[Haskell-cafe] Haskell spec vs FlexibleInstances

Dan Doel dan.doel at gmail.com
Fri Feb 28 16:14:02 UTC 2014

On Fri, Feb 28, 2014 at 9:44 AM, Casey McCann <cam at uptoisomorphism.net>wrote:

> The restriction to distinct type variables, and the resulting issues
> with unification, apply equally to instances for applied type
> constructors. This disallows e.g. writing a Monoid instance for (a ->
> a) directly without a newtype wrapper, not that we'd want to do that
> anyway.

This was once the instance defined in Data.Monoid. It was changed to the
current one in GHC 6.6.

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