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

[Sylvain Henry]

Hi,

I've proposed something like this in the past (which was rightfully 
rejected). You may be interested in the discussion here:

https://github.com/ghc-proposals/ghc-proposals/pull/22

Sylvain


On 12/07/2021 20:24, 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?
>
> With best regards.
>
>
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