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, 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, 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, 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

Take-aways:

• The common containers are ordered by:

  • Array ⊏ List ⊏ Seq
  • Buffer ⊏ Bytes ⊏ String ⊏ Seq
  • {Hashtbl, Map, Queue, Set, Stack} ⊏ Seq

• To convert between containers Foo.t → Bar.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

Take-aways:

• The numeric modules are ordered by Int64 ⊏ Nativeint ⊏ Int32 ⊏ {Int, Float}. When coercing between them, you should check modules in this order.