Reification of out-of-scope variables?
Richard Eisenberg
eir at cis.upenn.edu
Sat Apr 16 14:02:12 UTC 2016
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
>
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