[Haskell-cafe] Ambiguous type variable with subclass instance
(also: is there a better way to do this?)
daniel.is.fischer at web.de
Thu Sep 17 11:50:27 EDT 2009
Am Donnerstag 17 September 2009 15:40:10 schrieb Andy Gimblett:
> > instance (Enumerated a) => Target a where
> > convert n | n `elem` [0..len-1] = Just $ constructors !! n
> > | otherwise = Nothing
> > where len = length constructors
Yes, the second appearance of 'constructors' is at an unspecified type.
instance (Enumerated a) => Target a where
| n < 0 = Nothing
| otherwise = case drop n constructors of
(x:_) -> Just x
_ -> Nothing
would make it compile.
But there'd be a risk that Target is unusable, depending on how instance resolution is
> I guess I see roughly what's going on; the question is "which
> constructors instance is meant?", right? In the "Just" part it's OK,
> because it can be inferred from the function's return type (right?).
> But in the guard we don't have that help, so it could be any
> Enumerated instance?
> Any advice appreciated! Particularly if this is just a dumb approach.
> For context, this is related to deserialisation of binary data (they'll
> actually be Word8's, not Int's) into a variety of data structures.
> Hmmm, maybe I should just be using Data.Binary...
> Many thanks,
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