Greg,
I apologize for the confusing terminology. I am not differentiating between sending a static file with sendfile and a streaming response. I am differentiating between 2 different use cases for sending static files (with sendfile). For all of my web applications, I know what all the static files are and they will never change until I deploy another web application. That means I can stat the files once when the application is deployed and keep that information in memory. So I already have the file length information to include in the header, even though I don't do a file stat when the file is requested. wai-app-static and yesod-static supports these techniques.
Thanks. I think I understand. :) So, do you support to *not* change the API (apps should add CL: by themselves)? --Kazu