Haskell Immutable data structure - the Map data type

Question

I am confused about the this data type in Haskell - Map. In particular, there is a function call insert (from the Data.Map module) which allows you to append new values to the Map data structure. So, here is my confusion. If haskell data structure is immutable, how can you insert new data into an existing Map data structure?

You can't, you construct a new map that contains the new key. — willeM_ Van Onsem
– willeM_ Van Onsem, Commented Apr 17, 2018 at 13:17

chepner · Accepted Answer · 2018-04-17 13:17:59Z

7

insert doesn't actually modify the input Map. It returns a new Map that contains the same entries as the original Map, plus the one you are inserting.

Underneath the hood, the compiler may not actually have to copy all the old entries to a new Map, though; it can reuse the original if it determines that nothing else is using the input Map. Immutability is a property of the language, not necessarily the implementation of the language.

answered Apr 17, 2018 at 13:17

chepner

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2 Comments

chi Over a year ago

Indeed, in a balanced BST-based implementation, insert only needs to copy O(log N) nodes of the tree, and reuse much of the old (immutable) tree.

Daniel Wagner Over a year ago

Compilers are allowed to modify a value in-place if it can determine that no references are held to the old value, but as far as I know none of them does. But not all is lost: Map is implemented as a tree, and unchanged subtrees can be (and are) shared between the old Map and the new one. This is permissible whether something else is using the input Map or not (because immutability guarantees that the sharing won't cause aliasing problems).

MathematicalOrchid · Accepted Answer · 2018-04-18 08:58:04Z

0

Perhaps an example will make this clearer.

Prelude> let map1 = empty
Prelude> map1
fromList []

Prelude> let map2 = insert "Lemon" 6 map1
Prelude> map2
fromList [("Lemon", 6)]

Prelude> map1
fromList []

Prelude> let map3 = insert "Lime" 7 map2
Prelude> map3
fromList [("Lime", 7), ("Lemon", 6)]

Prelude> map2
fromList [("Lemon", 6)]

Prelude> map1
fromList []

Each time we define a new variable (map1, map2, map3), the previous version of the Map remains completely unchanged. Once we have defined that (for example) map1 contains the key "Lemon", nothing can ever change that definition. We can create a new map that contains additional keys (or fewer keys, indeed), but we cannot change map1 itself.

answered Apr 18, 2018 at 8:58

MathematicalOrchid

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