On Dec 19, 2006, at 10:11 PM, Dan Weston wrote:
> > instance CommandFunction (Sh st ()) st where
> ^
> I think your first argument (on which the second has a functional
> dependence) does not determine the second argument, since it makes
> use of st in the first argument. This strikes me as a likely place
> to begin.
No, I'm pretty sure this isn't a problem. The second argument is
determined _because_ it is mentioned in the first. The functional
dependencies and instance declarations work, as long as I can make
the compiler accept them. They are only being rejected by the
termination-checking part of the algorithm.
That said, I'm open to the idea of reformulating these instances. In
fact, I don't really like the fact that I need FDs. It seems to me
that I should somehow be able to eliminate the second argument
altogether and thus the FD, but I can't seem to figure it out.
> Dan
>
> Robert wrote:
>> Fellow Haskellers,
>> I have a package that uses some light typeclass hackery to
>> automaticly
>> build parsing algorithms based on the type of a function.
>> I was recently informed that my package doesn't compile on GHC 6.6
>> due
>> to the new restrictions on FD resolution; in particular I have
>> instance
>> declarations which fail the coverage condition. I can use
>> undecidable
>> instances to make the package compile again, but I'd prefer not to
>> if I
>> can avoid it.
>> class CommandFunction f st | f -> st where
>> parseCommand :: String -> f -> CommandParser st
>> commandSyntax :: f -> [Doc]
>> instance CommandFunction (Sh st ()) st where
>
>
>
>> parseCommand wbc m str =
>> -- list monad
>> do (x,[]) <- runRegex (maybeSpaceBefore (Epsilon
>> (CompleteParse
>> m))) str
>> return x
>> commandSyntax _ = []
>> instance CommandFunction r st
>> => CommandFunction (Int -> r) st where
>> parseCommand = doParseCommand Nothing intRegex id
>> commandSyntax f = text (show intRegex) : commandSyntax (f
>> undefined)
>> instance CommandFunction r st
>> => CommandFunction (Integer -> r) st where
>> parseCommand = doParseCommand Nothing intRegex id
>> commandSyntax f = text (show intRegex) : commandSyntax (f
>> undefined)
>
>
Speak softly and drive a Sherman tank.
Laugh hard; it's a long way to the bank.
-- TMBG