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epsilonhalbe
epsilonhalbe

Reputation: 15959

Property Based Testing with Exceptions

I am writing an introduction to Haskell for my local functional programming group. As a base I am using the Tasty-testing framework and I want to test the indexing function (!!).

MinimalExample.hs

module MinimalExample where

myIndex :: Int -> [a] -> a                                               
myIndex _ [] = error "index too large"                                   
myIndex 0 (x:_) = x                                                      
myIndex n (_:xs) = myIndex (n-1) xs

MinimalTests.hs

module MinimalTests where

import Test.Tasty
import Test.Tasty.SmallCheck as SC
import Test.SmallCheck.Series

import MinimalExample

main :: IO ()
main = defaultMain tests

tests :: TestTree
tests = testGroup "Tests" [scProps]

scProps ::  TestTree
scProps = testGroup "(checked by SmallCheck)"
  [ SC.testProperty "(!!) == myIndex" $ \lst n ->
      lst !! n == myIndex (n::Int) (lst::[Int])
  ]

The tests should not fail on "too large indices" as the Errors/Exceptions are the same.

The tests should fail with negative input - which could be resolved by adding NonNegative as a constraint on the input, or adding the respective clause in the myIndex-function.

Upvotes: 2

Views: 327

Answers (1)

Roman Cheplyaka
Roman Cheplyaka

Reputation: 38758

You could use the spoon package, or write a similar function that would return the exception if you want to test that the exceptions are the same.

Upvotes: 1

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