#11990: Custom Type Error not getting triggered in the nested Type function call -------------------------------------+------------------------------------- Reporter: magesh.b | Owner: Type: bug | Status: merge Priority: normal | Milestone: 8.0.2 Component: Compiler | Version: 8.0.1-rc3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by magesh.b):
If there are multiple `TypeErrors`, then we just report one of them.
Consider the following class, whose instance has a contrived TypeError {{{ class ShowHL (xs :: [*]) where showHL :: HList xs -> String instance (Show x, ShowHL xs, TypeError (ShowType x)) => ShowHL (x ': xs) where showHL (x :& xs) = show x ++ showHL xs }}} The error message emitted when this type class method is called with **showHL ('a' :& True :& Nil)** contains all the errors in one shot. This behavior is very useful and would this be retained after this fix? {{{ MutiCE.hs:26:8: error: • Bool • In the expression: showHL testRec In an equation for ‘test’: test = showHL testRec MutiCE.hs:26:8: error: • Char • In the expression: showHL testRec In an equation for ‘test’: test = showHL testRec Failed, modules loaded: none. }}} I have attached the full source code which contains the test case for this behaviour -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11990#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler