# [Haskell-beginners] Referring to several constructors at once

Tomer Libal shaolintl at gmail.com
Sun Jan 11 14:47:44 EST 2009

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
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