Hi,

I understood one more point. (I share here.)

The Prelude library document for ghc 8.0 is already well described for beginners/newcomers.

* The ($)'s signature of 8.0.1 is already simple (not include forall ...).

* The Bool's kind of 8.0.1 is already represented with "TYPE Lifted" (changed from '*').

ghc7.8.4 [1]:

data Bool :: *

foldr :: (a -> b -> b) -> b -> [a] -> b

($) :: (a -> b) -> a -> b

ghc7.10.4 [2]:

data Bool :: *

foldr :: (a -> b -> b) -> b -> t a -> b

($) :: (a -> b) -> a -> b

ghc8.0.1-rc2 [3]:

data Bool :: TYPE Lifted

foldr :: (a -> b -> b) -> b -> t a -> b

($) :: (a -> b) -> a -> b

[1] https://downloads.haskell.org/~ghc/7.8.4/docs/html/libraries/base-4.7.0.2/Prelude.html

[2] https://downloads.haskell.org/~ghc/latest/docs/html/libraries/base-4.8.2.0/Prelude.html

[3] https://downloads.haskell.org/~ghc/8.0.1-rc2/docs/html/libraries/base-4.9.0.0/Prelude.html

Regards,

Takenobu

2016-02-08 19:08 GMT+09:00 Takenobu Tani <takenobu.hs@gmail.com>:

Hi Richard and devs,

What a wonderful (#11549) !
This is a beautiful solution for beginners/newcomers.
Beginners will not confuse and they can gradually go ahead.

I extremely appreciate that you are continuously improving the ghc for us.

Thank you very much,
Takenobu

2016-02-07 0:17 GMT+09:00 Richard Eisenberg <eir@cis.upenn.edu>:
I have made a ticket #11549 (https://ghc.haskell.org/trac/ghc/ticket/11549) requesting a -fshow-runtime-rep flag (recalling that the name levity will soon be outdated) as described in this thread. I will make sure this gets in for the release of 8.0.

Other points:

- You're quite right that (.) could be generalized. But I'll wait for someone to really want this.

- I don't have a non-contrived example of the use of ($) with unlifted types. It's quite possible that when adding the dirty runST hack, it was observed that an unlifted type would be OK. At that point, the type of ($) didn't need to become so elaborate. And now we're just trying not to change old (but perhaps unrequested) behavior.

- For the record, this debate is entirely unrelated to the runST impredicativity hack. (Except, as noted above, perhaps in history.) That hack remains, basically unchanged.

- On Feb 6, 2016, at 9:55 AM, Roman Cheplyaka <roma@ro-che.info> wrote:
>
> I would call this a simplification rather than a lie.

This is a very convincing argument.

- Thanks, also, for the voice of support. What I love about the Haskell community is that we can have an impassioned debate full of strong opinions, and it all very rarely devolves into a proper flame war. All the posts I've seen in this thread have been constructive and helpful. Thanks.

Richard

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