fromList package:containers

Create a map from a list of key/value pairs.

fromList [] == empty
fromList [(5,"a"), (3,"b"), (5, "c")] == fromList [(5,"c"), (3,"b")]
fromList [(5,"c"), (3,"b"), (5, "a")] == fromList [(5,"a"), (3,"b")]

Create a set from a list of integers.

Build a map from a list of key/value pairs. See also fromAscList. If the list contains more than one value for the same key, the last value for the key is retained. If the keys are in non-decreasing order, this function takes <math> time.

fromList [] == empty
fromList [(5,"a"), (3,"b"), (5, "c")] == fromList [(5,"c"), (3,"b")]
fromList [(5,"c"), (3,"b"), (5, "a")] == fromList [(5,"a"), (3,"b")]

Create a sequence from a finite list of elements. There is a function toList in the opposite direction for all instances of the Foldable class, including Seq.

Create a set from a list of elements. If the elements are in non-decreasing order, this function takes <math> time.

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

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

Note the reverse ordering of "cba" in the example. The symmetric combining function f is applied in a left-fold over the list, as f new old.

Performance

You should ensure that the given f is fast with this order of arguments. Symmetric functions may be slow in one order, and fast in another. For the common case of collecting values of matching keys in a list, as above: The complexity of (++) a b is <math>, so it is fast when given a short list as its first argument. Thus:

fromListWith       (++)  (replicate 1000000 (3, "x"))   -- O(n),  fast
fromListWith (flip (++)) (replicate 1000000 (3, "x"))   -- O(n²), extremely slow

because they evaluate as, respectively:

fromList [(3, "x" ++ ("x" ++ "xxxxx..xxxxx"))]   -- O(n)
fromList [(3, ("xxxxx..xxxxx" ++ "x") ++ "x")]   -- O(n²)

Thus, to get good performance with an operation like (++) while also preserving the same order as in the input list, reverse the input:

fromListWith (++) (reverse [(5,"a"), (5,"b"), (5,"c")]) == fromList [(5, "abc")]

and it is always fast to combine singleton-list values [v] with fromListWith (++), as in:

fromListWith (++) $ reverse $ map (\(k, v) -> (k, [v])) someListOfTuples

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

Also see the performance note on fromListWith.

Build a map from a list of key/value pairs with a combining function. See also fromAscListWith. If the keys are in non-decreasing order, this function takes <math> time.

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

Note the reverse ordering of "cba" in the example. The symmetric combining function f is applied in a left-fold over the list, as f new old.

Performance

You should ensure that the given f is fast with this order of arguments. Symmetric functions may be slow in one order, and fast in another. For the common case of collecting values of matching keys in a list, as above: The complexity of (++) a b is <math>, so it is fast when given a short list as its first argument. Thus:

fromListWith       (++)  (replicate 1000000 (3, "x"))   -- O(n),  fast
fromListWith (flip (++)) (replicate 1000000 (3, "x"))   -- O(n²), extremely slow

because they evaluate as, respectively:

fromList [(3, "x" ++ ("x" ++ "xxxxx..xxxxx"))]   -- O(n)
fromList [(3, ("xxxxx..xxxxx" ++ "x") ++ "x")]   -- O(n²)

Thus, to get good performance with an operation like (++) while also preserving the same order as in the input list, reverse the input:

fromListWith (++) (reverse [(5,"a"), (5,"b"), (5,"c")]) == fromList [(5, "abc")]

and it is always fast to combine singleton-list values [v] with fromListWith (++), as in:

fromListWith (++) $ reverse $ map (\(k, v) -> (k, [v])) someListOfTuples

Build a map from a list of key/value pairs with a combining function. See also fromAscListWithKey. If the keys are in non-decreasing order, this function takes <math> time.

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

Also see the performance note on fromListWith.