Proposal: specs for Maybe, Either, Tuple, Task, ...

Sorry if it was discussed before, after scanning the issues I didn’t find anything on the subject.

The motivation comes from the last addition: the spec for chainRec which I find hard to understand

chainRec :: ChainRec m => ((a -> c, b -> c, a) -> m c, a) -> m b

with respect to

class Monad m <= MonadRec m where
  tailRecM :: forall a b. (a -> m (Either a b)) -> a -> m b

Relevant issues

• https://github.com/fantasyland/fantasy-land/issues/151
• https://github.com/fantasyland/fantasy-land/pull/152

I understand the benefits of not depending on a particular implementation of Either but why fantasy-land doesn’t provide a spec for Either in the first place?

For example writing the spec for Unfoldable would be problematic (Maybe AND Tuple)

class Unfoldable t where
  unfoldr :: forall a b. (b -> Maybe (Tuple a b)) -> b -> t a

We have great libraries out there for Maybe, Either, Task, etc… but there’s no standard for them.

Would it be a good idea to add specs for these concrete types?

[puffnfresh]

@gcanti those data-types can be Church-encoded (pedantically, Boehm-Berarducci encoded, if we’re using types) meaning they can be represented as functions.

type Maybe a = forall b. b -> (a -> b) -> b
type Either a b = forall c. (a -> c) -> (b -> c) -> c

function nothing(b, f) {
  return b;
}

function just(a) {
  return function(b, f) {
    return f(a);
  };
}

function left(a) {
  return function(l, r) {
    return l(a);
  };
}

function right(b) {
  return function(l, r) {
    return r(b);
  };
}

// Example
function eitherToMaybe(e) {
  return e(
    function(ignored) { return nothing; },
    just
  );
}

[gabejohnson]

@robotlolita I’ve made some changes to #280 that would make interop very simple.

That being said, I think any specification is better than none.

we should just implement the data structures in Fantasy Land, we could even have a fantasy-land-runtime library that provides operations on those values

Agreed. In fact, why have more than one ADT library? Currently we have:

• https://github.com/fantasyland/daggy
• https://github.com/origamitower/folktale/tree/master/packages/base/source/adt
• https://github.com/paldepind/union-type
• https://github.com/sanctuary-js/sanctuary-union-type/tree/davidchambers/everything
• https://github.com/JAForbes/sum-type

And these are just the libraries in the Fantasy Land ecosystem. There seems to be a lot of duplicated effort.

@robotlolita a simple spec, as you mentioned above, could be implemented once using daggy (with contributions from all interested lib authors/maintainers).

I’ve noticed a lot of confusion amongst those new to FL concerning where they should get their data types. It creates an unnecessary barrier to entry for beginners and hurts the ecosystem as a whole.

I’m not saying every data type should come from this org, but the basic ones are so simple there’s no good reason to do so repeatedly.