#14307: NamedFieldPuns should allow "ambiguous" field names -------------------------------------+------------------------------------- Reporter: mgsloan | Owner: (none) Type: feature request | Status: new Priority: low | Milestone: Component: Compiler | Version: 8.2.1 Resolution: | 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):
This seems like poor behavior, because since a particular constructor is used, it is unambiguous which field is intended.
Yes, but you need `-XDisambiguateRecordFields` for that ([http://downloads.haskell.org/~ghc/master/users-guide/glasgow_exts.html #record-field-disambiguation user manual entry]). Haskell 2010 specifies that the code is should be rejected. So I think GHC is behaving right here. Incidentally, `-XDisambiguateRecordFields` is also implied by `-XDuplicateRecordFields`.
I actually encountered this issue in a bit of a different usecase.
Yes, here's the code {{{ {-# LANGUAGE DuplicateRecordFields #-} {-# LANGUAGE NamedFieldPuns #-} module T14307 where data A = A { field :: Int } data B = B { field :: Int } f (C { field }) = field }}} You get two errors with 8.2 {{{ T14307.hs:10:4: error: Not in scope: data constructor ‘C’ | 10 | f (C { field }) = field | ^ T14307.hs:10:8: error: Ambiguous occurrence ‘field’ It could refer to either the field ‘field’, defined at T14307.hs:7:14 or the field ‘field’, defined at T14307.hs:6:14 | 10 | f (C { field }) = field | ^^^^^ }}} I think your point is that you'd like the second to be suppressed. I see the point. Are you also ok with getting just one error message from {{{ f (A { fld = x }) = ... }}} namely `'A' is not in scope`; but no `fld is not in scope`? That is: if the data constructor is not in scope, suppress out-of-scope or ambiguity messages for the fields. I think that'd be fine. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14307#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler