Why does Foldable inherit from Functor in Frege?

Question

In Haskell, the class Foldable is standalone i.e.

class Foldable t where ...

However, in Frege:

class Functor t => Foldable t where ...

Why was this constraint introduced? What ill-formed programs/ideas can it rule out?

It prevented me from defining an instance Foldable HashSet, where HashSet is a custom data type which wraps Data.HashMap to provide a set-based interface. (It can be worked around by applying toList first, so it's not a serious issue.)

score 3 · Answer 1 · answered Dec 04 '17 at 11:16

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I think this discussion give you the whole feedback on the topic (especially part about Sets) Haskell : An example of a Foldable which is not a Functor (or not Traversable)?

answered Dec 04 '17 at 11:16

Pawel Szulc

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In terms why Frege chooses to limit its `Foldable`s to `Functor`s - I don't know – Pawel Szulc Dec 04 '17 at 11:17

score 2 · Accepted Answer · answered Dec 25 '17 at 23:27

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I can only guess that it used to be the case that Haskell's Foldable did have Functor as a superclass once and that Frege inherited the Foldable code at this time. Could've been 2012 or so.

I'm not sure I understand your concern as Data.HashMap is indeed a Functor. But anyway, I invite you to design a patch and make a pull request.

answered Dec 25 '17 at 23:27

Ingo

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Thank you for your answer. I opened a [PR](https://github.com/Frege/frege/pull/334). – Ohashi Jan 04 '18 at 04:28

Why does Foldable inherit from Functor in Frege?

2 Answers2