Ada 2012 中的自定义条件失败消息

Question

有没有办法为前置条件和后置条件指定自定义 error/on 失败消息，类比为谓词 Predicate_Failure？我似乎无法在官方文档中找到任何内容。 TIA.

Answer 1

您可以使用加注表达式（参见 here），如下例所示。

main.adb

pragma Assertion_Policy (Check);

with Ada.Text_IO;
with Ada.Float_Text_IO;

procedure Main is
   
   package TIO renames Ada.Text_IO;
   package FIO renames Ada.Float_Text_IO;
   
   
   function Reciprocal (X : Float) return Float is (1.0 / X)
     with Pre => (X /= 0.0 or else
                    raise Constraint_Error with "X must not be 0.");
   
begin
   FIO.Put (Reciprocal (2.0));
   TIO.New_Line;
   
   FIO.Put (Reciprocal (0.0));
   TIO.New_Line;   
end Main;

输出

$ ./obj/main
 5.00000E-01

raised CONSTRAINT_ERROR : X must not be 0.
[2020-07-03 22:20:25] process exited with status 1, elapsed time: 00.32s