{-# OPTIONS -fallow-undecidable-instances #-}

{- |

Module : $Header$

Description : Instance of class Logic for propositional logic

Copyright : (c) Dominik Luecke, Uni Bremen 2007

License : similar to LGPL, see HetCATS/LICENSE.txt or LIZENZ.txt

Maintainer : luecke@tzi.de

Stability : experimental

Portability : portable

Definition of signatures for propositional logic

-}

{-

Ref.

Till Mossakowski, Joseph Goguen, Razvan Diaconescu, Andrzej Tarlecki.

What is a Logic?.

In Jean-Yves Beziau (Ed.), Logica Universalis, pp. 113-@133. Birkhäuser.

2005.

-}

module Propositional.Sign

(

Sign (..) -- Propositional Signatures

,pretty -- pretty printing

,isLegalSignature -- is a signature ok?

) where

import qualified Data.Set as Set

import Common.Id

import Common.Doc

import Common.DocUtils

-- | Datatype for propositional Signatures

-- Signatures are just sets

data Sign = Sign {items :: Set.Set Id}

deriving (Eq, Show)

instance Pretty Sign where

pretty = printSign

-- | determines whether a signature is vaild

-- all sets are ok, so glued to true

isLegalSignature :: Sign -> Bool

isLegalSignature _ = True

printSign :: Sign -> Doc

printSign s = specBraces $ (sepByCommas $ map idDoc (Set.toList $ items s))