Daniel Carrera
Is there a way to tell Haskell that a list or array must have exactly (say) 256 elements? I'd love to have Haskell make sure that the array I build is the correct size.
Yes, you can build lists with a maximum size. The simplest approach I've seen is BoundedList.hs in the HaskellDB library: http://cvs.sourceforge.net/viewcvs.py/haskelldb/haskelldb/src/Database/Haske... The more general question is, "How to encode values into types in such a way that I can use them?" One neat answer is here: http://pobox.com/~oleg/ftp/Haskell/number-parameterized-types.html There are a bunch of neat type tricks like this, but the next step up in encoding information in types is dependent type systems like the one used in Epigram: http://www.dur.ac.uk/CARG/epigram/ I recently asked on http://lambda-the-ultimate.org/ what else can be done with type systems other than program safety. I got a neat answer from David Teller: <quote> Static type-checking in Camelot guarantees RAM usage -- not only its safety but also its amount. Static type-checking in my controlled pi-calculus (definitely not a real programming language) guarantees statically properties of access control ("only X can use the printer", "I can only read this information for the purpose of transferring it to you", ...) or resource management ("this program will never require more than x threads, y RAM and z printers"). Static type-checking in TyPiCal guarantees the respect of some protocols. In Acute, you can check the safety of communications... </quote> Type systems can do a lot of neat stuff these days, I've been able to use HaskellDB in my paying work already! -- It seems I've been living two lives. One life is a self-employed web developer In the other life, I'm shapr, functional programmer. | www.ScannedInAvian.com One of these lives has futures (and subcontinuations!)| --Shae Matijs Erisson