Map natural transformation for FreeT about free HOT 4 CLOSED

I'm not actively against adding this. =)

Let me check the extensions used, and if it doesn't extend the set then it'll definitely go in.

from free.

Like I said, it's not a blocking issue. If worse comes to worse I'll
just include it as a utility function in pipes.

On 09/07/2012 02:31 PM, Edward A. Kmett wrote:

I'm not actively against adding this. =)

Let me check the extensions used, and if it doesn't extend the set
then it'll definitely go in.

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#5 (comment).

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Oh, I just realized that if the Rank2Types extension is a problem you can copy what Ross does for transformers. Instead of providing a natural transformation he specializes the first argument to the concrete function that the compiler infers:

mapFreeT
  :: (Monad m, Functor f)
  => (m (FreeF f a (FreeT g n b)) -> n (FreeF g b (FreeT g n b)))
  -> FreeT f m a -> FreeT g n b

This still works correctly for monad morphisms, at the expense of an uglier type signature that hides the purpose of the function. However, you could add a documentation note saying something like:

"The typical use is to pass mapFreeT a monad morphism of type forall r . m r -> n r as its first argument so that you get a new monad morphism that modifies the base monad: forall r . FreeT f m r -> FreeT f n r"

from free.

I just decided to add the Rank2Types requirement. This will let me move the Church encoded free monad from kan-extensions to here, which reduces the amount of crap people need to install in order to use it considerably.

from free.