#9433: Partially applied type family allowed but unusable -------------------------------------+------------------------------------- Reporter: hesselink | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 7.9 checker) | Operating System: Keywords: | Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: GHC Difficulty: Unknown | accepts invalid program Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- I was playing around with the following code, which tries to map a type family over a type (something I thought would not be allowed): {{{#!hs {-# LANGUAGE TypeFamilies , KindSignatures #-} type family Id x :: * type instance Id a = a type family Map (f :: * -> *) x :: * type instance Map f [a] = [f a] x :: Map Id [Bool] x = [] }}} Both GHC 7.8.3 and the current HEAD (7.9.20140811) accept this program! However, changing the definition of `x` to `[True]` gives an error: {{{ Couldn't match type ‘Id Bool’ with ‘Bool’ Expected type: Map Id [Bool] Actual type: [Bool] In the expression: [True] In an equation for ‘x’: x = [True] }}} If I define `y :: Id Bool` with value `True` (which works fine) and define `x = [y]`, I even get this error: {{{ Couldn't match type ‘Bool’ with ‘Id Bool’ Expected type: Id Bool Actual type: Id Bool In the expression: y In the expression: [y] }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9433 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler