[Haskell-beginners] Haskell Help

Andrei M. andresocrates90 at yahoo.com
Thu Feb 10 18:24:44 CET 2011

I need to write two functions for a coursework that will behave as follows...
unction 1)removeTautologies :: Formula->Formula 
If in a clause, a literal and its negation are found, it means that the clause will be true, regardless of the value
finally assigned to that propositional variable. Consider the following example:
(A v B v -A) ^ (B v C v A)
The first clause contains the literals A and -A. This means that the clause will always be true, in which case
it can be simplify the whole set to simply (B v C v A) .
I was tinking of using something like
 removeTautologies (f:fs)=filter rTf:removeTautologies fs
 where rT-is supposed to take the firs Literal from the clasue and 
search for a similar one,if one si found we compare the values if not 
the we go to the second literal.
->Function 2)pureLiteralDeletion :: Formula->Formula
This is a little bit complicate but from What I get this function is suppose to implement a simplification step that assumes 
as true any atom in a formula that appears exclusively in a positive or
negative form (not both). Consider the formula:
(P v Q v R) ^ (P v Q v -R) ^ (-Q v R)
Note that in this formula P is present but -P is not. Using Pure Literal
 Deletion  it can be assumed that the value of P will be True thus 
simplifying the formula to (-Q v R). If the literal were false then the 
literal would simply be deleted from the clauses it appears in. In that 
case any satisfying model for the resulting formula would also be a 
satisfying model for the formula when we assume that the literal is true.

removeTautologies :: Formula -> Formula
removeTautologies = map rt.map head.group.sort
     where rt ((x, x') : (y, y') : rest) | x' == y' = rt rest
                              | otherwise = (x, x') : rt ((y, y') : rest)

pureLiteralDeletion :: Formula -> Formula
pureLiteralDeletion = map lD.map head.group.sort.concat
     where lD ((x, x') : (y, y') : rest) | x' == y' && x/=y = lD rest
                                         | otherwise = (x, x') : lD ((y, y') : rest)

this is what I've witten so far but they don't wok poperly....any sugestions?

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