One :Thank you Carsten Schultz,Daniel Fischer,Mihai and all the other for your help. Two:After my last post I wrote some function that should help me in the future,but I need some help with the followint as I'm tired and have to fit in a schedule.I know it is long so I'll try and explain it as short and consice as I can. The DPLL procedure has two main stages: a simplification stage and a search stage. In the simplification stage, functions are applied to the formula to assign truth values to certain propositional variables. The simplifications are made because if there is a satisfying assignment for the simplified formula then it is also a satisfying assignment for the original formula. This reduces the need for search which can take a long time. The search stage is performed when no more simplifications can be made. In this stage a literal is chosen and is assigned true or false leading to two new branches in the search space. I wrote some function wich should be helpfull,but I have to fit in a schedule and I'm getting tired I need some additional help if possible. 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], Model)) -- This function takess a Clause and return the set of Atoms of that Clause. atomsClause :: Clause -> [Atom] -- This function takes a Formula returns the set of Atoms of a Formula atoms :: Formula -> [Atom] -- This function returns True if the given Literal can be found within -- the Clause. isLiteral :: Literal -> Clause -> Bool -- this function takes a Model and an Atom and flip the truthvalue of -- the atom in the model flipSymbol :: Model -> Atom -> Model -- is this ok? Additional functions that I wrote: remove :: (Eq a) )a ->[a] ->[a] -This function removes an item from a list. neg :: Literal->Literal -This function flips a literal (ie. from P to :P and from :P to P). falseClause :: Model -> Clause -> Bool -This function takes a Model and a Clause and returns True if the clause is unsatisfied by the model or False otherwise. falseClauses :: Formula -> Model -> [Clause] -This function takes a Formula and a Model and returns the list of clauses of the formula that are not satisfied. assignModel :: Model -> Formula -> Formula -This function applies the assign function for all the assignments of a given model. checkFormula :: Formula -> Maybe Bool This function checks whether a formula can be decided to be satisfiable or unsatisfiable based on the effects of the assign function. satisfies :: Model -> Formula -. Bool This function checks whether a model satisfies a formula. This is done with the combination of the assignModel and checkFormula functions. --Where do I need help: removeTautologies :: Formula->Formula This function should output a simplified formula if tautologies can be found in one or more clauses in the input Notes: 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) (the second clause alone) pureLiteralDeletion :: Formula->Formula 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. Hence this simplification is sound in that if there is a solution to the simplified formula then there is a solution to the original formula. propagateUnits :: Formula->Formula If a clause in a propositional formula contains only one literal, then that literal must be true (so that the particular clause can be satisfied). When this happens,we can remove the unit clauses (the ones that contain only one literal), all the clauses where the literal appears and also, from the remaining clauses, we can delete the negation of the literal (because if P is true, -P will be false).For example, in the formula (P v Q v R) ^ (-P v Q v -R) ^ (P) we have one unit clause (the third clause(P) ). Because this one has to be true for the whole formula to be true we assign True to P and try to find a satisfying assignment for the remaining formula. Finally because -P cannot be true (given the assigned value of P) then the second clause is reduced by eliminating the symbol -P . This simplification results in the revised formula (Q v -R). The resulting simplification can create other unit clauses. For example in the formula (-P v Q) ^ (P) is simplified to (Q) when the unit clause (P) is propagated. This makes (Q) a unit clause which can now also be simplified to give a satisfying assignment to the formula. The function should apply unit propagation until it can no longer make any further simplifications. Note that if both P and -P are unit clauses then the formula is unsatisfiable. In this case the function should return a formula with an empty clause in it to indicate that the formula could not be satisfied. update :: Node -> [Node] The update function should take in a Node and return a list of the Nodes that result from assigning True to an unassigned atom in one case and False in the other (ie. a case split). So the list returned should have two nodes as elements. One node should contain the formula with an atom assigned True and the model updated with this assignment, and the other should contain the formula with the atom assigned False and the model updated to show this. The lists of unassigned atoms of each node should also be updated accordingly. This function should use your implemented assign function to make the assignments. It should also use the chooseAtom function provided to select the literal to assign. search :: (Node -> [Node]) -> [Node] -> Int -> (Bool, Int) The search function should perform a backtracking search. The function takes the update function as input and uses it to generate nodes in the search space. The search function also takes in a list which has one element, the initial node consisting of the formula along with an initial model. It should generate nodes using the update function and check nodes using the given check function. If a node is unsatisfiable then it should abandon that branch of the search. If a node is satisfiable then a satisfying assignment has been found and so it should return True. If a node is neither satisfiable or unsatisfiable then it should generate new nodes from this node. If all possible branches of the search space have been tried, i.e. the list of nodes to try has become the empty list, then it should return False since all possible assignments have been tried. The search function also has an Int argument which should be an integer that tracks how many calls to the update function have been made. The search function should return a pair consisting of the truth value of the formula and the number of calls to update made. Thank you(because ,I'm anticipating your help),and until I get a reply I will continue to try an squize my exhausted brain. -- View this message in context: http://haskell.1045720.n5.nabble.com/Haskell-procedure-tp3369005p3369005.htm... Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.