[Haskell-cafe] Re: Remember the future

[ChrisK]

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