A local type synonym should arguably be expanded, and only be in scope in its where clause, or in general in the lexical scope of the declaration. Probably the synonym should be expanded when exiting the synonym’s scope, so that error messages for type errors in the block use

#4020: Please consider adding support for local type synonyms -------------------------------------+------------------------------------- Reporter: nr | Owner: (none) Type: feature request | Status: new Priority: low | Milestone: Component: Compiler (Type | Version: 6.12.2 checker) | Resolution: | Keywords: type synonym 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 AntC): Replying to [comment:17 pggiarrusso]: the synonym and not its expansion. GHC constantly strives to provide error messages that pinpoint a problem as closely as possible. Consider {{{ x :: String x = "hello" y :: [Char] y = "world" xy = x + y -- error: no instance for (Num String) yx = y + x -- error: no instance for (Num [Char]) }}} So the error message uses the declared type/synonym where it can. (I imagine that's rather fragile.) It doesn't look for a synonym that would be equivalent to some type. Why not: declare the synonym at top level, as usual; omit it from the module's export list so its scope is only the module; if you want a let- bound var declared with the synonym you can, and error messages will use that. Yes, lambda-lift for lexically scoped type vars. I'm not seeing that's a big burden. But if it is, perhaps it'll spur someone to put in a properly explained proposal on github. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/4020#comment:19 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler