Warning for incomplete record field label used as function

[goldfire]

Consider the following definition:

data Foo = Bar { frob :: Int }
         | Baz { frob :: Int, quux :: Bool }

It would be great if GHC could produce a warning if I use quux as a function somewhere in my code, as calling quux on a Foo constructed with Bar will cause a runtime error. On the other hand, every use of frob is OK.

Currently, to find such problems, I have to first figure out which field labels are incomplete and then search for those, ignoring their uses as a proper field label (in construction or pattern matching).

[simonpj]

But there are lots of partial functions (eg head). Why warn for quux but not for head.

[goldfire]

Because I think it's easier to make quux partial by mistake during refactoring. The particular example that motivated this post was refactoring TyClDecl in GHC to make family declarations a separate type. Much later, I noticed that the refactor made tcdLName into a partial function and I had to perform the search mentioned above.

[igloo]

We do warn when you define

head (x : _) = x

though. It would be nice if we also warned about

data Foo = Bar { frob :: Int }
         | Baz { frob :: Int, quux :: Bool }

but unfortunately

data Foo = Bar { frob :: Int }
         | Baz { frob :: Int, _ :: Bool }

is currently a parse error. Perhaps it shouldn't be?

[goldfire]

I don't think that last question directly relates to this issue... We want to be able to have fields such as quux that are used only with some (but not all) constructors of a type, but I think a warning when such field names are used as functions would be helpful.

Perhaps I'm misunderstanding your post?