#12048: Allow CustomTypeErrors in type synonyms (+ evaluate nested type family?) -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: Type: feature | Status: new request | Priority: normal | Milestone: Component: Compiler | Version: 8.1 Keywords: | Operating System: Unknown/Multiple CustomTypeErrors | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- I didn't find a ticket with this, but with wiki:CustomTypeErrors you can't define a type synonym without it erroring: {{{#!hs -- error: … -- • onetwothree -- • In the type synonym declaration for ‘ERROR’ -- Compilation failed. type ERROR = TypeError (Text "onetwothree") }}} but I often want to abstract over error messages since they are clunky to write at the type level and I often use a similar message in many different type families, especially when they can be determined by a type family. Here's a hypothetical example: {{{#!hs type family Whoami (ty :: Type) :: Symbol where Whoami Int = "a number" Whoami Float = "a number" Whoami [_] = "a list of things" Whoami _ = "something else" }}} I would like to write {{{#!hs type Error ty = TypeError (Text "Expected a <...> but got ":<>: Text (Whoami ty)) }}} ---- Even when ‘inlined’, it displays `Expected a GRUE but got 'Text (Whoami Int)` and not `• Expected a GRUE but got a number.`: {{{#!hs a :: TypeError (Text "Expected a GRUE but got ":<>: Text (Whoami Int)) a = 'a' }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12048 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler