Ocaml, replace all specified elements with a given element in a list

Question

I am writing a ocaml project, in which I have a function that replace all '' in a char-list with 'E'. Here's my code for this propose:

    let rec string_lst_change_E lst = 
    match lst with
        [] -> let a ='E'; a::[]
        |(h::t) if (h = '') -> 'E'::(string_lst_change_E t) 
        |(h::t) ->  h::(string_lst_change_E t)
;;

It says I have a syntax error... But I cannot figure out by myself. I tried to modify it like this:

    let rec string_lst_change_E lst = 
    match lst with
        [] -> 'E'::[]
        |(h::t) ->if (h = '') then 'E'::(string_lst_change_E t) else h::(string_lst_change_E t)
;;

but still there's syntax error...(on the line |(h::t) -> .... char 18-21)

Please help me to take a look at it. Thank you!

Answer 1

This is where the first error lies: [] -> let a ='E'; a::[] If you want to use a after declaring it, you should instead write [] -> let a = 'E' in a ::[]. Obviously, [] -> ['E'] is simpler.

The second is the use of if in a pattern match. You should use when instead: |(h::t) when h = '' -> 'E'::(string_lst_change_E t)

But what's '' anyway? The empty character? How would you get this in a string? Typing '' is itself a syntax error. Try it in the toplevel! To make your code compile, I replaced '' by ' '.

let rec string_lst_change_E lst =
    match lst with
        | [] -> let a ='E' in a::[]
        | (h::t) when h = ' ' -> 'E'::(string_lst_change_E t) 
        | (h::t) ->  h::(string_lst_change_E t)

Note that you can simply use function here:

let rec string_lst_change_E = function
    | [] -> let a ='E' in a::[]
    | (h::t) when h = ' ' -> 'E'::(string_lst_change_E t) 
    | (h::t) ->  h::(string_lst_change_E t)