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 Digest: sig .. end
   This module provides functions to compute 128-bit 'digests' of
   arbitrary-length strings or files. The digests are of cryptographic
   quality: it is very hard, given a digest, to forge a string having
   that digest. The algorithm used is MD5. This module should not be
   used for secure and sensitive cryptographic applications. For these
   kind of applications more recent and stronger cryptographic
   primitives should be used instead.
type t = string 
val compare : t -> t -> int
Pervasives.compare and the implementation
    shared with String.compare. Along with the type t, this
    function compare allows the module Digest to be passed as
    argument to the functors Set.Make and Map.Make.val string : string -> t
val bytes : bytes -> t
val substring : string -> int -> int -> t
Digest.substring s ofs len returns the digest of the substring
   of s starting at index ofs and containing len characters.val subbytes : bytes -> int -> int -> t
Digest.subbytes s ofs len returns the digest of the subsequence
    of s starting at index ofs and containing len bytes.val channel : Pervasives.in_channel -> int -> t
len is nonnegative, Digest.channel ic len reads len
   characters from channel ic and returns their digest, or raises
   End_of_file if end-of-file is reached before len characters
   are read.  If len is negative, Digest.channel ic len reads
   all characters from ic until end-of-file is reached and return
   their digest.val file : string -> t
val output : Pervasives.out_channel -> t -> unit
val input : Pervasives.in_channel -> t
val to_hex : t -> string
val from_hex : string -> t
Invalid_argument if the argument is not exactly 32 hexadecimal
   characters.