Adding an ignore function to Control.Monad

[Edward Kmett]

I've used this same function with parser combinators as well. I would prefer
a version that just relied on the fact that you have a Functor however, so
you can use it with Applicatives or any other Functor you happen to have
lying around.

ignore :: Functor f => f a -> f ()
ignore = fmap (const ())



On Wed, Jun 10, 2009 at 12:56 PM, Michael Snoyman <michael at snoyman.com>wrote:

> +1.
>
> I also remember this post by Neil Mitchell which seems appropriate:
> http://neilmitchell.blogspot.com/2008/12/mapm-mapm-and-monadic-statements.html.
> He also uses the name "ignore" for your function.
>
> Michael
>
>
> On Wed, Jun 10, 2009 at 7:53 PM, Gwern Branwen <gwern0 at gmail.com> wrote:
>
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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.
>>
>> - --
>> gwern
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