[Haskell-cafe] OPPS, missing attachment (was Re: howto mix <- within do?

[Daniel Fischer]

Am Freitag, 17. Oktober 2008 14:42 schrieb Larry Evans:
> On 10/17/08 07:39, Larry Evans wrote:
> > The attached code produces error:
> > <-- cut here --
>
> [snip]
> {- 
> Purpose:
>   Explore how to mix 'assignments' inside do.
> Motivation:
>   Instead of:
>     let 
>       { v0 = e0
>       ; v1 = e1 
>       }
>     in do
>       { print v0
>       ; print v1
>       }
>   which is hard to read, recode above as:
>     do
>       { v0 = e0
>       ; print v0
>       ; v1 = e1
>      ; print v1
>       }

That would have to be
do let v0 = e0
   print v0
   let v1 = e1
   print v1

or
do v0 <- return e0
   print v0
   v1 <- return e1
   print v1

(better (?):
do print $ e0
   print $ e1
)
>   which is easier to read and more intuitive for
>   c++ programmers.

"intuitive for c++ programmers" is dangerous, the languages are very 
different, and one shouldn't gloss over that. The different syntax should 
help to not confound the languages.

> Example_code_suggesting_should_work:
>   http://www.nabble.com/List-as-input-p19987726.html
> -}
> module Main where
> import Data.List
> main = do
> { v0 <- [999]
> ; putStr "v0="
> ; print v0
> }

remember that in

do value <- action
   statements -- using value (or not)

action and statements must belong to the same monad. In your code above, the 
'action' [999] has type Num a => [a], so the monad is [], but putStr "v0=" 
has type IO (), the monad is IO. So the binding v0 <- [999] and the statement 
putStr "v0=" can't appear in the same do-block.

If what you want is "for every member of some list, do some monadic action", 
you need mapM/mapM_ resp forM(_):

mapM(_) (\v -> putStr "v0=" >> print v) [999]

or
forM(_) [999] $ (putStr "v0=" >>) . print

especially

forM_ list $ \v -> do
	some
	actions
	with v

looks pretty imperative.