Conversion from decimal to binary in Ocaml

Programmer Question

I am trying to convert a given decimal value its corresponding binary form. I am using Ocaml about which I don't know much and am quite confused. So far I have the following code

let dec_to_bin_helper function 1->'T' | 0->'F'
let dec_to_bin x =
List.fold_left(fun a z -> z mod 2 dec_to_bin_helper a) [] a ;;

I must include here that I want my output to be in the form of a list of T's and F's where T's represent the binary 1's and F's represent binary 0's
If I try to run the above code it gives me an error saying "Error: This expression is not a function; it cannot be applied"
I understand that the part where I am calling the helper function is wrong... Any help in the matter would be appreciated!

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