(It's to do with pattern matching/deconstructing, rather than applying
#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 goldfire): Replying to [comment:2 AntC]: them as functions/constructing.) Precisely.
So ''prima facie'' `MkDF, MkDG`'s types are the same; `MkDG2`'s is different.
This is true.
But actually `MkDG` is the same as `MkDG2`.
In most contexts, yes. But you can say `MkDG2 @Int` while you can't do that with `MkDG`. Their types are subtly different.
(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.
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.)
Does that help?
Perhaps `:t` should behave precisely like `:i` if it is given a single identifier as its argument. That would be non-uniform and ad-hoc, but
Your code does not contain an explicit `forall` in `MkDG2`. It contains an implicit one, implied by the presence of the type variable `a`. perhaps useful in practice. But that would be non-uniform and ad-hoc. :) `:type <expr>` gives you the type that is assigned to `it` if you had `let it = <expr>`. It's a uniform rule that always works.
Usually if I'm having trouble from error messages to do with
"untouchable" or "rigid", it's `:t` I go to. I just have to kick myself to go to `:i`. But even the type signature showing in `:i` is misleading. Perhaps: if `<name>` is a data constructor, and not a H98 constructor, and `-fprint-explicit-foralls` is set, show the foralls explicitly? I'm not sure what `forall`s you're looking for. There isn't one in `MkDG`. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15674#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler