#15674: GADT's displayed type is misleading -------------------------------------+------------------------------------- Reporter: AntC | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.6.1-beta1 Resolution: | Keywords: Operating System: Windows | Architecture: x86_64 Type of failure: Poor/confusing | (amd64) error message | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by AntC): Replying to [comment:3 goldfire]:
... you can say `MkDG2 @Int` while you can't do that with `MkDG`. Their types are subtly different.
Aagh. My brain just blew a fuse.
(And setting `-fprint-explicit-foralls` doesn't explicitly show the `forall` in `MkDG`, although it does in `MkDG2`.
There is no `forall` in `MkDG`, which doesn't quantify over any variables.
Then I totally ignore the `a` in the `data DG a where ...`? (I don't mean its specific variable name, I mean the fact that `MkDG`'s return type is more specific that the `data`'s head, which is what makes it a GADT.)
But `MkDG2`'s decl doesn't have an explicit `forall`, neither do I need `-XExplicitForAll` to compile it -- that's not implied by `-XGADTs`, surprisingly.)
Your code does not contain an explicit `forall` in `MkDG2`. It contains an implicit one, implied by the presence of the type variable `a`.
For `MkDG2`, with `-fprint-explicit-foralls` `:i` shows `MkDG2 :: forall a. (a ~ Int) => a -> DG a` `:t` shows `MkDG2 :: Int -> DG Int` With `-fno-print-explicit-foralls` `:i` shows `MkDG2 :: (a ~ Int) => a -> DG a` `:t` shows `MkDG2 :: Int -> DG Int` Is that intended behaviour? It doesn't seem either internally consistent, nor consistent with your explanation: why is the `forall` appearing at all? Why is it appearing for `:i` but not `:t`? For `:t`, why is the `(a ~ Int)` not showing, even with verbosity to the max? `MkDG2`'s decl does have that explicitly, and it does make a (subtle) difference to the type.
I'm not sure what `forall`s you're looking for. There isn't one in
`MkDG`. What I'm looking for with these mind-blowingly similar-but- "subtly different" nuances is strong help from the compiler to navigate error messages when I get my types wrong. (Or rather, when I get wrong the types I'm importing from some library whose internals I don't really want to tangle with.) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15674#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler