#13598: role annotation for newtype (partially?) ignored? -------------------------------------+------------------------------------- Reporter: edsko | Owner: (none) Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 8.2.1-rc2 Resolution: invalid | 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 simonpj):
even though I was not using those constructors anywhere in that code
The rationale (explained in the paper) is this. Without the built-in support for `Coercible` you could do everything by unpacking and repacking newtype constructors. Eg {{{ newtype Age = MkAge Pos newtype Pos = MkPos Int foo1, foo2 :: Age -> Int foo1 (MkAge (MkPos n)) = n foo2 n = coerce n }}} The `foo1` route requires `MkAge` and `MkPos` to be in scope; and you might hide them specifically to prevent you looking inside the abstraction (e.g. you might anticipate changing the representation of `Age` in the future). If so, then for the same reasons `foo2` should fail if `MkAge` or `MkPos` are not in scope. Does that help explain? Should we add something to the user manual? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13598#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler