Proposal: Add Semigroup and Monoid instances for (:~:)

[David Feuer]

(:~:) is from Data.Type.Equality.

data a :~: b where
  Refl :: a :~: a
On Jan 14, 2016 8:56 AM, "Andreas Abel" <andreas.abel at ifi.lmu.de> wrote:

> What does this smiley :~: stand for?
>
> On 13.01.2016 06:58, David Feuer wrote:
>
>> These are not likely to be *useful*, but they fall easily into the
>> general bucket of "exactly one sensible instance exists".
>>
>> instance Semigroup (a :~: b) where
>>    r <> _ = r
>>
>> instance a ~ b => Monoid (a :~: b) where
>>    mempty = Refl
>>    mappend r _ = r
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>
> --
> Andreas Abel  <><      Du bist der geliebte Mensch.
>
> Department of Computer Science and Engineering
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>
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>
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