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.