Dimitry Golubovsky wrote:
John & all,
I use HSH in my project where several external programs and Haskell functions need to be piped together: HSH is of great help here.
I however came across the situation when one of pipe-connected functions has signature IO (), yet it reads from stdin* and writes to stdout.
The documentation mentions "instance ShellCommand (Handle -> Handle -> IO ())" which could be of some help, but in the latest version of HSH on Hackage this instance is commented out.
What was the reason of doing that? Is this to be expected in the upcoming versions?
Yes; that's due to the new more flexible way of sending data between processes in HSH -- the Channel. You can replace it with a function that can take any Channel, and produce a result as a Channel of one particular sort. In particular, this instance: instance ShellCommand (Channel -> IO Channel) where is the direct replacement for what you were doing. A Channel is generally a String, a lazy ByteString, or a Handle. There are helper functions in HSH.Channel to deal with these: chanAsString, chanAsBSL, and chanToHandle. You can think of the first two as similar to hGetContents. The last will write the channel out literally to a passed-along handle. So, let's say that you wanted to process input as a String, and before you were given a Handle that you used hGetContents on. Now, you will get a Channel, on which you will call chanAsString. It will convert whatever type of Channel you were handed into a String, lazily. Does that make sense?