On Thursday 24 February 2005 19:51, Keean Schupke wrote:
> Benjamin Franksen wrote:
> >I still wonder if your TH generated code can handle higher ranked field
> > types; i.e. can I write
> >
> >$(ttypelift [| data Record = Record {
> > field1 :: Int,
> > field2 :: (forall a. a-> a) } |] )
> >
> >or does ghc give me an error?
>
> Well at the moment this would give an error, but remember the
> list is heterogeneous, so you can just not give the list a type, and
> simply append the specific function... admitedly this is not as
> type-safe.
>
> hUpdateAtLabel field2 someFunction myRecord
That is an advantage of hLists as compared to normal records.
A disadvantage is that each field access needs to traverse the list. I wonder
if this isn't rather less efficient than the random access provided by normal
records.
> Of course if the type of the function is unknown until runtime, we have
> to use existential quantification.
>
> It might be possible to automatically wrap such types, but my feeling at
> the moment is that it is best left to the programmer.
Automatic wrapping is easy. What is not so easy and I think not even possible,
is automatic _un_wrapping, so that the wrapping isn't exposed at all to the
programmer.
I am not a type system guru, so can anyone enlighten me on why higher ranked
types are not allowed in instances? Is there a principle problem, or is it
just difficult to implement? Does this have to do with this "higher order
unification" that Haskell is alleged not to have?
Ben
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