Re: Reification of out-of-scope variables?

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" 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 wrote:

...

On Apr 15, 2016, at 11:51 AM, Simon Peyton Jones 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