{- |

Description : infrastructure needed for presenting a logic to Hets

The folder "Logic" contains the infrastructure needed for

presenting a logic to Hets. A logic here is understood in

the theory of institutions. More precisely, a logic is

an entailment system in the sense of Meseguer.

An entailment system consists of a category

of signatures and signature morphisms, a set of sentences

for each signature, a sentence translation map of each

signature morphism, and an entailment relation

between sets of sentences and sentences for each signature.

The module "Logic.Logic" contains all the type classes for interfacing

entailment systems in this sense, including the type class

'Logic.Logic.Logic'. The entailment relation is given by one or more

theorem provers, rewriters, consistency checkers, model checkers. The

module "Logic.Prover" is for the interface to these. The data types

'Logic.Prover.Proof_Status' and 'Logic.Prover.Prover' provides the

interface to provers. In case of a successful proof, also the list of

axioms that have been used in the proof can be returned.

Note that the type class 'Logic.Logic.Logic' contains much more than

just an entailment system. There is infrastructure for parsing,

printing, static analysis (of both basic specifications and symbols

maps) , conversion to ATerms, sublogic recognition etc. You see that

in order to really work with it, one needs much more than just

an entailment system in the mathematical sense.

We will use the term /logic/ synonymously with an extended entailment

system in this sense.

The type class 'Logic.Logic.LogicalFramework' is an interface for the

logics which can be used as logical frameworks, in which object

logics can be specified by the user. Its methods are used in the

analysis of newlogic definitions, in 'Framework.Analysis'.

The method 'Logic.Logic.LogicalFramework.base_sig'

returns the base signature of the framework

The method 'Logic.Logic.LogicalFramework.write_logic'

constructs the contents of the Logic_L

file, where L is the name of the object logic passed as an argument.

Typically, this file will declare the lid of the object logic L and

instances of the classes Language, Syntax, Sentences, Logic, and

StaticAnalysis. The instance of Category is usually inherited from

the framework itself as the object logic reuses the signatures and

morphisms of the framework. The function "write_syntax" constructs

the contents of the file declaring the Ltruth morphism.

Currently we simply store the morphism using its representation as

a Haskell datatype value. If proofs and models are later integrated

into Hets, there should be analogous functions "write_proofs" and

"write_models" added, declaring the Lpf and

Lmod morphisms respectively.

Module "Logic.Comorphism" provides type classes for the various kinds

of mappings between logics, and module "Logic.Grothendieck"

realizes the Grothendieck logic and also contains a type

'Logic.Grothendieck.LogicGraph'.

/How to add a new logic to Hets/

A good idea is to look at an existing logic, say "Propositional" or

"CASL", and look how it realizes the abstract interface given in

"Logic.Logic" - this is done in "Propositional.Logic_Propositional"

resp. "CASL.Logic_CASL", where the whole of

the "Propositional" resp. "CASL" folder is imported.

You also should have a look at some of the "Common" modules,

providing data structures for identifiers ("Common.Id"),

results ("Common.Result"), and relations

("Common.Lib.Rel"). These are used quite frequently.

-}

module Logic where