How to implement `assertFail` in Haskell?

Question

在实现自己的工具模块和功能时，我想确保我的函数在特定输入上失败，例如assertFail (head []) ，但是该assertFail函数似乎无法在Haskell中实现，有什么方法可以破解它？

Answer 1

If I understand what you want correctly, you can use spoon :

import Control.Spoon

assertFail x = case spoon x of
    Nothing -> ()
    Just _ -> error "unexpected success"

A relevant question: Is spoon unsafe in Haskell?

Answer 2

One way to prototype assertFail could be assertFail = undefined , because evaluating it would raise a runtime exception . Another would be using error .

If you're building a test framework, a proper design would likely involve total functions (that never raise a runtime error) which return something like Maybe String for a possible error message.

If what you need is indeed assertRaises , take a look at Control.Exception which likely would help.