I think it might be impossible with type families. I don't think it's
possible to differentiate with type families something like T a a, and T a
b, with b different from a.
I think that you would need overlap to write this.
Here'shttp://www.haskell.org/ghc/docs/7.4.2/html/users_guide/type-families.html#da...the
GHC page on it. With type families you can do overlap, but in that
case, the result of unification must be the same. So basically :
T a a === T a b (with b set to a)
whereas the "intuitive" way of doing Find requires a way of differentiating
between these two cases.
On Wed, Feb 27, 2013 at 4:33 PM, 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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