Re: unsafeCoerce and type aliases under type applications

4 Dec 2011

      Hi,

Am Sonntag, den 04.12.2011, 00:50 -0500 schrieb wren ng thornton:
...
On 12/3/11 9:07 PM, Felipe Almeida Lessa wrote:
...
On Sat, Dec 3, 2011 at 5:39 PM, Joachim Breitner
  wrote:
...
have used unsafeCoerce to change the type inside a container to a "type"
alias in real code, but your post makes me wonder: Under what
circumstances is that safe? Is that documented somehow? Can a tool or
the compiler decide for us whether it is safe?
AFAIK, newtypes are safe, and for everything else you're on your own.  =)
N.B., newtypes are safe in the sense of congruent rewriting; i.e., if X 
is a newtype of Y, then we can rewrite X to Y (or Y to X) in any subterm 
of the type term (just like if X = Y or X ~ Y). It's not just at the 
top-level of the type term.
that is what I would expect at first glance, but at least some type
features break this behavior:

$ cat brokenNewtype.hs 
{-# LANGUAGE TypeFamilies #-}

import Unsafe.Coerce

newtype Int' = Int' Int

type family Break a
type instance Break Int = Int
type instance Break Int' = Int -> IO Int

list :: [Break Int]
list = [1..10]

list' :: [Break Int']
list' = unsafeCoerce list

main = do
    print list
    head list' 1 >>= print
$ ghc --make brokenNewtype.hs 
[1 of 1] Compiling Main             ( brokenNewtype.hs, brokenNewtype.o )
Linking brokenNewtype ...
$ ./brokenNewtype
[1,2,3,4,5,6,7,8,9,10]
brokenNewtype: internal error: stg_ap_v_ret
    (GHC version 7.0.4 for x86_64_unknown_linux)
    Please report this as a GHC bug:
http://www.haskell.org/ghc/reportabug
Abgebrochen

So the question remains: Under which circumstances is newtypes coercing
within a type term using unsafeCoerce safe?

And I find this not a purely academic question: If I have a huge data
structure of “[Tagged SomePhantomType Int]” and I need to run some
library function on it that only provides me with an operation of type
“[Int] -> [Int]”, I do not want to re-create the list twice even when I
_know_ the representation is the same.

Greetings,
Joachim

-- 
Joachim "nomeata" Breitner
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