Some "GHC-internal" types leak to users via TH, and their constructors
occasionally pick up new fields, causing breakage downstream. The extra
field often has a sensible default (Nothing, [], ...) and it should be
best practice to rename the constructor when adding the new field, while
replacing the original constructor with a pattern synonym with the "old"
signature.
data Foo = ...
| NewImprovedMkFoo X Y Z -- was MkFoo Y Z
pattern MkFoo :: Foo
pattern MkFoo Y Z = NewImprovedMkFoo Nothing Y Z
When pattern synonyms are used to maintain a backwards-compatible API,
there should of course be no special signalling to differentiate them
from "real" constructors.
The boundary between "GHC-internal" and external may not always be
obvious, some care is required to reduce leaking breakage via TH.
--
Viktor.
_______________________________________________
ghc-devs mailing list
ghc-devs@haskell.org
http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
