
#12048: Allow CustomTypeErrors in type synonyms (+ evaluate nested type family?) -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: diatchki Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.1 Resolution: | Keywords: | CustomTypeErrors 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 andrewthad): Just to add information about my current workaround, the example given can be rewritten to use a type family instead: {{{#!hs {-# LANGUAGE DataKinds #-} {-# LANGUAGE TypeOperators #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE UndecidableInstances #-} import GHC.TypeLits import Data.Kind -- Works just like a type synonym type family Fail where Fail = TypeError (Text "It failed") type family Bad a where Bad a = TypeError (Text "It messed up on " :<>: Text (Whoami a)) type family Whoami (ty :: Type) :: Symbol where Whoami Int = "a number" Whoami Float = "a number" Whoami [_] = "a list of things" Whoami _ = "something else" expr :: Bad [Bool] expr = 5 main :: IO () main = putStrLn "Hello" }}} Everyone on this ticket is probably already aware of this, but I just wanted to document it for passersby like myself who may be less familiar with the issue. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12048#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler