如何让QuickCheck只为参数生成有效的列表索引？ [英] How to tell QuickCheck to generate only valid list indices for a parameter?

问题描述

假设我想为（!!）函数编写一些单元测试。

  my_prop xs n = ... 
  

我想限制n只能有效索引，我知道我可以做一些像

  my_prop xs n =（not.null）（drop n xs）==> ; ... 
  

但是，这使得绝大多数生成的案例无效并且被抛出远。有没有一种方法可以让QuickCheck首先生成 xs 列表，并使用它的值仅生成 n ？

解决方案

使用 forAll ，您可以指定一个 generator

  my_prop（NonEmpty xs）= forAll（choose（ 0，length xs  -  1））$ \\\
  - > ... 
  




Say I want to write some unit tests for the (!!) function.

my_prop xs n = ...

I want to restrict n to only valid indexes and I know I could do something like

my_prop xs n = (not.null) (drop n xs) ==> ...

But this makes it so that the vast majority of the generated cases are invalid and get thrown away. Is there a way I can set things up so that QuickCheck generates the xs list first and uses its value to generate only valid cases of n?

解决方案

Using forAll, you can specify a generator for n which depends on the earlier arguments, e.g.

my_prop (NonEmpty xs) = forAll (choose (0, length xs - 1)) $ \n -> ...

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