[Haskell-cafe] Does this typeclass have a name?

[Andras Slemmer]

I think what you're looking for are monoid actions:
http://hackage.haskell.org/package/monoid-extras-0.2.2.0/docs/Data-Monoid-Action.html
And the laws the typeclass should satisfy:
foo bar = id
foo f . foo g = foo (f `mappend` g)

The identity law is optional, in whihc case your 'e' would be a semigroup
without a default 'bar'


On 21 April 2014 11:46, Tobias Brandt <tob.brandt at googlemail.com> wrote:

> Your class looks like Unfoldable from Data.Collections<http://hackage.haskell.org/package/collections-api-1.0.0.0/docs/Data-Collections.html>.
> foo is insert and bar is empty. The other three methods can be defined in
> terms of insert and empty.
>
>
> On 21 April 2014 07:09, Bardur Arantsson <spam at scientician.net> wrote:
>
>> Hi all,
>>
>> I was wondering if anyone out there knows if this type class has a name?
>>
>>   class Foo a e where
>>      foo :: e -> a -> a
>>      bar :: a
>>
>> (I also have a fundep a -> e, but that's not essential.)
>>
>> Essentially the usage is:
>>
>>    You have a sequence of "events" e_i and a starting value
>>    "bar" and can use the "foo" function to apply all events
>>    to that starting value
>>
>>       foo e_n $ foo e_{n-1} $ ... $ foo e_0 $ bar
>>
>>    and thus get the final value of the a induced by the
>>    events e_i.
>>
>> (I've included the "bar" starting value in the type class, but I suppose
>> it could be supplied by a Default instance.)
>>
>> Regards,
>>
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>
>
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