fold on Monad?

[Jon Fairbairn]

Suppose I have a task I want to do to each line of a file,
accumulate a result and output it, I can write

   main =3D do stuff <- getContents
	     print $ foldl process_line initial_value (lines stuff)

ie, it's obviously a fold

I can't see a way of doing the same thing directly on the
IO: I'd like to write something similar to

   main =3D do res <- foldX process_line initial_value getLine
	     print res

foldM almost does it:

   main =3D do res <- foldM process initial_value (repeat getLine)
	     print res

   process a g
    =3D do line <- g
	 return (process_line a line)

but that goes on forever (or some fixed amount if (replicate n/repeat))

I feel this ought to be straightforward -- the structure is
obviously some sort of fold, but I shouldn't have to use a
list -- so I must be missing something obvious. What is it?

  J=F3n

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J=F3n Fairbairn                                 Jon.Fairbairn@cl.cam.ac.u=
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