Hi everyone, I've recently been playing with a little mtl-like approach to streaming data, whereby I have a single type class - MonadYield - which can yield data, and then I use various implementations of this type class to implement operations. My usual approach to this is to implement this by newtyping around appropriate "off-the-shelf" monad transformers from the transformers library, but I found that this incurs a significant performance penalty. I've tried to put some fairly extensive benchmarks in place, which you can find at https://github.com/ocharles/monad-yield. In that repository is a README.md file that describes how I have been performing these benchmarks. The benchmarks are defined over a common interface that each implementation of MonadYield exports. The benchmarks are defined in "Benchmarks.hs", and the three implementations are "Transformers.hs" (using transformers from GHC), "TransformersInline.hs" (using transformers-ocharles from that repository, which has many more INLINE pragmas) and "Inline.hs" (which doesn't depend on anything other than base). There are three main benchmarks that are ran - one is benchmarking essentially the cost of ReaderT, the next the cost of StateT, and the last a composition of ReaderT over StateT over ReaderT. The results of the benchmark can be found here: https://ocharles.github.io/monad-yield/. It seems that the current darcs release of transformers loses every time, but if I sprinkle {-# INLINE #-} across the definition of lazy state, I get identical performance to just writing out the lazy state monad by hand. I was very surprised to see that I have to pay when I use transformers, and it seems like this cost can be removed at the cost of slightly larger interface files. Before I submit a patch, I'd love to hear others thoughts. Should {-# INLINE #-} be necessary? Is there any reason not to add it to every symbol in transformers? -- ocharles