Benjamin Franksen wrote:
Simon Peyton-Jones wrote:
| It is unfortunate that the [ghc] manual does not give the translation rules, or at | least the translation for the given example.
Hmm. OK. I've improved the manual with a URL to the main paper http://citeseer.ist.psu.edu/erk02recursive.html which is highly readable. And I've given the translation for the example as you suggest
Cool, thanks.
BTW, the Haskell' wiki says its adoption status is 'probably no' which I find unfortunate. IMHO recursive do is a /very/ useful and practical feature and the cons listed on http://hackage.haskell.org/trac/haskell-prime/wiki/RecursiveDo don't weigh enough against that. Ok, just my (relatively uninformed) 2 cents.
Cheers Ben
I will assume that the current compilers will keep the current "mdo" desugaring. It is incredibly valuable, and I use it in two different monad stacks in the regex-tdfa package I released. It has been an implemented extension for quite several version of GHC, and with the separate "mdo" keyword it does not interfere with other code. Why have a lazy language with added monad "do" sugaring support and balk at adding such a well tested and deployed way to use sugar for combining laziness and monads? Toy there g is an identity monadic version of f and h shows the kind of logic I tend to intersperse in an mdo block:
module Main where
import Control.Monad.Fix import Control.Monad.Identity import Control.Monad.Writer
f x = do let a = x*b b = x+1 return a
test_f = runIdentity (f 2) -- 6
g x = mdo a <- return (x*b) b <- return (x+1) return a
test_g = runIdentity (g 2) -- 6
h x = mdo a <- return (x*b) if even b then tell [('a',a)] else return () b <- return (x+1) tell [('b',b)] return a
test_h1 = (runWriter (h 1)) -- (2,[('a',2),('b',2)]) test_h2 = (runWriter (h 2)) -- (6,[('b',3)])
-- Chris