I wonder if I can have my cake and eat it too regarding `KnownNats`

. Can I write code that uses `Nats`

that may be both `KnownNats`

and `UnknownNats`

(`SomeNats`

?).

For example if I have a dependently typed vector `Vec (n :: Nat) a`

, can I write code that works both if the size is known at compile and at runtime? Thing is that I don't want to duplicate the whole code for statically and dynamically sized "things". And I don't want to lose static guarantees by storing sizes in the data structure.

Answer to András Kovács:

My specific usecase is reading images from disk (which are luckily of fixed size) and then extracting patches from that, so basically I have a function `extractPatch :: (KnownNat w2, KnownNat h2) => Image w1 h1 a -> Patch w2 h2`

a where both `Image`

and `Patch`

are instances of a common `Mat (w :: Nat) (h :: Nat)`

a type.

If I wouldn't know the image size I would have to encode this in "runtime types". Just wondering.

Answer:

Here's something potentially interesting...

```
{-# LANGUAGE DataKinds, KindSignatures, ScopedTypeVariables #-}
import GHC.TypeLits
import Data.Proxy
data Bar (n :: Nat) = Bar String deriving Show
bar :: KnownNat n => Bar n -> (String, Integer)
bar b@(Bar s) = (s, natVal b)
```

Ok, it's very pointless. But it's an example of using `KnownNat`

to get at compile-time information. But thanks to the other functions in `GHC.TypeLits`

, it can be used with run-time information as well.

Just add this on to the above code, and try it out.

```
main :: IO ()
main = do
i <- readLn
let Just someNat = someNatVal i
case someNat of
SomeNat (_ :: Proxy n) -> do
let b :: Bar n
b = Bar "hello!"
print $ bar b
```

Let's break down what happens here.

- Read an
`Integer`

from stdin. - Create a
`SomeNat`

-typed value from it, failing the pattern-match if the input was negative. For such a simple example, handling that error just gets in the way. - Here's the real magic. Pattern-match with a
`case`

expression, using`ScopedTypeVariables`

to bind the (statically unknown)`Nat`

-kinded type to the type variable`n`

. - Finally, create a
`Bar`

value with that particular`n`

as its type variable and then do things with it.

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