[Haskell-cafe] Help to write type-level function

[Dmitry Kulagin]

That seems to be very relevant to my problem (especially HList.Record).
Am I right that UndecidableInstances is required mostly because of eq on
types, like in this instances:


class HRLabelSet (ps :: [*])
instance HRLabelSet '[]
instance HRLabelSet '[x]

instance ( HEq l1 l2 leq
         , HRLabelSet' l1 l2 leq r
         ) => HRLabelSet (LVPair l1 v1 ': LVPair l2 v2 ': r)

so the usage of the extension is unavoidable for my purposes?

Thank you!


On Wed, Feb 27, 2013 at 12:28 PM, <oleg at okmij.org> wrote:

>
> Dmitry Kulagin wrote:
> > I try to implement typed C-like structures in my little dsl.
>
> HList essentially had those
>         http://code.haskell.org/HList/
>
> > I was unable to implement required type function:
> > type family Find (s :: Symbol) (xs :: [(Symbol,Ty)]) :: Ty
> > Which just finds a type in a associative list.
>
> HList also implemented records with named fields. Indeed, you need a
> type-level lookup in an associative list, and for that you need type
> equality. (The ordinary List.lookup has the Eq constraint, doesn't
> it?)
>
> Type equality can be implemented with type functions, right now.
>         http://okmij.org/ftp/Haskell/typeEQ.html#TTypeable
>
> (That page also defined a type-level list membership function).
>
>
>
>
>
>
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