Comments on: Riddle me this

By: Text.Printf and monad transformers « Communicating Haskell Processes

Text.Printf and monad transformers « Communicating Haskell Processes — Fri, 06 Nov 2009 13:35:48 +0000

[...] 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 [...]

By: Saizan

Saizan — Sat, 26 Sep 2009 16:46:45 +0000

well, the typeclass system is what’s doing most of the work

By: Bryan O'Sullivan

Bryan O'Sullivan — Sat, 26 Sep 2009 16:40:57 +0000

Saizan: Wow, that’s impressive. Type signatures longer than the code they describe? Eeek!

By: Saizan

Saizan — Sat, 26 Sep 2009 04:38:09 +0000

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.

By: brian

brian — Sat, 26 Sep 2009 04:09:44 +0000

Don’t forget package ‘xformat’.

By: sclv

sclv — Fri, 25 Sep 2009 19:51:21 +0000

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.