[Haskell-cafe] Haskell Help

Patrick M patrickm7860 at yahoo.co.uk
Fri Feb 11 18:23:34 CET 2011

 writting a function that will remove tautologies from a fomula.The 
basic idea is that 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.My appoach is to create a 
function that will remove this but for a clause and map it to the 
fomula.Of course I have to remove duplicates at the beginning.

    module Algorithm where

    import System.Random
    import Data.Maybe
    import Data.List

    type Atom = String
    type Literal = (Bool,Atom)
    type Clause = [Literal]
    type Formula = [Clause]
    type Model = [(Atom, Bool)]
    type Node = (Formula, ([Atom],
    removeTautologies :: Formula -> Formula
    removeTautologies = map tC.map head.group.sort
      where rt ((vx, x) : (vy, y) : clauses) | x == y = rt rest
                                          | otherwise = (vx, x) : rt ((vy, y) : clauses)
 I have problems  when I try to give it a formula (for example (A v B v 
-A) ^ (B v C v A)).Considering that example 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) . 
But I get the following

    Loading package old-locale- ... linking ... done.
 package time-1.1.4 ... linking ... done.
    Loading package random- ... linking ... done.
    [[(True,"A"),(True,"B")*** Exception: Assignment.hs:(165,11)-(166,83): Non-exhaustive patterns in function rt

What should I do?

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