[Tests] Ninety-nine haskell problems

NnhpTests.hs

import Test.QuickCheck
import Nnhp

{- For most of the problems, randomly choosen parameters
 - are not a good test, as some obvious combinations are
 - not likely to be generated, not matter how interresting
 - they are to the test case at hand.
 -}

-- P01
prop_myLastHasAtMostOneElement list =
  (length . myLast) list < 2

-- P02
prop_myButLastHasAtMostTwoElements list =
  (length . myButLast) list < 3

-- P03
prop_elementAtCorrectForPositiveIndices list i =
  (i < 1 || i >= length list ) || (elementAt list i == list !! i)

-- P04
prop_numberOfElementsEquivalentToLength list =
  numberOfElements list == length list

prop_numberOfElementsIterEquivalentToLength list =
  numberOfElements list == length list

-- P05
prop_myReverseSquaredEqualsIdentity list = 
  (myReverse . myReverse) list == list

-- P06
-- this is a stupid reimplementation of the function :-)
-- The type specification is important here, as quickCheck will
-- use nil-filled lists (always generating obvious
-- palindromes) if omitted. Restricting to numbers is better, but 
-- still unsatisfactory, as the odds to generate palindromes is 
-- rather low. Computing the odds is left as an exercice to the
-- reader :-)

-- TODO: read the documentation of QuickCheck, to learn how to
-- generate better test data
prop_palindromeListsAreEqualToTheirReverse :: Num a => [a] -> Bool
prop_palindromeListsAreEqualToTheirReverse list =
  isPalindrome list == (reverse list == list)

-- P08
prop_compressShouldBeIdempotent :: Num a => [a] -> Bool
prop_compressShouldBeIdempotent list =
  (compress . compress) list == compress list

-- P09
-- suggestions welcomed to test `pack' :-)
prop_maybePackIsCorrect :: Bool
prop_maybePackIsCorrect = 
  pack [1,1,1,2,5,3,7,7,7,8,8,8,9,1,5,5] == [[1,1,1],[2],[5],[3],[7,7,7],[8,8,8],[9],[1],[5,5]]

-- P10 & P12
prop_decodeRlIsTheInverseOfEncode :: Num a => [a] -> Bool
prop_decodeRlIsTheInverseOfEncode list =
  (decodeRl . encode) list == list