> a -> Bool) -> String -> Assoc a b -> a -> b
    assocMaybe        :: (Eq a) => Assoc a b -> a -> Maybe b
    assocDefaultUsing :: (a -> a -> Bool) -> b -> Assoc a b -> a -> b

    assocDefaultUsing _  deflt []             _   = deflt
    assocDefaultUsing eq deflt ((k,v) : rest) key
      | k `eq` key = v
      | otherwise  = assocDefaultUsing eq deflt rest key

    assoc crash_msg         list key = assocDefaultUsing (==) (panic ("Failed in assoc: " ++ crash_msg)) list key
    assocDefault deflt      list key = assocDefaultUsing (==) deflt list key
    assocUsing eq crash_msg list key = assocDefaultUsing eq (panic ("Failed in assoc: " ++ crash_msg)) list key

    assocMaybe alist key
      = lookup alist
      where
        lookup []             = Nothing
        lookup ((tv,ty):rest) = if key == tv then Just ty else lookup rest

    {-
    ************************************************************************
    *                                                                      *
    \subsection[Utils-dups]{Duplicate-handling}
    *                                                                      *
    ************************************************************************
    -}

    hasNoDups :: (Eq a) => [a] -> Bool

    hasNoDups xs = f [] xs
      where
        f _           []     = True
        f seen_so_far (x:xs) = if x `is_elem` seen_so_far
                               then False
                               else f (x:seen_so_far) xs

        is_elem = isIn "hasNoDups"

    equivClasses :: (a -> a -> Ordering) -- Comparison
                 -> [a]
                 -> [[a]]

    equivClasses _         []  = []
    equivClasses _   stuff@[_] = [stuff]
    equivClasses cmp items     = runs eq (sortBy cmp items)
      where
        eq a b = case cmp a b of { EQ -> True; _ -> False }

    {-
    The first cases in @equivClasses@ above are just to cut to the point
    more quickly...

    @runs@ groups a list into a list of lists, each sublist being a run of
    identical elements of the input list. It is passed a predicate @p@ which
    tells when two elements are equal.
    -}

    runs :: (a -> a -> Bool) -- Equality
         -> [a]
         -> [[a]]

    runs _ []     = []
    runs p (x:xs) = case (span (p x) xs) of
                    (first, rest) -> (x:first) : (runs p rest)

    removeDups :: (a -> a -> Ordering) -- Comparison function
               -> [a]
               -> ([a],     -- List with no duplicates
                   [[a]])   -- List of duplicate groups.  One representative from
                            -- each group appears in the first result

    removeDups _   []  = ([], [])
    removeDups _   [x] = ([x],[])
    removeDups cmp xs
      = case (mapAccumR collect_dups [] (equivClasses cmp xs)) of { (dups, xs') ->
        (xs', dups) }
      where
        collect_dups _           []         = panic "ListSetOps: removeDups"
        collect_dups dups_so_far [x]        = (dups_so_far,      x)
        collect_dups dups_so_far dups@(x:_) = (dups:dups_so_far, x)

    findDupsEq :: (a->a->Bool) -> [a] -> [[a]]
    findDupsEq _  [] = []
    findDupsEq eq (x:xs) | null eq_xs  = findDupsEq eq xs
                         | otherwise   = (x:eq_xs) : findDupsEq eq neq_xs
        where (eq_xs, neq_xs) = partition (eq x) xs