I am currently teaching myself ocaml for a programming language class and I am trying to figure out how to specify a function parameter and return type as a List.
I have create a program that reads a file char by char stores each char in a List, reverses the list then returns the List.
Current code:
(*
Creating a function that will read all the chars
in a file passed in from the command argument.
This function takes a parameter of type List.
This function will return a List.
*)
let read_file (char_List : List) : List =
let char_in = open_in Sys.argv.(1) in (* Creating a file point/in_channel *)
try
while true do
let c = input_char char_in in (* Getting char from the file *)
char_List := c :: !char_List (* Storing the char in the list *)
done
with End_of_file ->
char_List := List.rev !char_List; (* End of file was reaching reversing char list *)
close_in char_in; (* Closing the file pointer/in_channel *)
;;
(* Storing the result of read_file to buffer which buffer is of type list *)
let buffer = ref [] in
read_file(buffer);
print_string "\nThe length of the buffer is: ";
print_int (List.length !buffer); (* Printing length of the list *)
print_string ("\n\n");
List.iter print_char !buffer; (* Iterating through the list and print each element *)
If I remove specifying the parameter type and return type of List the code runs as intended. However; I would like to specify the type of the parameter and return type as a List.
How do you specify the function parameter and return type to be a List?
First, List is a module not a type, so you probably meant list. However, you cannot annotate with only list, because a list is not a type by itself: you do not want to have a list of unknowable things, but a list of elements which have themselves a known type. For instance, in your case, you have a list of characters, which can be written as char list. Similarly, a list of integers will be typed int list.
More precisely, list is not a type by itself but a type constructor which takes as argument a type for the elements of the list and returns a type for the list of such elements.
p.s. : if you are learning OCaml you could try to rewrite your code without using references to get used to a more functional style.
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