:set -XTypeInType class C a where f :: a b :t f f ∷ ∀ {k} {b ∷ k} {a ∷ k → ★}. C k a ⇒ a b class C a where f :: b a :t f f ∷ ∀ {k} {a ∷ k} {b ∷ k → ★}. C k a ⇒ b a }}} However at 3 variables every combination fails (I've used +v here to
#12549: Panic on ":t datatypeName" ---------------------------------+-------------------------------------- Reporter: johnleo | Owner: johnleo Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.1 Resolution: | Keywords: Operating System: MacOS X | Architecture: x86_64 (amd64) Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | ---------------------------------+-------------------------------------- Comment (by johnleo): Info after further investigation. The following both work fine. {{{ prevent deep instantiation, which is where the failure occurs). {{{
class C a where f :: a b c :t +v f f ∷ ∀ {k1} {k2} (a ∷ k1 → k2 → ★). C k1 k2 a ⇒ ∀ (b ∷ k1) (c ∷ k2). a b c class C a where f :: b a c :t +v f f ∷ ∀ {k} (a ∷ k). C k a ⇒ ∀ {k1} (b ∷ k → k1 → ★) (c ∷ k1). b a c class C a where f :: b c a :t +v f f ∷ ∀ {k} (a ∷ k). C k a ⇒ ∀ {k1} (b ∷ k1 → k → ★) (c ∷ k1). b c a }}} All three of these panic without the +v. Note that with no TypeInType everything is fine {{{ class C a where f :: a b c :t f f ∷ ∀ {b} {a ∷ ★ → ★ → ★} {c}. C a ⇒ a b c }}}
The problem seems to be that in the TypeInType case with 3 variables, the quantification of some variables is moving to the right of the class constraint. Variables are instantiated in independent groups, with separate groups before and after the constraint, and here the types after the constraint are referring to kinds defined before the constraint, causing the panic. I will investigate further -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12549#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler