ECArule.hs

{-# OPTIONS_GHC -Wall #-}
module DatabaseDesign.Ampersand.ADL1.ECArule ( isAll
                                             , isCHC
                                             , isBlk
                                             , isNop
                                             , isDo
                                             , dos
 
                                             )
where
import DatabaseDesign.Ampersand.Fspec.Fspec
import DatabaseDesign.Ampersand.Basics     (fatalMsg)

  --   Ampersand derives the process logic from the static logic by interpreting an expression in relation algebra as an invariant.
  --   So how does Ampersand derive dynamic behaviour from static rules? An example may clarify this:
  --   Suppose you have large shoes that do not fit through your trousers in any way.
  --   Does this have any consequences for the process of dressing in the morning?
  --   Well sure it has!
  --   Since your shoes won't fit through your trousers, you must first put on your trousers, and then put on your shoes.
  --   So the order of putting on trousers and putting on shoes is dictated by the (static) fact that your shoes are too big to fit through your trousers.
  --   When undressing, the order is reversed: you must take off your shoes before taking off your trousers.
  --   This example ilustrates how the order of activities is restricted by an invariant property.
  --   So it is possible to derive some dynamic behaviour from static properties.
  --   The following datatypes form a process algebra.
fatal :: Int -> String -> a
fatal = fatalMsg "ADL1.ECArule"

  

isAll :: PAclause -> Bool
isAll ALL{} = True
isAll _     = False
  
isCHC :: PAclause -> Bool
isCHC CHC{} = True
isCHC _     = False
  
isBlk :: PAclause -> Bool
isBlk Blk{} = True
isBlk _     = False
  
isNop :: PAclause -> Bool
isNop Nop{} = True
isNop _     = False
  
isDo :: PAclause -> Bool
isDo Do{}   = True
isDo _      = False

dos :: PAclause -> [PAclause]   -- gather all Do's from a PAclause
dos p@CHC{} = concatMap dos (paCls p)
dos p@ALL{} = concatMap dos (paCls p)
dos p@Do{}  = [p]
dos p@Sel{} = dos (paCl p "x")
dos p@New{} = dos (paCl p "x")
dos p@Rmv{} = dos (paCl p "x")
dos Nop{}   = []
dos Blk{}   = []
dos Let{}   = fatal 56 "dos not defined for `Let` constructor of PAclause"
dos Ref{}   = fatal 57 "dos not defined for `Ref` constructor of PAclause"