Ocaml exception handling for opening input channel

Question

As a beginner in Ocaml, I have this current working code:

...
let ch_in = open_in input_file in
try
    proc_lines ch_in
with End_of_file -> close_in ch_in;;

Now I would like to add error handling for non-existing input files, I wrote this:

let ch_in = try Some (open_in input_file) with _ -> None in
match ch_in with
| Some x -> try proc_lines x with End_of_file -> close_in x
| None -> () ;;

and get an error message: This pattern matches values of type 'a option but is here used to match values of type exn for the last line. If I substitute None for _, I get an error about incomplete matching.

I read that exn is the exception type. I'm sure I don't understand what is really going on here, so please point me to the right direction. Thanks!

Answer 1

When embedding pattern matches inside other pattern matches you need to encase the embedded match with either ( ... ) or begin ... end (syntactic sugar for parentheses):

let ch_in = try Some (open_in input_file) with _ -> None in
match ch_in with
| Some x -> (try proc_lines x with End_of_file -> close_in x)
| None -> () ;;