=======================================================================
This is a brief description of the Haskell implementation of Feature
Algebra (Institut für Informatik, Universität Augsburg)
(c) June 2006 by Peter Höfner, Ridha Khedri, Bernhard Möller
=======================================================================
0. Prerequisites
----------------
The Package requires a Haskell implementation; please see
http://www.haskell.org for download and installation guides.
We will assume that users of our implementation have at least
a superficial knowledge of Haskell.
1. Package Structure
--------------------
The basic module is FeatureAlg.hs . It is a proper Haskell
module that needs to be imported into the definitions of
particular feature algebras. Samples of usage are provided by
the files
DVD.hs
Robot.hs
These provide implementations of the DVD player and Robot examples
discussed in the accompanying report "Feature Algebra" available from
http://www.informatik.uni-augsburg.de/lehrstuehle/dbis/pmi/publications/all_pmi_tech-reports/tr-2006-4_hoe_khe_moe/Downloads/FeatureAlgebra.pdf
To use any of these examples, simply load the respective *.hs file
into your Haskell interpreter or compiler, e.g.
:l DVD.hs
2. Main Types and Functions
---------------------------
The main data type, describing the abstract syntax of feature expressions
is the following:
data Feature = Zero
| One
| Basic BaseFeature
| Sum Feature Feature
| Product Feature Feature
| Excl Feature Feature
It uses the auxiliary types
type BaseFeature = String
type Product = [BaseFeature] -- interpreted as a bag of BaseFeatures
type ProdFamily = [Product] -- interpreted as a set of Products
One of the main tasks of the module is to normalise feature expressions
into a sum-of-products form. To speed up this process, we memoize this
normal form and use pairs each consisting of a feature expression
and its normal form. This yields the "working type"
type MemFeature = (Feature, ProdFamily)
To mimic the algebraic notation we introduce the following infix
operators that work on objects of type MemFeature
infixl 5 .+., .-. -- sum, difference
infixl 6 .*. -- composition
infixl 7 .^., .^<=. -- power, sum of powers
Moreover, zero and one are the MemFeature representations of the
Feature expressions Zero and One .
For abbreviation we introduce an operation for expression optionality
of several features:
opt :: [MemFeature] -> MemFeature
The expression opt [mf1,...,mfn] is equivalent to
(mf1 .+. one) .*. ... .*. (mfn .+. one)
The refinement relation between product families is
ref :: MemFeature -> MemFeature -> Bool
After MemFeature expressions have been built up with the above
operations, there are two ways of (moderately) pretty-printing them.
The pure sum-of-products form is output by
printfeat :: MemFeature -> IO()
In the interpreter you simply type printfeat mf , where mf is (the
name of) a feature expression, followed by ENTER, to get the normal
form displayed.
A more structured output where the features common to all members of a
product family are printed up front is obtained by
print_struct :: MemFeature -> IO()
3. Further Usage
----------------
For outputting the result of a normalisation into a file and the like
the standard Haskell operations of result type IO() may be used;
currently we do not provide special operations for this.
4. Feedback
-----------
We are very interested in hearing from users. Please send your comments to
feature_algebra@informatik.uni-augsburg.de