% Testing Call-Time Choice of Functional-Logic Operations
% Sebastian Fischer (sebf@informatik.uni-kiel.de)

This module defines tests that specify the intended behaviour of
functional-logic programs w.r.t. laziness and sharing. Although
non-deterministic computations must be executed on demand, their
results have to be as if they were executed eagerly.

> module Control.CFLP.Tests.CallTimeChoice where
>
> import Control.CFLP
> import Control.CFLP.Tests
> import Test.HUnit
>
> import Control.CFLP
>
> import Prelude hiding ( not, null, head )
>
> tests :: Test
> tests = "call-time choice" ~: test
>  [ "ignore first, narrow second" ~: ignoreFirstNarrowSecond
>  , "shared vars are equal" ~: sharedVarsAreEqual
>  , "no demand on shared var" ~: noDemandOnSharedVar
>  , "shared compound terms" ~: sharedCompoundTerms
>  ]

Every module under `Control.CFLP.Tests` defines a constant `tests`
that collects all defined tests.

> ignoreFirstNarrowSecond :: Assertion
> ignoreFirstNarrowSecond = assertResults comp [True,False]
>  where
>   comp cs u = ignot (error "illegal demand") (unknown cs u) cs
>
> ignot :: CFLP cs m
>       => Nondet cs m a -> Nondet cs m Bool -> cs -> Nondet cs m Bool
> ignot _ x = not x

This test checks a function with two arguments, where the first must
be ignored. Any changes in the translation scheme must not lead to
demand on the first argument of `ignot`. I have no better idea to
check demand than with using `error`. So an *error* is considered a
*failure* in this test case.

> sharedVarsAreEqual :: Assertion
> sharedVarsAreEqual = assertResults comp [[False,False],[True,True]]
>  where
>   comp cs u = two (unknown cs u)
>
> two :: Monad m => Nondet cs m a -> Nondet cs m [a]
> two x = x ^: x ^: nil

This test checks call-time choice semantics: variables represent
identical ground values. The elements of the constructed list must be
equal although they are computed from a free variable. 

In the current translation scheme sharing is implicit (we have no
special combinator to express sharing but use Haskell's sharing
directly). In case we introduce such a combinator, the following tests
are interesting.

> noDemandOnSharedVar :: Assertion
> noDemandOnSharedVar = assertResults comp [False]
>  where
>   comp cs _ = null (two (error "illegal demand" :: Nondet cs m Bool)) cs

Even with an explicit combinator for sharing (to be used, e.g., in the
definition of the function `two`) there must not be demand on
something that is shared.

> sharedCompoundTerms :: Assertion
> sharedCompoundTerms = assertResults comp [[True,False],[False,True]]
>  where
>   comp cs u = negHeads (unknown cs u) cs
>
> negHeads :: CFLP cs m => Nondet cs m [Bool] -> cs -> Nondet cs m [Bool]
> negHeads l cs = not (head l cs) cs ^: head l cs ^: nil

This test checks whether sharing is ensured on aruments of compound
terms even if they are consumed. In this example, the variable `l` is
shared, so the heads that are computed twice must be equal and the
negated head must be different from the head.