#11376: Inconsistent specified type variables among functions and datatypes/classes when using -XTypeApplications -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: goldfire Type: bug | Status: new Priority: normal | Milestone: 8.0.1 Component: Compiler (Type | Version: 8.1 checker) | Keywords: Resolution: | TypeApplications Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: Other | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by goldfire): What about for data constructors? Without your patch: {{{
:type Proxy Proxy :: forall {k} (t :: k). Proxy t }}}
This is now useful information that `k` is not specified but `t` is. With your patch (assumedly -- I don't have this locally): {{{
:type Proxy Proxy :: forall {k} {t :: k}. Proxy t }}}
I now don't know which is specified and which is generalized. So I could use `:info`: {{{
:info Proxy type role Proxy phantom data Proxy (t :: k) = Proxy -- Defined in ‘Data.Proxy’ instance forall k (s :: k). Bounded (Proxy s) -- Defined in ‘Data.Proxy’ instance forall k (s :: k). Enum (Proxy s) -- Defined in ‘Data.Proxy’ instance forall k (s :: k). Eq (Proxy s) -- Defined in ‘Data.Proxy’ instance Monad Proxy -- Defined in ‘Data.Proxy’ instance Functor Proxy -- Defined in ‘Data.Proxy’ instance forall k (s :: k). Ord (Proxy s) -- Defined in ‘Data.Proxy’ instance forall k (s :: k). Read (Proxy s) -- Defined in ‘Data.Proxy’ instance forall k (s :: k). Show (Proxy s) -- Defined in ‘Data.Proxy’ instance Applicative Proxy -- Defined in ‘Data.Proxy’ instance Foldable Proxy -- Defined in ‘Data.Foldable’ instance Traversable Proxy -- Defined in ‘Data.Traversable’ instance forall k (s :: k). Monoid (Proxy s) -- Defined in ‘Data.Proxy’ }}}
There's an awful lot of information I have no interest in, and the very piece of information I do want -- the specificity of the variables -- is missing. It's missing because GHC helpfully says `data Proxy (t :: k) = ...` even though `Data.Proxy` declares `data Proxy t = ...`. The former would make `k` specified. But the latter, actual definition, means that `k` is not specified. I can exhibit the same problems with a class method. Take `Control.Category`'s `id`: Without your patch: {{{
:type C.id C.id :: forall {k} (cat :: k -> k -> *). Category cat => forall (a :: k). cat a a }}} Very helpful info.
With your patch (assumedly): {{{
:type C.id C.id :: forall {k} {a :: k} {cat :: k -> k -> *}. Category cat => cat a a }}} Here I decided to swap the order of `cat` and `a`, because it's quite conceivable that this would happen.
And now `:info`: {{{
:info C.id class Category (cat :: k -> k -> *) where Control.Category.id :: forall (a :: k). cat a a ... }}} Once again, I can't tell that `k` is unspecified. And even if I could somehow, I'd still have to do some work to reconstruct the type from this information.
I'm sure record selectors would have the same problem. Bottom line: I don't think Simon's proposal to use `:info` for bare names quite does it. I'm sympathetic to disliking (1) and I, too, desire to keep things simple. But I don't think we've quite figured this out yet. I actually think there's a really easy solution: have `:type` print ''both'' the uninstantiated type and the instantiated one (if they're different, by more than the ordering of invisible things). `tcRnExpr` could return two `Type`s -- it has both types we want to hand. We could even only do this new behavior when `-fprint-explicit-foralls` is enabled (because otherwise, the user isn't seeing specificity anyway). -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11376#comment:31 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler