+1.
I also remember this post by Neil Mitchell which seems appropriate:
http://neilmitchell.blogspot.com/2008/12/mapm-mapm-and-monadic-statements.ht....
He also uses the name "ignore" for your function.
Michael
On Wed, Jun 10, 2009 at 7:53 PM, Gwern Branwen
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So while writing my wp-archivebot, I ran into the issue that forkIO requires IO () but returns IO ThreadId, and that many useful IO functions will return IO a instead of IO ().
This forces some awkward contortions. Suppose I want to ping the WebCite website at a particular address, and this request makes WebCite archive a URL embedded in that address. Presumably I could venture into the depths of Network.HTTP to figure out how to ping an URL without also pulling down the server's HTML, but why do that when I already have obviously 'openURL :: String -> IO String'? Much easier to do something like 'openURL "webcite.org" ++ foo ++ "other stuff" '.
But my bot needs to handle quite a few URLs; one at a time, what with all the waits and timeouts, isn't going to hack it. So for a given link, I forkIO the openURL request. But of course, forkIO demands IO (), so I toss in a '>> return ()'. Fair enough.
So I examine the performance, and it's still too slow. Recent Changes has hundreds of different pages a minute. I'd better fork each page (and then fork for each link). But wait, all those forkIOs are returning IO ThreadIds, and my top-level forkIO call demands IO ()... So another >> return (). At this point, the code is starting to look pretty silly - something like '...stuff >> return ()) >> return ())'.
So I see the repeated pattern, and by the rule of 3, factor it out to:
- -- | Convenience function. 'forkIO' and 'forM_' demand return types of 'IO ()', but most interesting - -- IO functions don't return void. So one adds a call to 'return ()'; this just factors it out. ignore ∷ (Monad m) ⇒ m a → m () ignore x = x >> return ()
Not the most complex convenience function I've ever written, but not any simpler than, say Control.Monad.forever or for that matter, most of the stuff in Control.Monad.
I'd think it'd be useful for more than just me. Agda is lousy with calls to '>> return ()'; and then there's ZMachine, arrayref, whim, the barracuda packages, binary, bnfc, buddha, bytestring, c2hs, cabal, chesslibrary, comas, conjure, curl, darcs, darcs-benchmark, dbus-haskell, ddc, dephd, derive, dhs, drift, easyvision, ehc, filestore, folkung, geni, geordi, gtk2hs, gnuplot, ginsu, halfs, happstack, haskeline, hback, hbeat... You get the picture.
I realize the specific name of 'ignore' can be bikeshedded to death, but it's clear, it's short, Hoogle turns up one other function with ignore in its name (Distribution.ParseUtils ignoreUnrec), and it's been independently named 'ignore' by another Haskeller (lilac).
Existing uses of the string 'ignore are rare - it's in a few places as a variable, cabal and cabal-install and ehc and tar have where definitions of an ignore, a test for directory defines an ignore and imports Control.Monad unqualified, fit defines an 'ignore' but doesn't seem to use it in any module that also imports Control.Monad unqualified, halipeto defines an ignore but doesn't import Control.Monad, shim/yi has a let definition of an ignore, yhc has a where definition of an ignore. And that's about it. One of directory's tests would break, and the rest might have an additional -Wall warning.
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