#13833: Instances with GHC.TypeLits.Nat/Symbol should be possible without FlexibleInstances. -------------------------------------+------------------------------------- Reporter: Hjulle | Owner: (none) Type: bug | Status: new Priority: low | Milestone: Component: Compiler | Version: 8.0.2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Yes I know, `0` and `"B"` are not technically type constructors, but it's as close as you can get for the kinds Nat and Symbol. Test cases: {{{#!hs {-# LANGUAGE DataKinds, KindSignatures #-} import GHC.TypeLits (Nat, Symbol) class A (n::Nat) instance A 0 class B (s::Symbol) instance B "B" }}} Result: {{{ A.hs:6:10: error: • Illegal instance declaration for ‘A 0’ (All instance types must be of the form (T a1 ... an) where a1 ... an are *distinct type variables*, and each type variable appears at most once in the instance head. Use FlexibleInstances if you want to disable this.) • In the instance declaration for ‘A 0’ A.hs:9:10: error: • Illegal instance declaration for ‘B "B"’ (All instance types must be of the form (T a1 ... an) where a1 ... an are *distinct type variables*, and each type variable appears at most once in the instance head. Use FlexibleInstances if you want to disable this.) • In the instance declaration for ‘B "B"’ }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13833 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler