prune-lang 0.2.2

Prune is a constraint logic programming language with branching heuristic.
Documentation 
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datatype Expr where
| Var(Int)
| Abs(Int, Expr)
| App(Expr, Expr)
end

datatype List[a] where
| Nil
| Cons(a, List[a])
end

function remove(xs: List[Int], x: Int) -> List[Int]
begin
    match xs with
    | Nil => Nil
    | Cons(head, tail) =>
        if head == x then remove(tail, x)
        else Cons(head, remove(tail, x))
    end
end

function concat[a](xs: List[a], ys: List[a]) -> List[a]
begin
    match xs with
    | Nil => ys
    | Cons(head, tail) => Cons(head, concat(tail, ys))
    end
end

function free_vars(expr: Expr) -> List[Int]
begin
    match expr with
    | Var(x) => Cons(x, Nil)
    | Abs(x, e) => remove(free_vars(e), x)
    | App(e1, e2) => 
        let v1 = free_vars(e1);
        let v2 = free_vars(e2);
        concat(v1, v2)
    end
end

function example(expr: Expr)
begin
    guard free_vars(expr) = Nil;
end

query example(depth_step=5, depth_limit=30, answer_limit=100)