Reification of out-of-scope variables?

[Richard Eisenberg]

I don't have a better idea. I've added a few small comments to the bottom of that wiki page, which still, for me, represents the best solution to this problem.

Richard

On Apr 18, 2016, at 5:01 PM, "Boespflug, Mathieu" <m at tweag.io> wrote:

> Hi all,
> 
> For our use case, namely automatic bindings generation via
> quasiquotation à la language-c-inline, typed quasiquotation alone
> wouldn't solve much. Because in order to be sensible it would likely
> have to be restricted (one way or another) to only allowing
> reification of variables in the previous declaration groups (since we
> AFAICT all agree that the current permissiveness of reification in
> typed splices is a bug). That won't fit the use case, since it's
> precisely the types of the locally bound variables we're interested in
> (see examples on the updated wiki page:
> https://ghc.haskell.org/trac/ghc/wiki/TemplateHaskell/Reify).
> 
> So that brings us back to the question of allowing addModFinalizer to
> capture the local type environment of the call site somehow. There's a
> proposal for this that Facundo came up with and wrote up at the end of
> the aforementioned wiki page, but does anyone else have a better way
> of doing this in mind?
> 
> Best,
> --
> Mathieu Boespflug
> Founder at http://tweag.io.
> 
> 
> On 16 April 2016 at 16:02, Richard Eisenberg <eir at cis.upenn.edu> wrote:
>> 
>> On Apr 15, 2016, at 11:51 AM, Simon Peyton Jones <simonpj at microsoft.com> wrote:
>>> 
>>> Hang on!  The design for typed splices, describe here,
>>> https://ghc.haskell.org/trac/ghc/wiki/TemplateHaskell/BlogPostChanges
>>> says "Unlike TH, the only way to construct a value of type TExp is with a quote. You cannot drop into do-notation, nor use explicit construction of the values in the Exp algebraic data type. That restriction limits expressiveness, but it enables the strong typing guarantees."
>>> 
>>> So why does the above work?  $$(e) requires a TExp, and do-notation doesn’t produce a TExp.
>> 
>> Indeed this is true -- this is what that page says. But it's not what's implemented: when I say $$( _ ) :: Bool, GHC tells me that the hole has type Q (TExp Bool).
>> 
>> There still is no way to create a TExp other than to use a type TH quote.
>> 
>> Addressing your other message: a typed quasiquoter would be somewhat limited, but not utterly silly. For example, this works:
>> 
>>> bool :: String -> Q (TExp Bool)
>>> bool "true"  = [|| True ||]
>>> bool "false" = [|| False ||]
>>> bool _ = fail "not a bool"
>>> 
>>> -- and then, in another module because of the stage restriction:
>>> yes :: Bool
>>> yes = $$(bool "true")
>> 
>> Now `bool` could be a typed quasiquoter.
>> 
>> I don't know whether any of this is worth implementing, but it's not, a priori, a terrible idea.
>> 
>> Richard
>> 
>>> 
>>> |  * Should we consider it a bug (and file a ticket) that reification in
>>> |  typed splices is able to observe the order of type checking, just like
>>> |  reify used to do in untyped splices?
>>> 
>>> Yes I think so!!!
>>> 
>>> Simon
>>> 
>> 
>