Re: Make many insertions in long string

Thank you for your feedback. It helped me a lot ! In reality, using String.concat is not that hard (see the relatively short&simple code below), you just have to be careful a little bit about the order of the operations.
 I was amazed by the efficiency of that solution : it executes my initial computation in less than a second!
 
 
  let adjacent_elements_pair_list l=
        let rec sub_f=
            (function (accu,a,rl)->match rl with
            []->List.rev(accu)
            |b::x->sub_f((a,b)::accu,b,x)
            ) in
        match l with
        []->[]
        |u::v->sub_f([],u,v);;
  
  let interval s a b=String.sub s (a-1) (b-a+1);;
  
  
  let insert_at_points s l=
     let n=String.length s in
     let temp1=(n,"")::(List.rev_append l [0,""]) in
     let temp2=adjacent_elements_pair_list temp1 in
     let temp3=List.rev_map (fun ((j2,w2),(j1,w1))->(interval s (j1+1) j2)^w2) temp2 in
     String.concat "" temp3;;
     
  let example= insert_at_points "123456789" [3,"aaa";6,"bb"];;