What's the recommended way to have module-private record types without setting off -Wunused-top-binds?

Question

This module

module Foo (Foo, qux) where

data Foo = Foo {bla::Int}

qux :: Foo
qux = Foo 37

causes a warning when compiled with -Wall:

/tmp/wtmpf-file12937.hs:3:17: warning: [-Wunused-top-binds]
    Defined but not used: ‘bla’
  |
3 | data Foo = Foo {bla::Int}
  |                 ^^^

Ok – if bla were just a standalone function, this would be easy and should be fixed by removing bla. But for a record, the fields do more than just provide a name that can be used, they also serve as documentation in the code.

What is the preferred way to get rid of the warning?

It should be a permanent solution, should preferrably keep the record as-is, and preferrably not disable any warnings for the rest of the module.

Answer 1

To avoid these, I usually add a definition like this to the module:

_unused :: a
_unused = error "don't complain" bla

The nice thing is you can chain them, like so:

_unused :: a
_unused = error "don't complain" bla bah foo bar

It's kind of crude, but gets the job done.