An implementation of a bimap, a map that is bi-directional and which any client code can use to look up keys if given values as well as traditional lookup of values given keys. Implemented using functors as a class and a module, with support for multi-maps as well as well as single-valued maps. Master branch uses Core. A no-core branch is a work-in-progress and needs re-writing. OUnit testing also implemented.

With a moderate amount of non-trivial logic, this seems to be a prime for property-based testing and in particular Crowbar fuzzing. Have you tried to use crowbar?

There seems to be a fair amount of almost-duplication of logic between your `module`

version (which implements an immutable interface) and your `class`

version (which implements an imperative interface). Would you be able to refactor the code to eliminate the duplication? One approach would be to call the `module`

implementation from your `class`

module (which becomes simply an adapter); you could do this by hand (adding `!`

and `:=`

in the right places), but it could even be possible to write generaic adapter combinators, along the lines of

```
let find = Pure.find |> (id @-> source @!-> id)
let change = Pure.change |> (id @-> id @-> source @!-> sink)
let iter = Pure.iter |> (id @-> source @!-> id)
```

in the following proof-of-concept

```
module Pure : sig
type key = bool
type 'a data
val create : 'a * 'a -> 'a data
val find : key -> 'a data -> 'a
val change : key -> 'a -> 'a data -> 'a data
val iter : ('a -> unit) -> 'a data -> unit
end = struct
type key = bool
type 'a data = 'a * 'a
let create (v1, v2) = (v1, v2)
let find b (v1, v2) =
if b then v1 else v2
let change b v (v1, v2) =
if b then (v, v2) else (v1, v)
let iter f (v1, v2) =
f v1;
f v2;
()
end
let test (type a) (v1, v2) =
let module Impure : sig
val find : Pure.key -> a
val change : Pure.key -> a -> unit
val iter : (a -> unit) -> unit
end = struct
let state : a Pure.data ref = ref (Pure.create (v1, v2))
let id x = x
let sink result = state := result
let source () = !state
let (@->) c1 c2 f = fun x -> c2 (f (c1 x))
let (@!->) c1 c2 f = c2 (f (c1 ()))
let find = Pure.find |> (id @-> source @!-> id)
let change = Pure.change |> (id @-> id @-> source @!-> sink)
let iter = Pure.iter |> (id @-> source @!-> id)
end in
()
```

I just took a look at Crowbarâ€¦had not heard of it. But at first glance it seems to be the way to go. I will incorporate it as soon as I get the chance.

Thank you for the excellent suggestions to reduce code duplication. I should have thought of that. Code duplication is the source of all evil (in coding). I like your first suggestion to make the class module an adapter for the other implementation if only because Iâ€™m not familiar with generic adapter combinatorsâ€¦yet.