We have defined types:
 
type position = {line : int; col : int };;
type 'a ast = {data : 'a; kids : 'a ast list; posf : position; post: position};;
 
 
And then we transform string to ast:
 
let construct_common_ast s=
    let len = String.length s in
    let rec aux i j=
      if i>j then []
      else (
        let k = find_first s "(,)" i in
        let str = trim (String.sub s i (k-i)) in
        if k>j || s.[k]=')' then (
          if str="" then []
          else {data=str; kids=[]; posf=(calc_pos s i); post=(calc_pos s j)}::[]
        )
        else if s.[k]='(' then (
          let l = next_matched s '(' ')' (k+1) in
          let m = find_first_not s " \t\n\r" (l+1) in
          if m<=j && s.[m]<>',' then 
              fail_ast (calc_pos s k) (calc_pos s l) "construct_common_ast: invalid input string"
          else
              {data=str; kids=aux (k+1) (l-1); posf=(calc_pos s i); post=(calc_pos s l)} :: aux (m+1) j
        )
        else {data=str; kids=[]; posf=(calc_pos s i); post=(calc_pos s k)} :: aux (k+1) j
      )
    in
    let trees=aux 0 (len-1) in
List.hd trees;;
 
For construct_common_ast : 
# construct_common_ast;;
- : string -> string ast = <fun>
# construct_common_ast "node (function, params(), returns())";;
- : string ast =
{data = "node";
 kids =
  [{data = "function"; kids = []; posf = {line = 1; col = 8};
    post = {line = 1; col = 16}};
   {data = "params"; kids = []; posf = {line = 1; col = 17};
    post = {line = 1; col = 25}};
   {data = "returns"; kids = []; posf = {line = 1; col = 27};
    post = {line = 1; col = 36}}];
 posf = {line = 1; col = 2}; post = {line = 1; col = 37}}.
 
But, it is intolerable to deal with long string. how to increase coding efficiency of this function?
 
Thank you anyway. 
Best wishes!