Harry likes Sally

type Entity = Entity of string
type Relationship = Relationship of string

let entity name = Entity(name)
let relationship name = Relationship(name)
let __ = entity("__")

let facts = ref []

let fact (rel:Relationship) (xs:Entity list) =
    facts := (rel,xs) :: !facts

let question (Relationship(rel)) (clause:Entity list) =
    let find clause clause' =
        let xs = List.zip clause clause'
        if xs |> List.forall (fun (a,b) -> a = __ || a = b)
        then xs |> List.tryPick (fun (a,b) -> if a = __ then Some b else None)
        else None
    [for (Relationship(rel'),clause') in !facts do
        if rel = rel' then
            match find clause clause' with
            | Some(answer) -> yield answer
            | None -> ()
    ]

let sally = entity "sally"
let harry = entity "harry"
let likes = relationship "likes"

fact likes [harry;sally]

question likes [harry;__]
// val it : Entity list = [Entity "sally"]
question likes [__;sally]
// val it : Entity list = [Entity "harry"]