Hi Dmitry,

You can effectively think of the type-level language of Haskell as a strict functional language with almost no syntactic sugar, and recursion is the *only* way of getting the behaviour of a loop. The best you can do to improve the performance of that type family is to write it in tail-recursive form using a helper function and which will make the type family reduction look more linear vs a tree like it is now and make it easier for the type checker to quickly spit out the result.

type family Fib (n::Nat) :: Nat where
  Fib n = GoFib 0 1 n

type family GoFib (a :: Nat) (b :: Nat) (n :: Nat) where
  GoFib a _ 0 = a
  GoFib a b n = GoFib b (a + b) (n - 1)

Hope that helps,
Rahul

On Mon, Jul 17, 2017 at 11:19 AM, Dmitry Olshansky <olshanskydr@gmail.com> wrote:
Hello, cafe!

I wonder is there some possibility to get Memoization in Type Calculation (e.g. in closed Type Families)?

Can we make something more efficient for famous Fib function then

type family Fib (n::Nat) :: Nat where
  Fib 1 = 1
  Fib 2 = 1
  Fib n = Fib (n-1) + Fib (n-2)

This Fib has obviously exponential calculation time and we need some memoization.

If it is impossible right now, maybe there is a ticket for this? It seems to me very important things, no?

Probably in this case we can write TypeChecker Plugin, but it looks like the common problem.

Best regards,
Dmitry




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--
Rahul Muttineni