haskell@gentoo.org
Gentoo Haskell
Do you ever feel the need to test code involving bottoms (e.g. calls to
the @error@ function), or code involving infinite values? Then this
library could be useful for you.
It is usually easy to get a grip on bottoms by showing a value and
waiting to see how much gets printed before the first exception is
encountered. However, that quickly gets tiresome and is hard to automate
using e.g. QuickCheck
(<http://www.cse.chalmers.se/~rjmh/QuickCheck/>). With this library you
can do the tests as simply as the following examples show.
Testing explicitly for bottoms:
[@> isBottom (head [\])@] @True@
[@> isBottom bottom@] @True@
[@> isBottom (\\_ -> bottom)@] @False@
[@> isBottom (bottom, bottom)@] @False@
Comparing finite, partial values:
[@> ((bottom, 3) :: (Bool, Int)) ==! (bottom, 2+5-4)@] @True@
[@> ((bottom, bottom) :: (Bool, Int)) <! (bottom, 8)@] @True@
Showing partial and infinite values (@\\\/!@ is join and @\/\\!@ is meet):
[@> approxShow 4 $ (True, bottom) \\\/! (bottom, \'b\')@] @\"Just (True, \'b\')\"@
[@> approxShow 4 $ (True, bottom) \/\\! (bottom, \'b\')@] @\"(_|_, _|_)\"@
[@> approxShow 4 $ ([1..\] :: [Int\])@] @\"[1, 2, 3, _\"@
[@> approxShow 4 $ (cycle [bottom\] :: [Bool\])@] @\"[_|_, _|_, _|_, _\"@
Approximately comparing infinite, partial values:
[@> approx 100 [2,4..\] ==! approx 100 (filter even [1..\] :: [Int\])@] @True@
[@> approx 100 [2,4..\] \/=! approx 100 (filter even [bottom..\] :: [Int\])@] @True@
The code above relies on the fact that @bottom@, just as @error
\"...\"@, @undefined@ and pattern match failures, yield
exceptions. Sometimes we are dealing with properly non-terminating
computations, such as the following example, and then it can be nice to
be able to apply a time-out:
[@> timeOut' 1 (reverse [1..5\])@] @Value [5,4,3,2,1]@
[@> timeOut' 1 (reverse [1..\])@] @NonTermination@
The time-out functionality can be used to treat \"slow\" computations as
bottoms:
[@> let tweak = Tweak { approxDepth = Just 5, timeOutLimit = Just 2 }@]
[@> semanticEq tweak (reverse [1..\], [1..\]) (bottom :: [Int\], [1..\] :: [Int\])@] @True@
[@> let tweak = noTweak { timeOutLimit = Just 2 }@]
[@> semanticJoin tweak (reverse [1..\], True) ([\] :: [Int\], bottom)@] @Just ([],True)@
This can of course be dangerous:
[@> let tweak = noTweak { timeOutLimit = Just 0 }@]
[@> semanticEq tweak (reverse [1..100000000\]) (bottom :: [Integer\])@] @True@
Timeouts can also be applied to @IO@ computations:
[@> let primes = unfoldr (\\(x:xs) -> Just (x, filter ((\/= 0) . (\`mod\` x)) xs)) [2..\]@]
[@> timeOutMicro 100 (print $ filter ((== 1) . (\`mod\` 83)) primes)@] @[167,499,9NonTermination@
[@> timeOutMicro 100 (print $ take 6 $ filter ((== 1) . (\`mod\` 83)) primes)@] @[167,499,997,1163,1993NonTermination@
[@> timeOutMicro 100 (print $ take 6 $ filter ((== 1) . (\`mod\` 83)) primes)@] @[167,499,997,1163,1993,2657]@
[@ @] @Value ()@
For the underlying theory and a larger example involving use of
QuickCheck, see the article \"Chasing Bottoms, A Case Study in Program
Verification in the Presence of Partial and Infinite Values\"
(<http://www.cse.chalmers.se/~nad/publications/danielsson-jansson-mpc2004.html>).
The code has been tested using GHC. Most parts can probably be
ported to other Haskell compilers, but this would require some work.
The @TimeOut@ functions require preemptive scheduling, and most of
the rest requires @Data.Generics@; @isBottom@ only requires
exceptions, though.