#15850: coercion errors don't always mention that data constructors are not in scope -------------------------------------+------------------------------------- Reporter: gelisam | Owner: (none) Type: feature | Status: new request | Priority: normal | Milestone: Component: Compiler | Version: 8.6.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Poor/confusing Unknown/Multiple | error message Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Suppose a library defines a `newtype` with a `nominal` type parameter: {{{#!hs {-# LANGUAGE RoleAnnotations #-} module Lib where type role LibIdentity nominal newtype LibIdentity a = LibIdentity a }}} (in my code, that `newtype` is `ExceptT`, whose last argument is inferred to be `nominal`) In a different module, I also define a `newtype`, and then I try to use `coerce` to remove some of the `newtype` wrappers. Unfortunately I forgot to import `LibIdentity`'s data constructor, and so the coercions fail. Sometimes, the error message helpfully explains that I need that import... but sometimes it doesn't! {{{#!hs {-# LANGUAGE GeneralizedNewtypeDeriving #-} module Local where import Data.Coerce (coerce) import Lib (LibIdentity) newtype LocalIdentity a = LocalIdentity a -- error: -- • Couldn't match representation of type ‘()’ -- with that of ‘LibIdentity ()’ -- arising from a use of ‘coerce’ -- The data constructor ‘Lib.LibIdentity’ -- of newtype ‘LibIdentity’ is not in scope helpfulError :: LocalIdentity (LibIdentity ()) -> LocalIdentity () helpfulError = coerce -- error: -- • Couldn't match type ‘LocalIdentity ()’ with ‘()’ -- arising from a use of ‘coerce’ difficultError :: LibIdentity (LocalIdentity ()) -> LibIdentity () difficultError = coerce }}} Expected: both uses of `coerce` remind me to import the data constructor of `LibIdentity`. Observed: only `helpfulError` reminds me, while `difficultError` doesn't. (in my code, the `coerce` is generated by `GeneralizedNewtypeDeriving` and the error message does not include the type whose data constructor needs to be imported, making it quite difficult to guess how to fix the error) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15850 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler