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

[Lana Black]

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?

With best regards.