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Riddle me this

September 25th, 2009 by Bryan O'Sullivan

on 25 Sep 2009 at 15:511 sclv

this is a terrible hack, but no more of a hack than printf itself:

test :: (HPrintfType r, NoOp r) => Bool -> Handle -> String -> r
test b = if b then hPrintf
else nopr

class NoOp a where
nopr :: a

instance NoOp (IO a) where
nopr = return undefined

instance (NoOp r) => NoOp (a -> r) where
nopr = const $ nopr

don’t have an answer for the second bit at the moment tho.
on 26 Sep 2009 at 00:092 brian

Don’t forget package ‘xformat’.
on 26 Sep 2009 at 00:383 Saizan

I got a solution requiring more extensions but that it looks more general, here’s the paste:
http://hpaste.org/fastcgi/hpaste.fcgi/view?id=9857

The types, especially those inferred like in test, aren’t so pretty but not horrible either.
on 26 Sep 2009 at 12:404 Bryan O'Sullivan

Saizan: Wow, that’s impressive. Type signatures longer than the code they describe? Eeek!
on 26 Sep 2009 at 12:465 Saizan

well, the typeclass system is what’s doing most of the work
on 06 Nov 2009 at 08:356 Text.Printf and monad transformers « Communicating Haskell Processes

[...] little while ago, Bryan O’Sullivan was developing his Criterion benchmark suite, and had trouble with using the Text.Printf module in a monad transformer on top of IO. I thought I knew how to solve this, but my first idea didn’t work — and nor did my [...]