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

[Dmitry Kulagin]

Hi,

I try to implement typed C-like structures in my little dsl.
I was able to express structures using type-level naturals (type Ty is
promoted):

> data Ty = TInt | TBool | TStruct Symbol [Ty]

That allowed to implement all needed functions, including type-level
function:

> type family Get (n :: Nat) (xs :: [Ty]) :: Ty

But it is not very convenient to identify struct's fields using naturals,
and I wanted to change Ty definition to:

> data Ty = TInt | TBool | TStruct Symbol [(Symbol, Ty)]

It is much closer to how C-struct looks, but 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.

Could someone give me a hint, how to do it?
Or perhaps, is it just impossible thing to do?

Thanks!
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