| > 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 Humph. I suppose that provided you can't forge a TExp, there's no way to splice in a type-incorrect program; and having Q available lets you (say) do input/output or consult the context to decide which of two quotes to return. We could do this selectively. For example we'd certainly need (TExp t) to be able to fail
bool :: String -> TExp Bool bool "true" = [|| True ||] bool "false" = [|| False ||] bool _ = failTexp "not a bool"
My instinct is to make it less expressive, though, and only allow (TExp t) as the argument of $$. Does anyone care either way? I suppose we'd better open a ticket for this. Simon | | quote. | | Addressing your other message: a typed quasiquoter would be somewhat | limited, but not utterly silly. For example, this works: | | > bool :: String -> Q (TExp Bool) | > bool "true" = [|| True ||] | > bool "false" = [|| False ||] | > bool _ = fail "not a bool" | > | > -- and then, in another module because of the stage restriction: | > yes :: Bool | > yes = $$(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 | >