What `reify` sees in Template Haskell

[Simon Peyton Jones]

|    1. would it be possible to revert to the old behavior easily, or is
|  doing that difficult or impossible now given the overall changes to
|  the Template Haskell code?

I don’t think it would be difficult to recover the old behaviour, but it's not clear to me that it would be a Good Thing.  A program that works today, and then does not work tomorrow because of some incidental change to the way type inference works, would not be a happy state of affairs.

Perhaps if 'x' had a type annotation that would be ok.  Or maybe there is something else about your intended application that makes it solidly predictable.

By all means start a wiki page to sketch a design for what you think should happen, and why it should be predicable.


Meanwhile, if you would care to draft the bit of user-manual material that you wish had been there, I could review it and put it in.

Thanks!

Simon

|  -----Original Message-----
|  From: Francesco Mazzoli [mailto:f at mazzo.li]
|  Sent: 13 February 2015 12:54
|  To: ghc-devs at haskell.org
|  Cc: Simon Peyton Jones
|  Subject: What `reify` sees in Template Haskell
|  
|  Hi,
|  
|  I recently stumbled upon the issue of `reify` not being able to give
|  information about local variables, as of GHC 7.8.  For example, this
|  won't work:
|  
|      foo x = $(do {xInfo <- reify 'x; ...})
|  
|  The motivation for this change is described here:
|  <https://ghc.haskell.org/trac/ghc/wiki/TemplateHaskell/BlogPostChanges
|  #PartC:Reificationandtypechecking>.
|  
|  While I agree that getting the type of local variables is going to be
|  brittle, I also think that offering that functionality is extremely
|  useful in certain cases, such as in the case of the language-c-inline
|  library, that used this facility to avoid having to type verbose type
|  annotations -- see <https://github.com/mchakravarty/language-c-
|  inline/issues/25>.
|  
|  My two part question is:
|  
|    1. would it be possible to revert to the old behavior easily, or is
|  doing that difficult or impossible now given the overall changes to
|  the Template Haskell code?
|    2. if 1, wouldn't it make more sense to allow reifying all names if
|  the user really wants, maybe with a specific `unsafeReify` function or
|  similar?
|  
|  It should be made clear in the docs that by using `unsafeReify` the
|  user is relying on GHC internals (specifically the internals of type
|  checking).
|  
|  Also, I think that post should at least be referenced in the manual,
|  for lack of better documentation.  This behavior is not mentioned
|  anywhere else.
|  
|  Thanks,
|  Francesco