I’ve seen this question but it doesn’t help me because I’m writing functional code (Standard ML) and the answers there are heavily focused towards OOP (the OP is advised to use the Facade pattern, to use inheritance, etc — answers that are not relevant to an SML programmer).

In two separate, largely unrelated modules, Foo and Bar, I need to accomplish the same task: given functions equal and equals with the signatures:

val equal: t * t -> bool
val equals: t list * t list -> bool

make two new functions with the following signatures:

val equal': t * t -> t option
val equals': t list * t list -> t list option

Here, t stands for either Foo.t or Bar.t. The natural solution is to use a higher order function:

local
  fun boolToOpt (comparison: ('a * 'a) -> bool) (left, right) =
    if comparison (left, right) then
      SOME left
    else
      NONE
in
  val equal' = boolToOpt equal
  val equals' = boolToOpt equals
end

I need to repeat exactly this code snippet in both modules.

Now, this is a tiny, minor implementation detail, and it would be a bad idea to let it be publicly visible from either Foo or Bar. At the same time, it seems too small — and too tightly coupled to the two equal(s) functions — to be factored out in to a module of its own. Maybe in another language, the equals' functionality would merit its own class, say, ComparisonDecorator, that overrides equal(s) to give me functions with the return type t option, but that’s not an option here.

I run in to situations like these at least once per day. Is there a sane way to avoid just copy+pasting here?