Question

我正在编写一种策略来查找与绑定列表中的键相关联的值。类似的东西：

Require Import String List Program.

Ltac assoc needle haystack :=
  match haystack with
  | @nil (_ * ?T) => constr:(@None T)
  | cons (?k, ?v) ?t => let pr := constr:(eq_refl k : k = needle) in constr:(Some v)
  | cons _        ?t => let res := assoc needle t in constr:res
  end.

不幸的是，我不知道密钥的确切形式;相反，我知道一个匹配它的模式。更准确地说，我正在寻找的关键是调用类型类方法的结果，但我事先并不知道将使用哪个实例。在下面的示例中，我知道密钥是对show "a"的调用，但我不知道具体的实例：

Open Scope string_scope.
Open Scope list_scope.

Class Show A := { show: A -> string }.
Instance Show1 : Show string := {| show := fun x => x |}.
Instance Show2 : Show string := {| show := fun x => x ++ x |}.

Goal True.
  (* Works (poses Some 1) *)
  let v := assoc (show "a") [(show (Show := Show2) "a", 1); ("b", 2)] in pose v.
  (* Does not work (poses None) *)
  let v := assoc (show "a") [(show (Show := Show1) "a", 1); ("b", 2)] in pose v.

我是否可以使用这里的技巧，而不是通过assoc检查匹配的ltac？理想情况下，它看起来像(show (Show := _) "a")，或者(fun inst => show (Show := inst) "a")。

Answer 1

看起来传递函数的效果很好，实际上是：

Ltac assoc needlef haystack :=
  match haystack with
  | @nil (_ * ?T) => constr:(@None T)
  | cons (?k, ?v) ?t => let pr := constr:(eq_refl k : k = needlef _) in constr:(Some v)
  | cons _        ?t => let res := assoc needlef t in constr:res
  end.

Goal False.
  let v := assoc (fun i => show (Show := i) "a") [(show (Show := Show2) "a", 1); ("b", 2)] in pose v.
  let v := assoc (fun i => show (Show := i) "a") [(show (Show := Show1) "a", 1); ("b", 2)] in pose v.

将模式传递给战术

1 个答案: