Reification of out-of-scope variables?
eir at cis.upenn.edu
Tue Apr 19 14:49:58 UTC 2016
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.
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:
> 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?
> 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,
>>> 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.
>>> | * 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!!!
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