I'm experimenting with a Literate Haskell style to construct a tutorial about Description Logics [1][2]. I'm developing the material in small sections, and using examples in the document to test the code as I go along . I therefore find that I want to introduce the components of class and instance declarations in pieces, rather than having the entire declaration in a single place. Am I right in thinking this is not possible? For example, I find myself resorting to tricks like this: [[ So far, a single Description Logic (AL) has been considered. My next step is to generalize the function interface to a collection of type classes that can handle arbitrary description logics. The basic ideas of concepts, roles and interpretations are common to all description logics, so these are used as defined above. It is the concept descriptions, role descriptions and associated operations that need to be further abstracted: - class ConceptExpr c where - iConcept :: Ord a => TInterpretation a -> c -> Set a (The full and final definition of ConceptExpr is given later.) : ]] and later... [[ The class interface for concept expressions is therefore extended thus:

class ConceptExpr c where iConcept :: Ord a => TInterpretation a -> c -> Set a isSatisfiableWith :: TBox c -> c -> Bool isSatisfiableWith t c = not (isSubsumedByWith t c emptyClass) isSubsumedByWith :: TBox c -> c -> c -> Bool isSubsumedByWith t c d = not $ isSatisfiableWith t (intersectClass c (complementClass d)) emptyClass :: c complementClass :: c -> c intersectClass :: c -> c -> c ]]

What this suggests to me is that for Literate Haskell it is desirable to have multiple appearances of a class/instance declaration, constrained so that one version contains all of the definitions contained in any other appearance, and no two appearances contain differing definitions for any component (using a fairly strict syntactic comparison to determine difference here). Does this strike any chords with anyone? #g -- [1] http://dl.kr.org/ (Description Logics web page) [2] http://www.ninebynine.org/Software/HaskellDL/DLExploration.lhs (My tutorial, work-in-progress - implements "structural subsumption" for a simple Description Logic.) ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact