does this irritate anyone else?

[Hal Daume III]

Hi,

>   Is there really no way to say "This newtype supports all the operations
> of the type it was defined from."? Normally I would define it with type but
> I could not then write this code which is "the offending code" you asked for.

I think this pretty much defeats the purpose of newtypes.  Their primary
usage is to get the typechecker to tell you when you're using the old type
instead of the newtype.  That and to wrap stuff to make instance
declarations.

> class State a where
>   applyMove :: a -> Move -> a
> 
> type DetectiveTickets = Array Detective (Array Ticket Int)
> 
> instance State DetectiveTickets where
>   applyMove dT = ...

This produces a 'Cannot use type synonym in instance head' in hugs in 98
mode, but in mode with extensions on, works fine.  Similarly, in ghc,
without -fglasgow-exts, you get a similar error message, but it works fine
with -fglasgow-exts.

> On Wed, Feb 26, 2003 at 08:50:48AM -0800, Hal Daume III wrote:
> > One way to get around this is to do something like:
> > 
> > > newtype DetectiveLocations = DetectiveLocations SomeType1
> > > newtype FugitiveLocations  = FugitiveLocations SomeType2
> > >
> > > class MkUn nt t | nt -> t where
> > >    mk :: t -> nt
> > >    un :: nt -> t
> > >
> > > instance MkUn DetectiveLocations SomeType1 where
> > >    mk = DetectiveLocations
> > >    un (DetectiveLocations dl) = dl
> > >
> > > -- similarly for FugLocs
> > 
> > Then, your function is a bit cleaner with (note that the type sig is very
> > important now):
> > 
> > > findFugitive dL fL StartGame = 
> > >     mk (minusSet (un dL) (occupiedStops (un fL))
> > 
> > Another way to do it would be if 'minusSet' were a class method, then you
> > could use GHC's newtype deriving feature (or in Hugs just define the
> > obvious instances).  For instance, if we had:
> > 
> > > class Set s where
> > >    minusSet :: Set a -> Set a -> Set a
> > >    ...
> > 
> > then you could do:
> > 
> > > newtype DetectiveLocations = DetectiveLocations SomeType1
> > >     deriving (Set)
> > > newtype FugitiveLocations  = FugitiveLocations SomeType2
> > >     deriving (Set)
> > 
> > assuming SomeType1 and SomeType2 were instances of Set; then your second
> > definition would be fine.
> > 
> > Finally, if you make your constructor names shorter, it doesn't look quite
> > so bad and you get nice error messages and helpful type checking.
> > 
> > As for why you get ambiguous variables, I'd need to see the offending data
> > and instance definitions.
> > 
> > HTH
> > 
> >  - Hal
> > 
> > --
> > Hal Daume III
> > 
> >  "Computer science is no more about computers    | hdaume@isi.edu
> >   than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume
> > 
> > On Wed, 26 Feb 2003, Mike T. Machenry wrote:
> > 
> > > I am trying to use type classes to get some percollate actions through a
> > > nested data structure. The problem is I can't make an instance of a class
> > > unless the type is defined with newtype or data. This means all of my
> > > functions look something like this:
> > > 
> > > findFugitive :: DetectiveLocations -> FugitiveLocations -> Move -> FugitiveLocat
> > > findFugitive (DetectiveLocations dL) (FugitiveLocations fL) StartGame =
> > >   (FugitiveLocations minusSet fL (occupiedStops dL))
> > > 
> > > instead of this:
> > > 
> > > findFugitive :: DetectiveLocations -> FugitiveLocations -> Move -> FugitiveLocat
> > > findFugitive dL fL StartGame = minusSet fL (occupiedStops dL)
> > > 
> > > which is much simpler to read. Why can't I make a normal type an instance of 
> > > a type class? I get errors that it's an ambiguous type.
> > > 
> > > thanks,
> > > -mike
> > > _______________________________________________
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> > > http://www.haskell.org/mailman/listinfo/haskell
> > > 
> > 
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