Converting between standard OCaml types

The OCaml standard library provides many convertion functions of the form Foo.{to,of}_bar between its various types. These are incomplete – and somewhat arbitrary – so it's often necessary to go via intermediate types to achieve the convertion you want. (Either that or roll your own with a fold or iter function.)

What's worse, you may need to mix forwards and backwards conversions (to_ and of_ style respectively), requiring you to search both forwards and backwards in the documentation to get the conversion you need. For example, consider the case of ('k, 'v) Hashtbl.t -> ('k * 'v) list:

  1. Looking in the docs for [Hashtbl][hashtbl-doc], we see that there's no
    direct conversion to an association list. We might give up and build our own
    one with Hashtbl.fold. On OCaml 4.07+, we have Hashtbl.to_seq : ('k, 'v) Hashtbl.t -> ('k, 'v) Seq.t, so let's start by using that.

  2. We look in the docs for [Seq][seq-doc], and find no conversions. Once again,
    we could use get distracted and use Seq.fold_left to build our own. Or, we
    could start searching backwards from the type we want...

  3. In the docs for [List][list-doc], we find the function List.of_seq : 'a Seq.t -> 'a List.t. Now we can complete the chain:

let hashtbl_to_list h = Hashtbl.to_seq h |> List.of_seq

Rather than keeping the entire Stdlib documentation open in browser tabs, it can
be helpful to view all of the available conversions as a directed graph. That's
what this repository provides. (These graphs show the conversions available on
OCaml 4.11.0; your mileage may vary.)

Container types



  • The common containers are ordered by:

    • ArrayListSeq
    • BufferBytesStringSeq
    • {Hashtbl, Map, Queue, Set, Stack} ⊏ Seq
  • To convert between containers Foo.tBar.t, it's usually necessary to go
    via Seq.t with Foo.to_seq >> Bar.of_seq.

  • There's no direct conversion between float Array.t and Float.Array.t.

Data types



  • The numeric modules are ordered by Int64NativeintInt32 ⊏ {Int,
    Float}. When coercing between them, you should check modules in this order.