Hello, I am trying to define a logical calculus using one data type for the rules. There are three types of rules and each has 0,1 or 2 assumptions (rule, rule1 and rule2 below). I have defined all the rules as different constructors so the difference between the types according to 0,1 or 2 assumptions is very weak. I would still like to be able to group the rules in types according to the number of assumptions in order to use pattern matching. Is there a simple way to do that or another way I should implement the data type such that I can refer to the rules both according to their number of assumptions and according to their type? 27 data Rule = Axiom {lowseq :: Sequent} 28 | WeakeningL {rule :: Rule, lowseq :: Sequent, foccur :: FormulaOccur} 29 | WeakeningR {rule :: Rule, lowseq :: Sequent, foccur :: FormulaOccur} 30 | ContractionL {rule :: Rule, lowseq :: Sequent, foccur :: FormulaOccur} 31 | ContractionR {rule :: Rule, lowseq :: Sequent, foccur :: FormulaOccur} 32 | PermutationL {rule :: Rule, lowseq :: Sequent} 33 | PermutationR {rule :: Rule, lowseq :: Sequent} 34 | Mix {rule1 :: Rule, rule2 :: Rule, lowseq :: Sequent, foccur :: FormulaOccur} 35 | NotL {rule :: Rule, lowseq :: Sequent, foccur :: FormulaOccur} 36 | NotR {rule :: Rule, lowseq :: Sequent, foccur :: FormulaOccur} 37 | AndL {rule1 :: Rule, rule2 :: Rule, lowseq :: Sequent, foccur :: FormulaOccur} 38 | AndR {rule1 :: Rule, rule2 :: Rule, lowseq :: Sequent, foccur :: FormulaOccur} 39 deriving (Eq, Show) Thanks, Tomer