OCaml observations

~~ Verbose https://www.reddit.com/r/adventofcode/comments/7hvtoq/2017_day_6_solutions/dqu8z8v/ (vs mine) ~~
~~Inconsistencies (fold - accumulator first in List.fold_left but second in Hashtbl)
  print_int (List.fold_left (fun acc line -> max acc (process line registers)) min_int lines);
  print_endline "";
  print_int (Hashtbl.fold (fun k v acc -> if v > acc then v else acc) registers min_int);~~
Documentation
 - lacking, samples very hard to find
 - http://caml.inria.fr/pub/docs/manual-ocaml/libref/Str.html - initally confusing than helpful
~~Regular expressions - match groups~~
~~Basis IO operations - too complicated/missing~~
~~Basic text/string processing - hard/takes a lot of time~~
Errors: 
  * additional semicolon
  * begin - end needed for if even if has semicolon
  * Syntax error 
    ```
    File "solution.ml", line 49, characters 0-3:
    Error: Syntax error
    Command exited with code 2.
    Compilation unsuccessful after building 1 target (0 cached) in 00:00:00.
    ```
    * missing "in"
    * using `=>` instead of `->`
    * using reserved words `val` `match` etc.
  * too many parameters due to lack of parenthesis
  * missing parameter - value converted to function impossible to find (how to repro? Day 7)
  * math expressions have to be in () (even though in samples they don't seem to have to be in ())
    match_list line group + 1
        Error: This expression has type int but an expression was expected of type
         'a list
    match_list line (group + 1)
        Compiles fine
  * Error: Syntax error: pattern expected. 
    missing `->` in match
  * Unbound value test 
    - the value does not exist (e.g. typo in name)
    - recursive function not marked with `rec`
  * list concat:
    print_list right@left fails
    Error: This expression has type unit but an expression was expected of type
         'a list
   print_list (right@left) works
  * abs requires brackets when passed as argument (and maybe if not?) e.g. `(abs(5))` and not `abs(4)` (Day11)
  * exceptions are lacking details (e.g. stack trace): ```Fatal error: exception Invalid_argument("index out of bounds")```
  * Passing negative value: (Day22)
    print_state infected_cells -5 5 row col; produces
    File "solution.ml", line 48, characters 4-30:
    Error: This expression has type int -> int -> int -> int -> unit
       but an expression was expected of type int
    `print_state infected_cells (-5) 5 row col;` - works
    
  * beware float to int conversion (Day 23):
  let is_prime n =
  let rec is_prime_aux n div =
    (* if div > int_of_float(sqrt (float_of_int n)) then *)
    if div > n then
      true
    else
      if n mod div = 0 then false else is_prime_aux n (div + 1)
  in
  is_prime_aux n 2
  works 
  `if div > sqrt n then`
  File "solution.ml", line 62, characters 9-10:
  Error: This expression has type float but an expression was expected of type
         int
  (note - error on a completely different line)
  `if div > int_of_float (sqrt n) then`
  Same error
  `if div > int_of_float (sqrt (float_of_int (n))) then`
  works but is there a better way?
  * List concat (Day 24)
   match Hashtbl.find_opt parts key with
    | Some v -> Hashtbl.replace parts key v@[value]
    | None -> Hashtbl.add parts key [value]
  File "solution.ml", line 18, characters 16-43:
Error: This expression has type unit but an expression was expected of type
         'a list
Command exited with code 2.
change to `    | Some v -> Hashtbl.replace parts key (v@[value])` - compiles
links to sources
sample code