add substFreeT

[safareli]

Adds FreeT version of substFree :: forall f g. (f ~> Free g) -> Free f ~> Free g

[safareli]

NOTE: Monad m is needed because of pure val

[garyb]

Not Applicative?

[safareli]

Monad m => Applicative (FreeT f m) because Apply is implemented using ap

[safareli]

ping

[safareli]

Is there any particular blocker for getting this in? if yes can we at least expose constructors from some Internal module?

[thomashoneyman]

@safareli I'm not familiar with the problem this addition is solving, nor of how to judge whether it's correct. Not because I don't think this is sensible, but simply because I'm not a heavy FreeT user :) Do you mind sharing a little more detail about what this is meant to accomplish?

Also, I'd appreciate hearing about why / whether a test case makes sense for this addition (where could it possibly go wrong? maybe a FreeT to Aff or Effect with something which would blow the stack?)

[safareli]

In haskell there is type class MMonad which represents:

A monad in the category of monads, using lift from MonadTrans as the analog of return and embed as the analog of (=<<):

embed :: MMonad t => Monad n => (forall a. m a -> t n a) -> t m b -> t n b is main function of interest for us.

Let's look at it's signature, aligned with signature of bind, substFree and substFreeT:

bind       :: forall m a      (          a   -> m         b) -> m         a -> m         b
embed      :: forall m n b t. (forall a. m a -> t     n   a) -> t     m   b -> t     n   b
substFree  :: forall m n b    (forall a. m a -> Free  n   a) -> Free  m   b -> Free  n   b
substFreeT :: forall m n b g. (forall a. m a -> FreeT n g a) -> FreeT m g b -> FreeT n g b```

substFreeT has similar usage as substFree and bind have, they allow you to "graft" trees at leaves.

Here is example of it's usage:
https://github.com/blinky3713/purescript-coroutine-transducers/pull/1/files#diff-950dba9d55bad1315168b59150f435f4R114

Note: interpret is like map where substFreeT is like bind.

[garyb]

There's a corresponding substFree for the non-T Free too.

[safareli]

yes

Adds FreeT version of substFree :: forall f g. (f ~> Free g) -> Free f ~> Free g

[thomashoneyman]

Ok! Sounds good, thank you @safareli.

[thomashoneyman]

Released as v4.1.0

[safareli]

Thanks!