[Haskell-cafe] How to call hUpdateAtHNat with a value known only at runtime

[Andras Slemmer]

You can only construct such a Proxy if you hide the type itself in an
existential type. Take a look at Data.Singletons, specifically the toSing
function for Nat/Integer.
However what you will probably find is that a Proxy like that would not be
useful at all in this case.
What are you *really* trying to do? If you will only know the index (and
the type of the element) at runtime then what you really need is dynamic
typechecking. With the Typeable constraint you can do exactly that. Take a
look at Data.Typeable's 'cast' function to dynamically safely coerce your
unknown data into a known element type of the HList.

On 23 December 2014 at 10:52, Erik Hesselink <hesselink at gmail.com> wrote:

> Hi Phil,
>
> As you kind of figured out, it's not clear what the type of your
> function intoToHNatProxy should be. The output type depends on the
> input *value*, which requires dependent types. Haskell doesn't have
> those (yet?). I think the best you can do is put the result in an
> existential, in effect saying "this has *some* type, but I don't know
> which one". Something like the code below. The problem is that what
> you can then do with the value inside the existential is more limited.
> It might help to know what the bigger picture is of what you're trying
> to do.
>
> {-# LANGUAGE GADTs #-}
> import Control.Applicative
> import Data.HList
>
> data SomeHList where
>   SomeHList :: HList l -> SomeHList
>
> updateAtSomeN :: Int -> e -> HList l -> Maybe SomeHList
> updateAtSomeN _ _ HNil         = Nothing
> updateAtSomeN 0 e (HCons _ xs) = Just $ SomeHList (HCons e xs)
> updateAtSomeN n e (HCons x xs) = consSomeHList x <$> updateAtSomeN (n - 1)
> e xs
>
> consSomeHList :: e -> SomeHList -> SomeHList
> consSomeHList x (SomeHList l) = SomeHList (HCons x l)
>
> Erik
>
> On Mon, Dec 22, 2014 at 1:59 AM, Phil Darnowsky <phil at darnowsky.com>
> wrote:
> > Hello all,
> >
> > I'm working on something that involves HLists where I'm making use of the
> > hUpdateAtHNat function for array-like semantics, which works great if you
> > know the index of the element you want at compile time:
> >
> > Prelude> :module +Data.HList
> >
> > Prelude Data.HList> let hl = hEnd $ hBuild "foobar" 123 True
> > Loading [a bunch of packages]
> >
> > Prelude Data.HList> hl
> > H["foobar", 123, True]
> >
> > Prelude Data.HList> hUpdateAtHNat hZero 90909 hl
> > H[90909, 123, True]
> >
> > Prelude Data.HList> hUpdateAtHNat (hSucc hZero) 90909 hl
> > H["foobar", 90909, True]
> >
> > Prelude Data.HList> hUpdateAtHNat (hSucc $ hSucc hZero) 90909 hl
> > H["foobar", 123, 90909]
> >
> >
> > So far so good. What's giving me trouble is figuring out how to update
> at an
> > arbitrary index, where that index is only known at runtime, because hZero
> > and hSucc are of different types:
> >
> > Prelude Data.HList> :t hZero
> > hZero :: Proxy 'HZero
> >
> > Prelude Data.HList> :t hSucc hZero
> > hSucc hZero :: Proxy ('HSucc 'HZero)
> >
> >
> > What I'd ideally like is some kind of function intToHNatProxy that would
> > take an Int and return a Proxy of the appropriate type, but I so far only
> > kinda sorta understand how proxies and lifted types work, so I haven't
> yet
> > succeeded in constructing one. Any advice would be very welcome.
> >
> > Thanks,
> > Phil
> > _______________________________________________
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