Swapping between different implementations of a signature

Learning
,
1

I have a library called ‘consensus’ in my project with a signature that is exported defined in the file ‘consensus.mli’. There are two files implementing this signature in ‘consensus_impl.ml’ and ‘consensus_chained_impl.ml’.

I then hardcode a selection of implementation in the file ‘consensus.ml’:

include Types
include Util
include Consensus_chained_impl (* hardcode to select this implementation *)

How can I allow users of the library to select an implementation in code rather than hardcoding the choice inside the library?

2

If you use dune to build your library, you might be interested in reading about virtual libraries and variants , which seem to be what you want.

3

You could provide a syntactic signature Consensus.S and let users input it in their functors, which will allow swapping the implementation after the fact. E.g. user code:

module Using_consensus(C : Consensus.S) = struct
  (* Now they can use the consensus signature with C *)
end

module Using_consensus_impl = Using_consensus(Consensus_impl)
(* Or if they want, *)
module Using_consensus_chained = Using_consensus(Consensus_chained)
4

This is one of the motivating use-cases for first-class modules. Simply do

include (val
 (if ... then
    (module Consensus_impl : S)
  else
    (module Consensus_chained_impl : S)))

where S is the signature of the modules.

See more examples over at the manual:

https://v2.ocaml.org/manual/firstclassmodules.html#p:fst-mod-example

Cheers,
Nicolas

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