如何将sexp_diff库适配为OUnit assert_equal的pp_diff参数

方法一：适配sexp_diff为OUnit2的pp_diff函数 #

OUnit2的pp_diff参数需要一个接收格式化器和两个s-expression元组的函数。你可以基于sexp_diff库的diff结果，封装出符合要求的打印函数：

open OUnit2
open Sexplib.Sexp
open Sexp_diff

let sexp_pp_diff fmt (expected, actual) =
  (* 计算两个s-expression的差异 *)
  let diff_result = diff expected actual in
  (* 将差异输出到格式化器，假设sexp_diff提供了pp_diff用于打印diff结构 *)
  pp_diff fmt diff_result

(* 使用示例 *)
let test_parse_ast ctx =
  let actual_ast_sexp = parse_and_convert_to_sexp "input" in
  let expected_sexp = Sexp.of_string "(expected (ast structure))" in
  assert_equal
    ~cmp:Sexp.equal
    ~pp_diff:sexp_pp_diff
    expected_sexp
    actual_ast_sexp

如果sexp_diff没有现成的pp_diff打印函数，你可以自行将diff结构转为可读字符串后输出：

let sexp_pp_diff fmt (expected, actual) =
  let diff_result = Sexp_diff.diff expected actual in
  let diff_str = Format.asprintf "%a" Sexp_diff.pp diff_result in
  Format.pp_print_string fmt diff_str

方法二：序列化差异为断言失败消息 #

如果不想适配pp_diff，可以自定义断言函数，仅当比较失败时计算差异并作为错误消息抛出，避免无意义的diff计算：

open OUnit2
open Sexplib.Sexp
open Sexp_diff

let assert_sexp_equal expected actual =
  if Sexp.equal expected actual then ()
  else
    let diff = diff expected actual in
    let diff_msg = Format.asprintf "Expected:\n%a\nActual:\n%a\nDifferences:\n%a"
        Sexp.pp expected
        Sexp.pp actual
        pp_diff diff
    in
    assert_failure diff_msg

(* 使用示例 *)
let test_parse_ast ctx =
  let actual_ast_sexp = parse_and_convert_to_sexp "input" in
  let expected_sexp = Sexp.of_string "(expected (ast structure))" in
  assert_sexp_equal expected_sexp actual_ast_sexp

这种方式会在断言失败时同时展示预期值、实际值和差异点，便于快速定位问题。

内容的提问来源于stack exchange，提问作者user2852699