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  1. fromListWith :: (Eq k, Hashable k) => (v -> v -> v) -> [(k, v)] -> HashMap k v

    unordered-containers Data.HashMap.Lazy

    Construct a map from a list of elements. Uses the provided function f to merge duplicate entries with (f newVal oldVal).

    Examples

    Given a list xs, create a map with the number of occurrences of each element in xs: 
    let xs = ['a', 'b', 'a']
in fromListWith (+) [ (x, 1) | x <- xs ]

= fromList [('a', 2), ('b', 1)]
    Given a list of key-value pairs xs :: [(k, v)], group all values by their keys and return a HashMap k [v]. 
    let xs = [('a', 1), ('b', 2), ('a', 3)]
in fromListWith (++) [ (k, [v]) | (k, v) <- xs ]

= fromList [('a', [3, 1]), ('b', [2])]
    Note that the lists in the resulting map contain elements in reverse order from their occurrences in the original list. More generally, duplicate entries are accumulated as follows; this matters when f is not commutative or not associative. 
    fromListWith f [(k, a), (k, b), (k, c), (k, d)]
= fromList [(k, f d (f c (f b a)))]

  2. fromListWithKey :: (Eq k, Hashable k) => (k -> v -> v -> v) -> [(k, v)] -> HashMap k v

    unordered-containers Data.HashMap.Lazy

    Construct a map from a list of elements. Uses the provided function to merge duplicate entries.

    Examples

    Given a list of key-value pairs where the keys are of different flavours, e.g: 
    data Key = Div | Sub
    and the values need to be combined differently when there are duplicates, depending on the key: 
    combine Div = div
combine Sub = (-)
    then fromListWithKey can be used as follows: 
    fromListWithKey combine [(Div, 2), (Div, 6), (Sub, 2), (Sub, 3)]
= fromList [(Div, 3), (Sub, 1)]
    More generally, duplicate entries are accumulated as follows; 
    fromListWith f [(k, a), (k, b), (k, c), (k, d)]
= fromList [(k, f k d (f k c (f k b a)))]

  3. fromListWith :: (Eq k, Hashable k) => (v -> v -> v) -> [(k, v)] -> HashMap k v

    unordered-containers Data.HashMap.Strict

    Construct a map from a list of elements. Uses the provided function f to merge duplicate entries with (f newVal oldVal).

    Examples

    Given a list xs, create a map with the number of occurrences of each element in xs: 
    let xs = ['a', 'b', 'a']
in fromListWith (+) [ (x, 1) | x <- xs ]

= fromList [('a', 2), ('b', 1)]
    Given a list of key-value pairs xs :: [(k, v)], group all values by their keys and return a HashMap k [v]. 
    let xs = ('a', 1), ('b', 2), ('a', 3)]
in fromListWith (++) [ (k, [v]) | (k, v) <- xs ]

= fromList [('a', [3, 1]), ('b', [2])]
    Note that the lists in the resulting map contain elements in reverse order from their occurrences in the original list. More generally, duplicate entries are accumulated as follows; this matters when f is not commutative or not associative. 
    fromListWith f [(k, a), (k, b), (k, c), (k, d)]
= fromList [(k, f d (f c (f b a)))]

  4. fromListWithKey :: (Eq k, Hashable k) => (k -> v -> v -> v) -> [(k, v)] -> HashMap k v

    unordered-containers Data.HashMap.Strict

    Construct a map from a list of elements. Uses the provided function to merge duplicate entries.

    Examples

    Given a list of key-value pairs where the keys are of different flavours, e.g: 
    data Key = Div | Sub
    and the values need to be combined differently when there are duplicates, depending on the key: 
    combine Div = div
combine Sub = (-)
    then fromListWithKey can be used as follows: 
    fromListWithKey combine [(Div, 2), (Div, 6), (Sub, 2), (Sub, 3)]
= fromList [(Div, 3), (Sub, 1)]
    More generally, duplicate entries are accumulated as follows; 
    fromListWith f [(k, a), (k, b), (k, c), (k, d)]
= fromList [(k, f k d (f k c (f k b a)))]

  5. fromListClassOpKey :: Unique

    ghc GHC.Builtin.Names

    No documentation available.

  6. fromListNClassOpKey :: Unique

    ghc GHC.Builtin.Names

    No documentation available.

  7. fromListNName :: Name

    ghc GHC.Builtin.Names

    No documentation available.

  8. fromListName :: Name

    ghc GHC.Builtin.Names

    No documentation available.

  9. fromListWith :: (a -> a -> a) -> [(Key, a)] -> Word64Map a

    ghc GHC.Data.Word64Map.Internal

    Create a map from a list of key/value pairs with a combining function. See also fromAscListWith. 

    fromListWith (++) [(5,"a"), (5,"b"), (3,"b"), (3,"a"), (5,"c")] == fromList [(3, "ab"), (5, "cba")]
fromListWith (++) [] == empty

  10. fromListWithKey :: (Key -> a -> a -> a) -> [(Key, a)] -> Word64Map a

    ghc GHC.Data.Word64Map.Internal

    Build a map from a list of key/value pairs with a combining function. See also fromAscListWithKey'. 

    let f key new_value old_value = show key ++ ":" ++ new_value ++ "|" ++ old_value
fromListWithKey f [(5,"a"), (5,"b"), (3,"b"), (3,"a"), (5,"c")] == fromList [(3, "3:a|b"), (5, "5:c|5:b|a")]
fromListWithKey f [] == empty

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