[Haskell-cafe] Re: Implementing fixed-sized vectors (using datatype algebra?)

[Dan Weston]

This may be a GHC bug, but even though in the module 
Data.TypeLevel.Num.Reps has the header

{-# LANGUAGE EmptyDataDecls, TypeOperators #-}

I still get an error with both ghc and ghci version 6.8.2 unless I throw 
in the -XTypeOperators flag.

 > cat Go.hs
import Data.TypeLevel.Num.Reps
main = return (undefined :: D2 :+ D1) >> print "Done"

 > ghc --make Go.hs
[1 of 1] Compiling Main             ( Go.hs, Go.o )

Go.hs:3:31:
     Illegal operator `:+' in type `D2 :+ D1'
       (Use -XTypeOperators to allow operators in types)

 > ghc --make -XTypeOperators Go.hs
[1 of 1] Compiling Main             ( Go.hs, Go.o )
Linking Go ...

 > ghc --version
The Glorious Glasgow Haskell Compilation System, version 6.8.2

Dan

Alfonso Acosta wrote:
> Don't expect anything astonishing yet, but an initial version of the
> library can be found at
> 
> http:/code.haskell.org/type-level
> 
> To make reviewing easier I have put the haddock-generated documentation at
> 
> http://code.haskell.org/~fons/type-level/doc/
> 
> Some remarks:
> 
> * Only Positive and Natural numerals in decimal representation are
> supported. It would be cool to add support for Integers though.
> 
> * The code is based on Oleg's implimentation of type-level binaries
> http://okmij.org/ftp/Computation/resource-aware-prog/BinaryNumber.hs
> 
> * exponentiation/log and GCD is not implemented yet
> 
> * multiplication is not relational and thus division is broken. I
> tried porting Oleg's multiplication algorithm without success.
> 
> * Aliases (in binary, octal decimal and hexadecimal form) for
> type-level values and their value-level reflection functions are
> generated with TH.
>   That implies:
>     * Long compilation time depending on your system
>     * Although errors will always be reported in decimal form, the end
> user can input values using other bases (only for values in the range
> of generated aliases of course)
> 
> * It would be cool to have "real" support for other bases apart from decimals
>   * It would imply having unlimited size of input for other bases
> (right now if we want to input a value out of the alises limit,
> decimal reprentation is mandatory)
>   * However, is it feasible? How could it be done without needing to
> implement the operations for each base? WOuld it be possible to
> "overload" the type-level operators so that they worked with different
> representations and mixed representation arguments?
> 
> * Booleans are not implemented (Wolfgang?)
> 
> I would be happy to hear any suggestions, get code reviews and/or contributions.
> 
> Cheers,
> 
> Fons
> 
> 
> 
> 
> 
> 
> 
> 
> On Feb 7, 2008 11:17 AM, Wolfgang Jeltsch <g9ks157k at acme.softbase.org> wrote:
>> Am Donnerstag, 7. Februar 2008 02:47 schrieb Alfonso Acosta:
>>>> The other library I use for type-level programming is HList. It has
>>>> type-level booleans already so you might what to take a look at it if
>>>> you're not already familiar with it.
>>> Thanks I'll have a look at it.
>> I have to admit that I don't like the names HBool, HTrue and HFalse.  What do
>> they mean?  Heterogenous booleans?  Heterogenous truth?  Why it's "Bool"
>> instead of "Boolean" and therefore not conforming to the Prelude convention?
>>
>> Heterogenous lists are not directly about type level computation.  A HList
>> type is usually inhabited.  On the other hand, types which denote type level
>> naturals or type-level booleans are empty data types.  So type level booleans
>> should go into some type level programming library like the one Alfonso and I
>> want to create.  HList should then use these booleans.  This is at least my
>> opinion.
>>
>> Best wishes,
>> Wolfgang
>>
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