One major problem is that a then falls into the module namespace and is not reusable. Having a record namespace as in GHC is preferable i.e., Foo.a.

There is nothing wrong with the definition syntax. The only problem I have with those are the "namespace-pollution" problem where you must have them in a separate module to isolate field names (which is kind of a small issue to me).

What I have a bigger problem with is the syntax for updating fields is so clumsy. It doesn't go very well with the rest of the language and I usually end up writing helper functions for updating specific combination of fields.