Below is the API for the OCaml standard library. It's directly copied over from the OCaml Manual, formatted to the Reason syntax and styled accordingly. The API docs are work-in-progress; we'll be polishing these gradually!
If you're targeting JavaScript, the API docs for BuckleScript includes all of below, plus JS-specific APIs.
module Filename: sig .. endval current_dir_name : string
. in Unix).val parent_dir_name : string
.. in Unix).val dir_sep : string
/ in Unix).val concat : string -> string -> string
concat dir file returns a file name that designates file
   file in directory dir.val is_relative : string -> bool
true if the file name is relative to the current
   directory, false if it is absolute (i.e. in Unix, starts
   with /).val is_implicit : string -> bool
true if the file name is relative and does not start
   with an explicit reference to the current directory (./ or
   ../ in Unix), false if it starts with an explicit reference
   to the root directory or the current directory.val check_suffix : string -> string -> bool
check_suffix name suff returns true if the filename name
   ends with the suffix suff.val chop_suffix : string -> string -> string
chop_suffix name suff removes the suffix suff from
   the filename name. The behavior is undefined if name does not
   end with the suffix suff.val chop_extension : string -> string
.xyz for instance.
   Raise Invalid_argument if the given name does not contain
   an extension.
val basename : string -> string
name is a valid file name, then concat (dirname name) (basename name)
   returns a file name which is equivalent to name. Moreover,
   after setting the current directory to dirname name (with Sys.chdir),
   references to basename name (which is a relative file name)
   designate the same file as name before the call to Sys.chdir.
   This function conforms to the specification of POSIX.1-2008 for the
   basename utility.
val dirname : string -> string
Filename.basename.
   This function conforms to the specification of POSIX.1-2008 for the
   dirname utility.val temp_file : ?temp_dir:string -> string -> string -> string
temp_file prefix suffix returns the name of a
   fresh temporary file in the temporary directory.
   The base name of the temporary file is formed by concatenating
   prefix, then a suitably chosen integer number, then suffix.
   The optional argument temp_dir indicates the temporary directory
   to use, defaulting to the current result of Filename.get_temp_dir_name.
   The temporary file is created empty, with permissions 0o600
   (readable and writable only by the file owner).  The file is
   guaranteed to be different from any other file that existed when
   temp_file was called.
   Raise Sys_error if the file could not be created.val open_temp_file : ?mode:Pervasives.open_flag list -> ?temp_dir:string -> string -> string -> string * Pervasives.out_channel
Filename.temp_file, but returns both the name of a fresh
   temporary file, and an output channel opened (atomically) on
   this file.  This function is more secure than temp_file: there
   is no risk that the temporary file will be modified (e.g. replaced
   by a symbolic link) before the program opens it.  The optional argument
   mode is a list of additional flags to control the opening of the file.
   It can contain one or several of Open_append, Open_binary,
   and Open_text.  The default is [Open_text] (open in text mode).
   Raise Sys_error if the file could not be opened.val get_temp_dir_name : unit -> string
TMPDIR environment variable, or "/tmp"
    if the variable is not set.
    Under Windows, the value of the TEMP environment variable, or "."
    if the variable is not set.
    The temporary directory can be changed with Filename.set_temp_dir_name.val set_temp_dir_name : string -> unit
Filename.get_temp_dir_name
    and used by Filename.temp_file and Filename.open_temp_file.val temp_dir_name : string
Filename.get_temp_dir_name instead.TMPDIR environment variable, or "/tmp"
    if the variable is not set.
    Under Windows, the value of the TEMP environment variable, or "."
    if the variable is not set.val quote : string -> string