Reification of out-of-scope variables?

[Richard Eisenberg]

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
>