Unfold type represents an effectful action that generates a stream of values from a single starting value often called a seed value. Values can be generated and pulled from the Unfold one at a time. It can also be called a producer or a source of stream. It is a data representation of the standard unfoldr function. An Unfold can be converted into a stream type using unfold by supplying the seed.

Performance Notes

Unfold representation is more efficient than using streams when combining streams. Unfold type allows multiple unfold actions to be composed into a single unfold function in an efficient manner by enabling the compiler to perform stream fusion optimization. Unfold m a b can be considered roughly equivalent to an action a -> t m b (where t is a stream type). Instead of using an Unfold one could just use a function of the shape a -> t m b. However, working with stream types like SerialT does not allow the compiler to perform stream fusion optimization when merging, appending or concatenating multiple streams. Even though stream based combinator have excellent performance, they are much less efficient when compared to combinators using Unfold. For example, the concatMap combinator which uses a -> t m b (where t is a stream type) to generate streams is much less efficient compared to concatUnfold.

On the other hand, transformation operations on stream types are as efficient as transformations on Unfold.

We should note that in some cases working with stream types may be more convenient compared to working with the Unfold type. However, if extra performance boost is important then Unfold based composition should be preferred compared to stream based composition when merging or concatenating streams.

Programmer Notes

import qualified Streamly.Data.Unfold as UF

More, not yet exposed, unfold combinators can be found in Streamly.Internal.Data.Unfold.