SML : Changing value of INT in a structure - algorithm

Question

I've got a small problem.
I made calculations and all my ids are not following each other anymore (because of some delete during the calculus). Unfortunately, to make my result valid, I need this order... :/
So in order to simply the task, I made an external function, that will "rename" all the IDs but I do not know how to perform this.
Here is the function that I've got :
fun setId (W {id, ...}) =
let
in
print( "[" ^ Int.toString (id) ^ "]");
print( "[" ^ Int.toString (!nextId) ^ "]\n");
Ref.incr nextId
end
(for the one who are wandering appis just a homemade function to perform the same calcul on each element of a list, but it's not important.)
When I'm execute this code, I obtain in output :
[0][0]
[1][1]
[2][2]
[3][3]
[4][4]
[5][5]
[6][6]
[7][7]
[8][8]
[9][9]
[10][10]
[11][11]
[12][12]
[13][13]
[14][14]
[15][15]
[16][16]
[17][17]
[18][18]
[19][19]
[20][20]
[21][21]
[22][22]
[39][23]
[40][24]
[41][25]
[42][26]
[43][27]
[44][28]
[45][29]
[46][30]
[47][31]
[48][32]
[49][33]
[50][34]
[51][35]
[52][36]
[53][37]
as you can see there is a big problem [23] [39] the list does not have following numbers. :/
Basically, I would like the function setIdto be able to modify the ID of the Node. But I don't know how :/
Here is the datatype Node for understanding purposes :
datatype node =
W of {
id : int
, predId : int option
, creationDepcy : Dependency.depcy
, nominalDepcies : Dependency.depcy list ref
, pattern : Termstore.store
, propositions : Propstore.pstore
, nominals : Propstore.pstore
, diamonds : Termstore.store
, boxes : Termstore.store
, disjunctions : Termstore.store
, nglstore : Termstore.store
, lazyProps : Lazystore.store
, lazyNoms : Lazynomstore.store
, lazyBoxes : Lazyboxstore.store
, blockedDiamonds : (Term.index * int) list ref
, branchPoints : int list ref
}
Thanks in advance for your help !
Best Regards.

Answer 1

Since id is typed int it cannot be modified. If you change it to int ref then you can modify it, but you will also have to change the accessors to dereference the ref.
An alternative solution is to create an array that maps from old IDs to new IDs and use this array for presentation, but that seems even more complicated.

Answer 2

Since ints are immutable -- you could take your list of nodes and replace it with a new list of nodes:
fun newID (W(x), i) =
W({
id = i
, predId = #predId x
, creationDepcy = #creationDepcy x
, nominalDepcies = #nominalDepcies x
, pattern = #pattern x
, propositions = #propositions x
, nominals = #nominals x
, diamonds = #diamonds x
, boxes = #boxes x
, disjunctions = #disjunctions x
, nglstore = #nglstore x
, lazyProps = #lazyProps x
, lazyNoms = #lazyNoms x
, lazyBoxes = #lazyBoxes x
, blockedDiamonds = #blockedDiamonds x
, branchPoints = #branchPoints x
});
fun imap _ [] _ = []
| imap f (x::xs) i = f(x,i):: (imap f xs (i+1));
(imap stands for "increment map")
Then if xs is a list of nodes the function call
imap newID xs 0
will yield a new list of nodes with the id fields consecutive ints starting with 0
Disclaimer: I didn't try this on your setup of course, by I created a datatype of records which had an id field and successfully used this approach. Obviously this isn't something you would want to do a lot of because of all the copying, but if it is at one specific point in your code it should be okay.

Answer 3

Mostly equivalent to John's solution, here is how one would update a list of nodes without using references, using a fold instead. The function setId is the same.
fun setIds firstId ws =
#1 (foldr (fn (w,(ws',nextId)) => (setId w nextId::ws', nextId+1)) ([],firstId) ws)
Running setIds 1 [w1, w2, w3, ...] would yield [w1', w2', w3', ...].