No Linear newtypes - Haskell

Hey! is anyone familiar with the reason for why newtype constructors must be linear? e.g.

{-# LANGUAGE LinearTypes #-}

newtype Ur a where
  Ur :: a -> Ur a

doesn't work

[1 of 1] Compiling Main             ( linear.hs, interpreted )

linear.hs:4:3: error:
    • A newtype constructor must be linear
    • In the definition of data constructor ‘Ur’
      In the newtype declaration for ‘Ur’
  |
4 |   Ur :: a -> Ur a
  |   ^^^^^^^^^^^^^^^
Failed, no modules loaded.

this hints to me that linear types also carry some runtime info?

crazy

or perhaps they didn't want to limit their possibilities (in some way??) for implementation details in the future, and so forbade this for that reason

given that you can't write an implementation for a constructor this seems like something that should be implicit

wdym by implicit?

well that they are linear by default

if you write data Bla a = MkBla a, MkBla is linear by default

hmhm

https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0111-linear-types.rst#unrestricted-newtypes
whoops

huh!