Re: [Haskell-cafe] Check a lack of a constraint?

On Mon, 12 Jul 2021, Lana Black wrote:

Hello cafe,

Is it possible in Haskell to check a lack of a certain constraint?

For example,

``` foo :: C => a foo = undefined

```

Here `foo` can only be compiled if called with C satisfied. How do I write the opposite, so that `foo` is only possible to use when C is not satisfied?

Do you mean "C a"? Is it sensible to want this? Any instance that you import transitively will reduce the applicability of 'foo'.