[Haskell-cafe] how to make a KnownNat at runtime?

Johannes Waldmann johannes.waldmann at htwk-leipzig.de
Tue Nov 14 14:01:52 UTC 2017

Dear Cafe,

I am using type-level numbers for
representing dimensions of vectors and matrices
(similar to https://github.com/ekmett/ersatz/blob/master/tests/Z001.hs )
Now when I indeed know all the dimensions at compile time,
(in the example code, line 20) everything works nicely.

But what do I do when the dimensions
will only become available at run time?

Here is a simplified example: print the null vector
where the size is read from the command line -
of course in my application "handle" does more,
but it essentially has the type given here.

I can extend the "case" in the "main" function
to include all the values I want to handle.
Looks ugly. Is there a better way?

- J.

{-# language KindSignatures, RankNTypes, LambdaCase,
    TypeApplications, DataKinds, ScopedTypeVariables

import GHC.TypeLits
import Data.Proxy
import System.Environment

main :: IO ()
main = getArgs >>= \ case
  [ "2" ] -> handle (Proxy :: Proxy 2)
  [ "3" ] -> handle (Proxy :: Proxy 3)

handle :: forall (n::Nat) . KnownNat n => Proxy n -> IO ()
handle (_ :: Proxy n) = print (zero :: V n Int)

data V (n::Nat) a = V [a] deriving Show

zero :: forall (n::Nat) a . (KnownNat n, Num a) => V n a
zero = V $ replicate (fromIntegral $ natVal (Proxy :: Proxy n)) 0

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