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.

Concern

I usually prefix them with underscore _blah – Seller

Possible duplicate of How to [temporarily] suppress "defined but not used" warnings? – Archaean

I like the underscore solution, since it's consistent with my use of underscore variables for unused function arguments that I want to document. – Hybridism

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.

Prieto

The infinite-curry-unfolding trick is really quite neat. I'm not sure like the :: a signature, but that can easily be changed. – Concern

Yeah, you can give it whatever type you want obviously. – Prieto

I'd recommend this solution only if you can't or don't want to put underscores on the record names themselves. For example I've used a similar trick with yesod/persistent data types generated with template haskell – Seller