Hoogle Search

Within LTS Haskell 24.4 (ghc-9.10.2)

Note that Stackage only displays results for the latest LTS and Nightly snapshot. Learn more.

Exact lookup

1. 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)))]
   

2. 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)))]
   

3. fromListClassOpKey :: Unique

   ghc	GHC.Builtin.Names

   No documentation available.

4. fromListNClassOpKey :: Unique

   ghc	GHC.Builtin.Names

   No documentation available.

5. fromListNName :: Name

   ghc	GHC.Builtin.Names

   No documentation available.

6. fromListName :: Name

   ghc	GHC.Builtin.Names

   No documentation available.

7. 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
   

8. 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
   

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

   ghc	GHC.Data.Word64Map.Lazy

   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.Lazy

    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
    

Page 41 of many | Previous | Next